[3/3] sched: Update ->next_balance correctly during newidle balance

  From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>

When newidle balancing triggers, we see that it constantly clobbers
rq->next_balance even when there is no newidle balance happening due to
the cost estimates.  Due to this, we see that periodic load balance
(rebalance_domains) may trigger way more often when the CPU is going in
and out of idle at a high rate but is no really idle. Repeatedly
triggering load balance there is a bad idea as it is a heavy operation.
It also causes increases in softirq.

Another issue is ->last_balance is not updated after newidle balance
causing mistakes in the ->next_balance calculations.

Fix by updating last_balance when a newidle load balance actually
happens and then updating next_balance. This is also how it is done in
other load balance paths.

Testing shows a significant drop in softirqs when running:
cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q

Goes from ~6k to ~800.

Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/fair.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)
  

* Joel Fernandes (Google) <joel@joelfernandes.org> wrote:

> From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> 
> When newidle balancing triggers, we see that it constantly clobbers 
> rq->next_balance even when there is no newidle balance happening due to 
> the cost estimates.  Due to this, we see that periodic load balance 
> (rebalance_domains) may trigger way more often when the CPU is going in 
> and out of idle at a high rate but is no really idle. Repeatedly 
> triggering load balance there is a bad idea as it is a heavy operation. 
> It also causes increases in softirq.
> 
> Another issue is ->last_balance is not updated after newidle balance 
> causing mistakes in the ->next_balance calculations.
> 
> Fix by updating last_balance when a newidle load balance actually happens 
> and then updating next_balance. This is also how it is done in other load 
> balance paths.
> 
> Testing shows a significant drop in softirqs when running:
> cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> 
> Goes from ~6k to ~800.
> 
> Cc: Suleiman Souhlal <suleiman@google.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/fair.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8e276d12c3cb..b147ad09126a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  
>  	if (!READ_ONCE(this_rq->rd->overload) ||
>  	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> -
> -		if (sd)
> -			update_next_balance(sd, &next_balance);
>  		rcu_read_unlock();
> -
>  		goto out;
>  	}
>  	rcu_read_unlock();
> @@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  		int continue_balancing = 1;
>  		u64 domain_cost;
>  
> -		update_next_balance(sd, &next_balance);
> -
>  		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>  			break;
>  
> @@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  			t1 = sched_clock_cpu(this_cpu);
>  			domain_cost = t1 - t0;
>  			update_newidle_cost(sd, domain_cost);
> +			sd->last_balance = jiffies;
> +			update_next_balance(sd, &next_balance);
>  
>  			curr_cost += domain_cost;
>  			t0 = t1;

Okay, I'm applying patches #2 and #3, without #1: it should be safe
out of order, but let me know if I missed something ...

Thanks,

	Ingo
  

On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
>
> When newidle balancing triggers, we see that it constantly clobbers
> rq->next_balance even when there is no newidle balance happening due to
> the cost estimates.  Due to this, we see that periodic load balance
> (rebalance_domains) may trigger way more often when the CPU is going in
> and out of idle at a high rate but is no really idle. Repeatedly
> triggering load balance there is a bad idea as it is a heavy operation.
> It also causes increases in softirq.

we have 2 balance intervals:
- one when idle based on the sd->balance_interval = sd_weight
- one when busy which increases the period by multiplying it with
busy_factor = 16

When becoming idle, the rq->next_balance can have been set using the
16*sd_weight period so load_balance can wait for a long time before
running idle load balance for this cpu.
As a typical example, instead of waiting at most 8ms, we will wait
128ms before we try to pull a task on the idle CPU.

That's the reason for updating rq->next_balance in newidle_balance()

>
> Another issue is ->last_balance is not updated after newidle balance
> causing mistakes in the ->next_balance calculations.

newly idle load balance is not equal to idle load balance. It's a
light load balance trying to pull one  task and you can't really
consider it to the normal load balance

>
> Fix by updating last_balance when a newidle load balance actually
> happens and then updating next_balance. This is also how it is done in
> other load balance paths.
>
> Testing shows a significant drop in softirqs when running:
> cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
>
> Goes from ~6k to ~800.

Even if your figures look interesting, your patch adds regression in
the load balance and the fairness.

We can probably do improve the current behavior for decreasing number
of ILB but your proposal is not the right solution IMO

>
> Cc: Suleiman Souhlal <suleiman@google.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/fair.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8e276d12c3cb..b147ad09126a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>
>         if (!READ_ONCE(this_rq->rd->overload) ||
>             (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> -
> -               if (sd)
> -                       update_next_balance(sd, &next_balance);
>                 rcu_read_unlock();
> -
>                 goto out;
>         }
>         rcu_read_unlock();
> @@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>                 int continue_balancing = 1;
>                 u64 domain_cost;
>
> -               update_next_balance(sd, &next_balance);
> -
>                 if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>                         break;
>
> @@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>                         t1 = sched_clock_cpu(this_cpu);
>                         domain_cost = t1 - t0;
>                         update_newidle_cost(sd, domain_cost);
> +                       sd->last_balance = jiffies;
> +                       update_next_balance(sd, &next_balance);
>
>                         curr_cost += domain_cost;
>                         t0 = t1;
> --
> 2.42.0.655.g421f12c284-goog
>
  

Hi Ingo,

On Fri, 20 Oct 2023 at 09:53, Ingo Molnar <mingo@kernel.org> wrote:
>
>

...

> >                       curr_cost += domain_cost;
> >                       t0 = t1;
>
> Okay, I'm applying patches #2 and #3, without #1: it should be safe
> out of order, but let me know if I missed something ...

Could you hold on for patch 3 ? As replied to Joel, the patch had
regression in the update of rq->next_balance

>
> Thanks,
>
>         Ingo
  

* Vincent Guittot <vincent.guittot@linaro.org> wrote:

> Even if your figures look interesting, your patch adds regression in
> the load balance and the fairness.

Indeed - I've removed it from tip:sched/core for the time being.

Thanks,

	Ingo
  

On Fri, Oct 20, 2023 at 9:57 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Vincent Guittot <vincent.guittot@linaro.org> wrote:
>
> > Even if your figures look interesting, your patch adds regression in
> > the load balance and the fairness.
>
> Indeed - I've removed it from tip:sched/core for the time being.

Sorry we should have marked it as RFC. We still feel there are issues
in the newidle balance code and the fix is in the right direction,
we'll continue discussing with Vincent on the other thread and try to
come up with a proper solution.

Thank you Ingo and Vincent!

 - Joel & Vineeth
  

* Vincent Guittot <vincent.guittot@linaro.org> wrote:

> Hi Ingo,
> 
> On Fri, 20 Oct 2023 at 09:53, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> 
> ...
> 
> > >                       curr_cost += domain_cost;
> > >                       t0 = t1;
> >
> > Okay, I'm applying patches #2 and #3, without #1: it should be safe
> > out of order, but let me know if I missed something ...
> 
> Could you hold on for patch 3 ? As replied to Joel, the patch had
> regression in the update of rq->next_balance

Yeah, patch #3 is gone already from tip:sched/core.

Thanks,

	Ingo
  

On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote:
> On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> >
> > From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> >
> > When newidle balancing triggers, we see that it constantly clobbers
> > rq->next_balance even when there is no newidle balance happening due to
> > the cost estimates.  Due to this, we see that periodic load balance
> > (rebalance_domains) may trigger way more often when the CPU is going in
> > and out of idle at a high rate but is no really idle. Repeatedly
> > triggering load balance there is a bad idea as it is a heavy operation.
> > It also causes increases in softirq.
> 
> we have 2 balance intervals:
> - one when idle based on the sd->balance_interval = sd_weight
> - one when busy which increases the period by multiplying it with
> busy_factor = 16

On my production system I see load balance triggering every 4 jiffies! In a
Qemu system (which I use for traces below), I see every 8 jiffies. I'll go
look into that more as well, it could be something going on in
get_sd_balance_interval().

> When becoming idle, the rq->next_balance can have been set using the
> 16*sd_weight period so load_balance can wait for a long time before
> running idle load balance for this cpu.

Got it.

> As a typical example, instead of waiting at most 8ms, we will wait
> 128ms before we try to pull a task on the idle CPU.
> 
> That's the reason for updating rq->next_balance in newidle_balance()

Got it, makes sense. But I still feel the mechanism is too aggressive, see
below.

> > Another issue is ->last_balance is not updated after newidle balance
> > causing mistakes in the ->next_balance calculations.
> 
> newly idle load balance is not equal to idle load balance. It's a
> light load balance trying to pull one  task and you can't really
> consider it to the normal load balance

True. However the point is that it is coupled with the other load balance
mechanism and the two are not independent. As you can see below, modifying
rq->next_balance in newidle also causes the periodic balance to happen more
aggressively as well if there is a high transition from busy to idle and
viceversa.

> > Fix by updating last_balance when a newidle load balance actually
> > happens and then updating next_balance. This is also how it is done in
> > other load balance paths.
> >
> > Testing shows a significant drop in softirqs when running:
> > cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> >
> > Goes from ~6k to ~800.
> 
> Even if your figures look interesting, your patch adds regression in
> the load balance and the fairness.

Yes I see that now. However it does illustrate the problem IMO.

> We can probably do improve the current behavior for decreasing number
> of ILB but your proposal is not the right solution IMO

One of the problems is if you have task goes idle a lot, then the
newidle_balance mechanism triggers the periodic balance every jiffie (once
per millisecond on HZ=1000).

Following are some traces I collected.

cyclictest-123   [003]   522.650574  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157)
   <idle>-0      [003]   522.651443  trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1
   <idle>-0      [003]   522.651461  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158)
cyclictest-123   [003]   522.651494  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158)
   <idle>-0      [003]   522.652522  trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1
   <idle>-0      [003]   522.652560  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159)
cyclictest-124   [003]   522.652586  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159)
   <idle>-0      [003]   522.654492  trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1
   <idle>-0      [003]   522.654534  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161)

Triggering it so aggressively likely that is useless, it increases softirq
count and hurts power when no real work is done IMHO. And it probably makes
things worse for power on ARM where you have uclamp stuff happening in the
load balance paths which is quite heavy when I last traced that..

Further, we have observed in our tracing on real device that the update of
rq->next_balance from the newidle path is itself buggy... we observed that
because newidle balance may not update rq->last_balance, it is possible that
rq->next_balance when updated by update_next_balance() will be updated to a
value that is in the past and it will be stuck there for a long time! Perhaps
we should investigate more and fix that bug separately. Vineeth could provide
more details on the "getting stuck in the past" behavior as well.

I hope these patches highlight some of the issues we find and can trigger any
improvements by us or others. From our side we'll continue to work on it and
thank you for explaining some of the load balance code!

thanks,

 - Joel & Vineeth


> 
> >
> > Cc: Suleiman Souhlal <suleiman@google.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> > Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  kernel/sched/fair.c | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 8e276d12c3cb..b147ad09126a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >
> >         if (!READ_ONCE(this_rq->rd->overload) ||
> >             (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> > -
> > -               if (sd)
> > -                       update_next_balance(sd, &next_balance);
> >                 rcu_read_unlock();
> > -
> >                 goto out;
> >         }
> >         rcu_read_unlock();
> > @@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >                 int continue_balancing = 1;
> >                 u64 domain_cost;
> >
> > -               update_next_balance(sd, &next_balance);
> > -
> >                 if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
> >                         break;
> >
> > @@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >                         t1 = sched_clock_cpu(this_cpu);
> >                         domain_cost = t1 - t0;
> >                         update_newidle_cost(sd, domain_cost);
> > +                       sd->last_balance = jiffies;
> > +                       update_next_balance(sd, &next_balance);
> >
> >                         curr_cost += domain_cost;
> >                         t0 = t1;
> > --
> > 2.42.0.655.g421f12c284-goog
> >
  

On Sun, 22 Oct 2023 at 02:28, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote:
> > On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
> > <joel@joelfernandes.org> wrote:
> > >
> > > From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> > >
> > > When newidle balancing triggers, we see that it constantly clobbers
> > > rq->next_balance even when there is no newidle balance happening due to
> > > the cost estimates.  Due to this, we see that periodic load balance
> > > (rebalance_domains) may trigger way more often when the CPU is going in
> > > and out of idle at a high rate but is no really idle. Repeatedly
> > > triggering load balance there is a bad idea as it is a heavy operation.
> > > It also causes increases in softirq.
> >
> > we have 2 balance intervals:
> > - one when idle based on the sd->balance_interval = sd_weight
> > - one when busy which increases the period by multiplying it with
> > busy_factor = 16
>
> On my production system I see load balance triggering every 4 jiffies! In a

Which kind of system do you have? sd->balance_interval is in ms

> Qemu system (which I use for traces below), I see every 8 jiffies. I'll go
> look into that more as well, it could be something going on in
> get_sd_balance_interval().
>
> > When becoming idle, the rq->next_balance can have been set using the
> > 16*sd_weight period so load_balance can wait for a long time before
> > running idle load balance for this cpu.
>
> Got it.
>
> > As a typical example, instead of waiting at most 8ms, we will wait
> > 128ms before we try to pull a task on the idle CPU.
> >
> > That's the reason for updating rq->next_balance in newidle_balance()
>
> Got it, makes sense. But I still feel the mechanism is too aggressive, see
> below.
>
> > > Another issue is ->last_balance is not updated after newidle balance
> > > causing mistakes in the ->next_balance calculations.
> >
> > newly idle load balance is not equal to idle load balance. It's a
> > light load balance trying to pull one  task and you can't really
> > consider it to the normal load balance
>
> True. However the point is that it is coupled with the other load balance
> mechanism and the two are not independent. As you can see below, modifying
> rq->next_balance in newidle also causes the periodic balance to happen more
> aggressively as well if there is a high transition from busy to idle and
> viceversa.

As mentioned, rq->next_balance is updated whenever cpu enters idle
(i.e. in newidle_balance() but it's not related with doing a newly
idle load balance. But your problem is more related with the fact that
nohz.needs_update is set when stopping cpu timer in order to update
nohz.next_balance which is then used to kick a "real" idle load
balance

>
> > > Fix by updating last_balance when a newidle load balance actually
> > > happens and then updating next_balance. This is also how it is done in
> > > other load balance paths.
> > >
> > > Testing shows a significant drop in softirqs when running:
> > > cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> > >
> > > Goes from ~6k to ~800.
> >
> > Even if your figures look interesting, your patch adds regression in
> > the load balance and the fairness.
>
> Yes I see that now. However it does illustrate the problem IMO.
>
> > We can probably do improve the current behavior for decreasing number
> > of ILB but your proposal is not the right solution IMO
>
> One of the problems is if you have task goes idle a lot, then the
> newidle_balance mechanism triggers the periodic balance every jiffie (once
> per millisecond on HZ=1000).

every msec seems quite a lot.

>
> Following are some traces I collected.
>
> cyclictest-123   [003]   522.650574  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157)
>    <idle>-0      [003]   522.651443  trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1
>    <idle>-0      [003]   522.651461  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158)
> cyclictest-123   [003]   522.651494  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158)
>    <idle>-0      [003]   522.652522  trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1
>    <idle>-0      [003]   522.652560  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159)
> cyclictest-124   [003]   522.652586  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159)
>    <idle>-0      [003]   522.654492  trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1
>    <idle>-0      [003]   522.654534  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161)

Ok, so IIUC your trace above, this happens because the tick is not
stop after entering idle so it continues to fire and triggers a load
balance without checking if there is a need like what is done for nohz
mode

>
> Triggering it so aggressively likely that is useless, it increases softirq
> count and hurts power when no real work is done IMHO. And it probably makes

In the case above, the tick has already fired because it was not
stopped. I mean that it's not the ilb which triggers this and woke up
the cpu but it takes advantage of the tick so most of the pain has
already happened (wakeup the cpu which was probably in a shallow
c-state to let its tick firing)

> things worse for power on ARM where you have uclamp stuff happening in the
> load balance paths which is quite heavy when I last traced that..
>
> Further, we have observed in our tracing on real device that the update of
> rq->next_balance from the newidle path is itself buggy... we observed that
> because newidle balance may not update rq->last_balance, it is possible that
> rq->next_balance when updated by update_next_balance() will be updated to a
> value that is in the past and it will be stuck there for a long time! Perhaps
> we should investigate more and fix that bug separately. Vineeth could provide
> more details on the "getting stuck in the past" behavior as well.

sd->last_balance reflects last time an idle/busy load_balance happened
(newly idle is out of the scope for the points that I mentioned
previously).  So if no load balance happens for a while, the
rq->next_balance can be in the past but I don't see a problem here. It
just means that a load balance hasn't happened for a while. It can
even move backward if it has been set when busy but the cpu is now
idle

>
> I hope these patches highlight some of the issues we find and can trigger any
> improvements by us or others. From our side we'll continue to work on it and
> thank you for explaining some of the load balance code!
>
> thanks,
>
>  - Joel & Vineeth
>
>
> >
> > >
> > > Cc: Suleiman Souhlal <suleiman@google.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Frederic Weisbecker <frederic@kernel.org>
> > > Cc: Paul E. McKenney <paulmck@kernel.org>
> > > Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> > > Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > ---
> > >  kernel/sched/fair.c | 8 ++------
> > >  1 file changed, 2 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 8e276d12c3cb..b147ad09126a 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> > >
> > >         if (!READ_ONCE(this_rq->rd->overload) ||
> > >             (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> > > -
> > > -               if (sd)
> > > -                       update_next_balance(sd, &next_balance);
> > >                 rcu_read_unlock();
> > > -
> > >                 goto out;
> > >         }
> > >         rcu_read_unlock();
> > > @@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> > >                 int continue_balancing = 1;
> > >                 u64 domain_cost;
> > >
> > > -               update_next_balance(sd, &next_balance);
> > > -
> > >                 if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
> > >                         break;
> > >
> > > @@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> > >                         t1 = sched_clock_cpu(this_cpu);
> > >                         domain_cost = t1 - t0;
> > >                         update_newidle_cost(sd, domain_cost);
> > > +                       sd->last_balance = jiffies;
> > > +                       update_next_balance(sd, &next_balance);
> > >
> > >                         curr_cost += domain_cost;
> > >                         t0 = t1;
> > > --
> > > 2.42.0.655.g421f12c284-goog
> > >
  

Hi Vincent,

Sorry for late reply, I was in Tokyo all these days and was waiting to get to
writing a proper reply. See my replies below:

On Thu, Oct 26, 2023 at 04:23:35PM +0200, Vincent Guittot wrote:
> On Sun, 22 Oct 2023 at 02:28, Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote:
> > > On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
> > > <joel@joelfernandes.org> wrote:
> > > >
> > > > From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> > > >
> > > > When newidle balancing triggers, we see that it constantly clobbers
> > > > rq->next_balance even when there is no newidle balance happening due to
> > > > the cost estimates.  Due to this, we see that periodic load balance
> > > > (rebalance_domains) may trigger way more often when the CPU is going in
> > > > and out of idle at a high rate but is no really idle. Repeatedly
> > > > triggering load balance there is a bad idea as it is a heavy operation.
> > > > It also causes increases in softirq.
> > >
> > > we have 2 balance intervals:
> > > - one when idle based on the sd->balance_interval = sd_weight
> > > - one when busy which increases the period by multiplying it with
> > > busy_factor = 16
> >
> > On my production system I see load balance triggering every 4 jiffies! In a
> 
> Which kind of system do you have? sd->balance_interval is in ms

Yes, sorry I meant it triggers every jiffies which is extreme sometimes. It
is an ADL SoC (12th gen Intel, 4 P cores 8 E cores) get_sd_balance_interval()
returns 4 jiffies there. On my Qemu system, I see 8 jiffies.

[...]
> > > > Another issue is ->last_balance is not updated after newidle balance
> > > > causing mistakes in the ->next_balance calculations.
> > >
> > > newly idle load balance is not equal to idle load balance. It's a
> > > light load balance trying to pull one  task and you can't really
> > > consider it to the normal load balance
> >
> > True. However the point is that it is coupled with the other load balance
> > mechanism and the two are not independent. As you can see below, modifying
> > rq->next_balance in newidle also causes the periodic balance to happen more
> > aggressively as well if there is a high transition from busy to idle and
> > viceversa.
> 
> As mentioned, rq->next_balance is updated whenever cpu enters idle
> (i.e. in newidle_balance() but it's not related with doing a newly
> idle load balance.

Yes, I understand that. But my point was that the update of rq->next_balance
from the newidle path is itself buggy and interferes with the load balance
happening from the tick (trigger_load_balance -> run_rebalance_domains).

> But your problem is more related with the fact that
> nohz.needs_update is set when stopping cpu timer in order to update
> nohz.next_balance which is then used to kick a "real" idle load
> balance

Well, independent of nohz idle balance, I think we need to fix this issue as
mentioned above. This effect the periodic one as mentioned in the commit log.

See here another trace I collected this time dumping the 'interval'. There is
a tug of war happening between the newidle balance and the periodic balance.

The periodic one sets rq->next_balance for cpu 0 to 760,143 and then the
newidle comes in pulls it back a 118 jiffies to 760,024. This is actually in
the past because jiffies is currently 760,045 !!

This triggers the periodic balance againwhich sets rq->next_balance back to
760,143.

Rinse and repeat. End result is you have periodic balance every jiffies. With
this patch the issue goes away but we could fix it differently as you
mentioned, we need to pull newidle balance back but perhaps not so
aggressively. How about something like the untested diff I enclosed at the
end of this email?

<idle>-0   [000]  13.081781: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,045)
cyclictest-120   [000]  13.081806: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
cyclictest-120   [000]  13.081807: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,045)
cyclictest-120   [000]  13.082130: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
cyclictest-120   [000]  13.082338: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
cyclictest-120   [000]  13.082636: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
<idle>-0   [000]  13.082823: trigger_load_balance: time_after_eq(jiffies=760,046, rq->next_balance=760,024) = 1
<idle>-0   [000]  13.082823: softirq_raise: vec=7 [action=SCHED]
<idle>-0   [000]  13.082871: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,046)
trace-cmd-114   [000]  13.082876: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
trace-cmd-114   [000]  13.082876: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,046)
cyclictest-120   [000]  13.083333: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
cyclictest-120   [000]  13.083633: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
<idle>-0   [000]  13.083656: trigger_load_balance: time_after_eq(jiffies=760,047, rq->next_balance=760,024) = 1
<idle>-0   [000]  13.083656: softirq_raise: vec=7 [action=SCHED]
<idle>-0   [000]  13.083702: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,047)
cyclictest-120   [000]  13.083729: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
cyclictest-120   [000]  13.083730: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,047)
cyclictest-120   [000]  13.083960: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
cyclictest-120   [000]  13.084069: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
cyclictest-120   [000]  13.084423: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
cyclictest-120   [000]  13.084633: trigger_load_balance: time_after_eq(jiffies=760,048, rq->next_balance=760,024) = 1
cyclictest-120   [000]  13.084633: softirq_raise: vec=7 [action=SCHED]
cyclictest-120   [000]  13.084678: rebalance_domains: rq[cpu=0]->next_balance:

> > > > Fix by updating last_balance when a newidle load balance actually
> > > > happens and then updating next_balance. This is also how it is done in
> > > > other load balance paths.
> > > >
> > > > Testing shows a significant drop in softirqs when running:
> > > > cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> > > >
> > > > Goes from ~6k to ~800.
> > >
> > > Even if your figures look interesting, your patch adds regression in
> > > the load balance and the fairness.
> >
> > Yes I see that now. However it does illustrate the problem IMO.
> >
> > > We can probably do improve the current behavior for decreasing number
> > > of ILB but your proposal is not the right solution IMO
> >
> > One of the problems is if you have task goes idle a lot, then the
> > newidle_balance mechanism triggers the periodic balance every jiffie (once
> > per millisecond on HZ=1000).
> 
> every msec seems quite a lot.

Yeah!

> >
> > Following are some traces I collected.
> >
> > cyclictest-123   [003]   522.650574  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157)
> >    <idle>-0      [003]   522.651443  trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1
> >    <idle>-0      [003]   522.651461  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158)
> > cyclictest-123   [003]   522.651494  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158)
> >    <idle>-0      [003]   522.652522  trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1
> >    <idle>-0      [003]   522.652560  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159)
> > cyclictest-124   [003]   522.652586  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159)
> >    <idle>-0      [003]   522.654492  trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1
> >    <idle>-0      [003]   522.654534  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161)
> 
> Ok, so IIUC your trace above, this happens because the tick is not
> stop after entering idle so it continues to fire and triggers a load
> balance without checking if there is a need like what is done for nohz
> mode

The tick is normally not stopped if the CPU is awakened too soon by a timer.
That's pretty normal AFAIK. As you can see in the traces above, cyclictest
keeps waking up.

> > things worse for power on ARM where you have uclamp stuff happening in the
> > load balance paths which is quite heavy when I last traced that..
> >
> > Further, we have observed in our tracing on real device that the update of
> > rq->next_balance from the newidle path is itself buggy... we observed that
> > because newidle balance may not update rq->last_balance, it is possible that
> > rq->next_balance when updated by update_next_balance() will be updated to a
> > value that is in the past and it will be stuck there for a long time! Perhaps
> > we should investigate more and fix that bug separately. Vineeth could provide
> > more details on the "getting stuck in the past" behavior as well.
> 
> sd->last_balance reflects last time an idle/busy load_balance happened
> (newly idle is out of the scope for the points that I mentioned
> previously).  So if no load balance happens for a while, the
> rq->next_balance can be in the past but I don't see a problem here. It
> just means that a load balance hasn't happened for a while. It can
> even move backward if it has been set when busy but the cpu is now
> idle

Sure, but I think it should at least set it by get_sd_balance_interval() into
the future. Like so (untested)? Let me know what you think and thanks!

---8<-----------------------

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a3318aeff9e8..0d6667d31c51 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11314,6 +11314,30 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)
 	return interval;
 }
 
+/*
+ * Update the next balance from newidle balance.
+ * The update of next_balance from newidle balance tries to make sure that
+ * we don't trigger periodic balance too far in the future on a now-idle
+ * system.  This is just like update_next_balance except that since
+ * sd->last_balance may not have been updated for a while, we're careful to
+ * not set next_balance in the past.
+ */
+static inline void
+update_next_balance_newidle(struct sched_domain *sd, unsigned long *next_balance)
+{
+	unsigned long interval, next;
+
+	/* used by new idle balance, so cpu_busy = 0 */
+	interval = get_sd_balance_interval(sd, 0);
+	next = sd->last_balance + interval;
+
+	next = max(next, jiffies + interval);
+
+	if (time_after(*next_balance, next)) {
+		*next_balance = next;
+	}
+}
+
 static inline void
 update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
 {
@@ -12107,7 +12131,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
 
 		if (sd)
-			update_next_balance(sd, &next_balance);
+			update_next_balance_newidle(sd, &next_balance);
 		rcu_read_unlock();
 
 		goto out;
@@ -12124,7 +12148,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 		int continue_balancing = 1;
 		u64 domain_cost;
 
-		update_next_balance(sd, &next_balance);
+		update_next_balance_newidle(sd, &next_balance);
 
 		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
 			break;
  

On Thu, Nov 9, 2023 at 5:02 AM Joel Fernandes <joel@joelfernandes.org> wrote:
[...]
> > > things worse for power on ARM where you have uclamp stuff happening in the
> > > load balance paths which is quite heavy when I last traced that..
> > >
> > > Further, we have observed in our tracing on real device that the update of
> > > rq->next_balance from the newidle path is itself buggy... we observed that
> > > because newidle balance may not update rq->last_balance, it is possible that
> > > rq->next_balance when updated by update_next_balance() will be updated to a
> > > value that is in the past and it will be stuck there for a long time! Perhaps
> > > we should investigate more and fix that bug separately. Vineeth could provide
> > > more details on the "getting stuck in the past" behavior as well.
> >
> > sd->last_balance reflects last time an idle/busy load_balance happened
> > (newly idle is out of the scope for the points that I mentioned
> > previously).  So if no load balance happens for a while, the
> > rq->next_balance can be in the past but I don't see a problem here. It
> > just means that a load balance hasn't happened for a while. It can
> > even move backward if it has been set when busy but the cpu is now
> > idle
>
> Sure, but I think it should at least set it by get_sd_balance_interval() into
> the future. Like so (untested)? Let me know what you think and thanks!

Btw, I also drew a graph showing the issue without patch:
https://i.imgur.com/RgTr45l.png

Each "x" mark is run_rebalance_domains() running on a CPU. As can be
seen, there were some 10 occurrences in a span of 15ms in one
instance.

Thanks,

 - Joel


> ---8<-----------------------
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a3318aeff9e8..0d6667d31c51 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11314,6 +11314,30 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)
>         return interval;
>  }
>
> +/*
> + * Update the next balance from newidle balance.
> + * The update of next_balance from newidle balance tries to make sure that
> + * we don't trigger periodic balance too far in the future on a now-idle
> + * system.  This is just like update_next_balance except that since
> + * sd->last_balance may not have been updated for a while, we're careful to
> + * not set next_balance in the past.
> + */
> +static inline void
> +update_next_balance_newidle(struct sched_domain *sd, unsigned long *next_balance)
> +{
> +       unsigned long interval, next;
> +
> +       /* used by new idle balance, so cpu_busy = 0 */
> +       interval = get_sd_balance_interval(sd, 0);
> +       next = sd->last_balance + interval;
> +
> +       next = max(next, jiffies + interval);
> +
> +       if (time_after(*next_balance, next)) {
> +               *next_balance = next;
> +       }
> +}
> +
>  static inline void
>  update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>  {
> @@ -12107,7 +12131,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>             (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>
>                 if (sd)
> -                       update_next_balance(sd, &next_balance);
> +                       update_next_balance_newidle(sd, &next_balance);
>                 rcu_read_unlock();
>
>                 goto out;
> @@ -12124,7 +12148,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>                 int continue_balancing = 1;
>                 u64 domain_cost;
>
> -               update_next_balance(sd, &next_balance);
> +               update_next_balance_newidle(sd, &next_balance);
>
>                 if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>                         break;
  

Le jeudi 09 nov. 2023 à 10:02:54 (+0000), Joel Fernandes a écrit :
> Hi Vincent,
> 
> Sorry for late reply, I was in Tokyo all these days and was waiting to get to
> writing a proper reply. See my replies below:
> 
> On Thu, Oct 26, 2023 at 04:23:35PM +0200, Vincent Guittot wrote:
> > On Sun, 22 Oct 2023 at 02:28, Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote:
> > > > On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
> > > > <joel@joelfernandes.org> wrote:
> > > > >
> > > > > From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> > > > >
> > > > > When newidle balancing triggers, we see that it constantly clobbers
> > > > > rq->next_balance even when there is no newidle balance happening due to
> > > > > the cost estimates.  Due to this, we see that periodic load balance
> > > > > (rebalance_domains) may trigger way more often when the CPU is going in
> > > > > and out of idle at a high rate but is no really idle. Repeatedly
> > > > > triggering load balance there is a bad idea as it is a heavy operation.
> > > > > It also causes increases in softirq.
> > > >
> > > > we have 2 balance intervals:
> > > > - one when idle based on the sd->balance_interval = sd_weight
> > > > - one when busy which increases the period by multiplying it with
> > > > busy_factor = 16
> > >
> > > On my production system I see load balance triggering every 4 jiffies! In a
> > 
> > Which kind of system do you have? sd->balance_interval is in ms
> 
> Yes, sorry I meant it triggers every jiffies which is extreme sometimes. It
> is an ADL SoC (12th gen Intel, 4 P cores 8 E cores) get_sd_balance_interval()
> returns 4 jiffies there. On my Qemu system, I see 8 jiffies.

Do you have details about the sched_domain  hierarchy ?
That could be part of your problem (see below)

> 
> [...]
> > > > > Another issue is ->last_balance is not updated after newidle balance
> > > > > causing mistakes in the ->next_balance calculations.
> > > >
> > > > newly idle load balance is not equal to idle load balance. It's a
> > > > light load balance trying to pull one  task and you can't really
> > > > consider it to the normal load balance
> > >
> > > True. However the point is that it is coupled with the other load balance
> > > mechanism and the two are not independent. As you can see below, modifying
> > > rq->next_balance in newidle also causes the periodic balance to happen more
> > > aggressively as well if there is a high transition from busy to idle and
> > > viceversa.
> > 
> > As mentioned, rq->next_balance is updated whenever cpu enters idle
> > (i.e. in newidle_balance() but it's not related with doing a newly
> > idle load balance.
> 
> Yes, I understand that. But my point was that the update of rq->next_balance
> from the newidle path is itself buggy and interferes with the load balance
> happening from the tick (trigger_load_balance -> run_rebalance_domains).

Newidle path is not buggy. It only uses sd->last_balance + interval to
estimate the next balance  which is the correct thing to do. Your problem
comes from the update of sd->last_balance which never happens and remains
in the past whereas you call run_rebalance_domains() which should
run load_balance for all domains with a sd->last_balance + interval in the
past.
Your problem most probably comes from the should_we_balance which always or
"almost always" returns false in your use case for some sched_domain and
prevents to updat sd->last_balance. Could you try the patch below ?
It should fix your problem of trying to rebalance every tick whereas
rebalance_domain is called.
At least this should show if it's your problem but I'm not sure it's the right
things to do all the time ...

---
 kernel/sched/fair.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3745ca289240..9ea1f42e5362 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11568,17 +11568,6 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 		need_decay = update_newidle_cost(sd, 0);
 		max_cost += sd->max_newidle_lb_cost;

-		/*
-		 * Stop the load balance at this level. There is another
-		 * CPU in our sched group which is doing load balancing more
-		 * actively.
-		 */
-		if (!continue_balancing) {
-			if (need_decay)
-				continue;
-			break;
-		}
-
 		interval = get_sd_balance_interval(sd, busy);

 		need_serialize = sd->flags & SD_SERIALIZE;
@@ -11588,7 +11577,12 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 		}

 		if (time_after_eq(jiffies, sd->last_balance + interval)) {
-			if (load_balance(cpu, rq, sd, idle, &continue_balancing)) {
+			/*
+			 * Stop the load balance at this level. There is another
+			 * CPU in our sched group which is doing load balancing more
+			 * actively.
+			 */
+			if (continue_balancing && load_balance(cpu, rq, sd, idle, &continue_balancing)) {
 				/*
 				 * The LBF_DST_PINNED logic could have changed
 				 * env->dst_cpu, so we can't know our idle
--
2.34.1

> 
> > But your problem is more related with the fact that
> > nohz.needs_update is set when stopping cpu timer in order to update
> > nohz.next_balance which is then used to kick a "real" idle load
> > balance
> 
> Well, independent of nohz idle balance, I think we need to fix this issue as
> mentioned above. This effect the periodic one as mentioned in the commit log.
> 
> See here another trace I collected this time dumping the 'interval'. There is
> a tug of war happening between the newidle balance and the periodic balance.
> 
> The periodic one sets rq->next_balance for cpu 0 to 760,143 and then the
> newidle comes in pulls it back a 118 jiffies to 760,024. This is actually in
> the past because jiffies is currently 760,045 !!
> 
> This triggers the periodic balance againwhich sets rq->next_balance back to
> 760,143.
> 
> Rinse and repeat. End result is you have periodic balance every jiffies. With
> this patch the issue goes away but we could fix it differently as you
> mentioned, we need to pull newidle balance back but perhaps not so
> aggressively. How about something like the untested diff I enclosed at the
> end of this email?
> 
> <idle>-0   [000]  13.081781: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,045)
> cyclictest-120   [000]  13.081806: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
> cyclictest-120   [000]  13.081807: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,045)
> cyclictest-120   [000]  13.082130: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
> cyclictest-120   [000]  13.082338: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
> cyclictest-120   [000]  13.082636: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
> <idle>-0   [000]  13.082823: trigger_load_balance: time_after_eq(jiffies=760,046, rq->next_balance=760,024) = 1
> <idle>-0   [000]  13.082823: softirq_raise: vec=7 [action=SCHED]
> <idle>-0   [000]  13.082871: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,046)
> trace-cmd-114   [000]  13.082876: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
> trace-cmd-114   [000]  13.082876: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,046)
> cyclictest-120   [000]  13.083333: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
> cyclictest-120   [000]  13.083633: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
> <idle>-0   [000]  13.083656: trigger_load_balance: time_after_eq(jiffies=760,047, rq->next_balance=760,024) = 1
> <idle>-0   [000]  13.083656: softirq_raise: vec=7 [action=SCHED]
> <idle>-0   [000]  13.083702: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,047)
> cyclictest-120   [000]  13.083729: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
> cyclictest-120   [000]  13.083730: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,047)
> cyclictest-120   [000]  13.083960: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
> cyclictest-120   [000]  13.084069: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
> cyclictest-120   [000]  13.084423: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
> cyclictest-120   [000]  13.084633: trigger_load_balance: time_after_eq(jiffies=760,048, rq->next_balance=760,024) = 1
> cyclictest-120   [000]  13.084633: softirq_raise: vec=7 [action=SCHED]
> cyclictest-120   [000]  13.084678: rebalance_domains: rq[cpu=0]->next_balance:
> 
> > > > > Fix by updating last_balance when a newidle load balance actually
> > > > > happens and then updating next_balance. This is also how it is done in
> > > > > other load balance paths.
> > > > >
> > > > > Testing shows a significant drop in softirqs when running:
> > > > > cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> > > > >
> > > > > Goes from ~6k to ~800.
> > > >
> > > > Even if your figures look interesting, your patch adds regression in
> > > > the load balance and the fairness.
> > >
> > > Yes I see that now. However it does illustrate the problem IMO.
> > >
> > > > We can probably do improve the current behavior for decreasing number
> > > > of ILB but your proposal is not the right solution IMO
> > >
> > > One of the problems is if you have task goes idle a lot, then the
> > > newidle_balance mechanism triggers the periodic balance every jiffie (once
> > > per millisecond on HZ=1000).
> > 
> > every msec seems quite a lot.
> 
> Yeah!
> 
> > >
> > > Following are some traces I collected.
> > >
> > > cyclictest-123   [003]   522.650574  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157)
> > >    <idle>-0      [003]   522.651443  trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1
> > >    <idle>-0      [003]   522.651461  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158)
> > > cyclictest-123   [003]   522.651494  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158)
> > >    <idle>-0      [003]   522.652522  trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1
> > >    <idle>-0      [003]   522.652560  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159)
> > > cyclictest-124   [003]   522.652586  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159)
> > >    <idle>-0      [003]   522.654492  trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1
> > >    <idle>-0      [003]   522.654534  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161)
> > 
> > Ok, so IIUC your trace above, this happens because the tick is not
> > stop after entering idle so it continues to fire and triggers a load
> > balance without checking if there is a need like what is done for nohz
> > mode
> 
> The tick is normally not stopped if the CPU is awakened too soon by a timer.
> That's pretty normal AFAIK. As you can see in the traces above, cyclictest
> keeps waking up.
> 
> > > things worse for power on ARM where you have uclamp stuff happening in the
> > > load balance paths which is quite heavy when I last traced that..
> > >
> > > Further, we have observed in our tracing on real device that the update of
> > > rq->next_balance from the newidle path is itself buggy... we observed that
> > > because newidle balance may not update rq->last_balance, it is possible that
> > > rq->next_balance when updated by update_next_balance() will be updated to a
> > > value that is in the past and it will be stuck there for a long time! Perhaps
> > > we should investigate more and fix that bug separately. Vineeth could provide
> > > more details on the "getting stuck in the past" behavior as well.
> > 
> > sd->last_balance reflects last time an idle/busy load_balance happened
> > (newly idle is out of the scope for the points that I mentioned
> > previously).  So if no load balance happens for a while, the
> > rq->next_balance can be in the past but I don't see a problem here. It
> > just means that a load balance hasn't happened for a while. It can
> > even move backward if it has been set when busy but the cpu is now
> > idle
> 
> Sure, but I think it should at least set it by get_sd_balance_interval() into
> the future. Like so (untested)? Let me know what you think and thanks!
> 
> ---8<-----------------------
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a3318aeff9e8..0d6667d31c51 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11314,6 +11314,30 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)
>  	return interval;
>  }
>  
> +/*
> + * Update the next balance from newidle balance.
> + * The update of next_balance from newidle balance tries to make sure that
> + * we don't trigger periodic balance too far in the future on a now-idle
> + * system.  This is just like update_next_balance except that since
> + * sd->last_balance may not have been updated for a while, we're careful to
> + * not set next_balance in the past.
> + */
> +static inline void
> +update_next_balance_newidle(struct sched_domain *sd, unsigned long *next_balance)
> +{
> +	unsigned long interval, next;
> +
> +	/* used by new idle balance, so cpu_busy = 0 */
> +	interval = get_sd_balance_interval(sd, 0);
> +	next = sd->last_balance + interval;
> +
> +	next = max(next, jiffies + interval);
> +
> +	if (time_after(*next_balance, next)) {
> +		*next_balance = next;
> +	}
> +}
> +
>  static inline void
>  update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>  {
> @@ -12107,7 +12131,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>  
>  		if (sd)
> -			update_next_balance(sd, &next_balance);
> +			update_next_balance_newidle(sd, &next_balance);
>  		rcu_read_unlock();
>  
>  		goto out;
> @@ -12124,7 +12148,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  		int continue_balancing = 1;
>  		u64 domain_cost;
>  
> -		update_next_balance(sd, &next_balance);
> +		update_next_balance_newidle(sd, &next_balance);
>  
>  		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>  			break;
  

Hi Vincent,

> On Nov 14, 2023, at 10:43 AM, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> 
> Le jeudi 09 nov. 2023 à 10:02:54 (+0000), Joel Fernandes a écrit :
>> Hi Vincent,
>> 
>> Sorry for late reply, I was in Tokyo all these days and was waiting to get to
>> writing a proper reply. See my replies below:
>> 
>>> On Thu, Oct 26, 2023 at 04:23:35PM +0200, Vincent Guittot wrote:
>>> On Sun, 22 Oct 2023 at 02:28, Joel Fernandes <joel@joelfernandes.org> wrote:
>>>> 
>>>> On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote:
>>>>> On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google)
>>>>> <joel@joelfernandes.org> wrote:
>>>>>> 
>>>>>> From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
>>>>>> 
>>>>>> When newidle balancing triggers, we see that it constantly clobbers
>>>>>> rq->next_balance even when there is no newidle balance happening due to
>>>>>> the cost estimates.  Due to this, we see that periodic load balance
>>>>>> (rebalance_domains) may trigger way more often when the CPU is going in
>>>>>> and out of idle at a high rate but is no really idle. Repeatedly
>>>>>> triggering load balance there is a bad idea as it is a heavy operation.
>>>>>> It also causes increases in softirq.
>>>>> 
>>>>> we have 2 balance intervals:
>>>>> - one when idle based on the sd->balance_interval = sd_weight
>>>>> - one when busy which increases the period by multiplying it with
>>>>> busy_factor = 16
>>>> 
>>>> On my production system I see load balance triggering every 4 jiffies! In a
>>> 
>>> Which kind of system do you have? sd->balance_interval is in ms
>> 
>> Yes, sorry I meant it triggers every jiffies which is extreme sometimes. It
>> is an ADL SoC (12th gen Intel, 4 P cores 8 E cores) get_sd_balance_interval()
>> returns 4 jiffies there. On my Qemu system, I see 8 jiffies.
> 
> Do you have details about the sched_domain  hierarchy ?
> That could be part of your problem (see below)

Since I am at LPC I am not at that machine right now but I could provide it next
week. I replied below:

> 
>> 
>> [...]
>>>>>> Another issue is ->last_balance is not updated after newidle balance
>>>>>> causing mistakes in the ->next_balance calculations.
>>>>> 
>>>>> newly idle load balance is not equal to idle load balance. It's a
>>>>> light load balance trying to pull one  task and you can't really
>>>>> consider it to the normal load balance
>>>> 
>>>> True. However the point is that it is coupled with the other load balance
>>>> mechanism and the two are not independent. As you can see below, modifying
>>>> rq->next_balance in newidle also causes the periodic balance to happen more
>>>> aggressively as well if there is a high transition from busy to idle and
>>>> viceversa.
>>> 
>>> As mentioned, rq->next_balance is updated whenever cpu enters idle
>>> (i.e. in newidle_balance() but it's not related with doing a newly
>>> idle load balance.
>> 
>> Yes, I understand that. But my point was that the update of rq->next_balance
>> from the newidle path is itself buggy and interferes with the load balance
>> happening from the tick (trigger_load_balance -> run_rebalance_domains).
> 
> Newidle path is not buggy.

Sure perhaps not directly newidle but something else is buggy as you
indicated below:

> It only uses sd->last_balance + interval to
> estimate the next balance  which is the correct thing to do. Your problem
> comes from the update of sd->last_balance which never happens and remains
> in the past whereas you call run_rebalance_domains() which should
> run load_balance for all domains with a sd->last_balance + interval in the
> past.
> Your problem most probably comes from the should_we_balance which always or
> "almost always" returns false in your use case for some sched_domain and
> prevents to updat sd->last_balance. Could you try the patch below ?
> It should fix your problem of trying to rebalance every tick whereas
> rebalance_domain is called.
> At least this should show if it's your problem but I'm not sure it's the right
> things to do all the time ...

You raise a good point, the root cause is indeed last_balance being
stuck in the past, or such.

I will try the patch below and let you know. Also my previous diff where
I cap the next balance setting also makes the issue go away, when I was last testing.

Thanks Vincent!

 - Joel 


> 
> ---
> kernel/sched/fair.c | 18 ++++++------------
> 1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3745ca289240..9ea1f42e5362 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11568,17 +11568,6 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>        need_decay = update_newidle_cost(sd, 0);
>        max_cost += sd->max_newidle_lb_cost;
> 
> -        /*
> -         * Stop the load balance at this level. There is another
> -         * CPU in our sched group which is doing load balancing more
> -         * actively.
> -         */
> -        if (!continue_balancing) {
> -            if (need_decay)
> -                continue;
> -            break;
> -        }
> -
>        interval = get_sd_balance_interval(sd, busy);
> 
>        need_serialize = sd->flags & SD_SERIALIZE;
> @@ -11588,7 +11577,12 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>        }
> 
>        if (time_after_eq(jiffies, sd->last_balance + interval)) {
> -            if (load_balance(cpu, rq, sd, idle, &continue_balancing)) {
> +            /*
> +             * Stop the load balance at this level. There is another
> +             * CPU in our sched group which is doing load balancing more
> +             * actively.
> +             */
> +            if (continue_balancing && load_balance(cpu, rq, sd, idle, &continue_balancing)) {
>                /*
>                 * The LBF_DST_PINNED logic could have changed
>                 * env->dst_cpu, so we can't know our idle
> --
> 2.34.1
> 
>> 
>>> But your problem is more related with the fact that
>>> nohz.needs_update is set when stopping cpu timer in order to update
>>> nohz.next_balance which is then used to kick a "real" idle load
>>> balance
>> 
>> Well, independent of nohz idle balance, I think we need to fix this issue as
>> mentioned above. This effect the periodic one as mentioned in the commit log.
>> 
>> See here another trace I collected this time dumping the 'interval'. There is
>> a tug of war happening between the newidle balance and the periodic balance.
>> 
>> The periodic one sets rq->next_balance for cpu 0 to 760,143 and then the
>> newidle comes in pulls it back a 118 jiffies to 760,024. This is actually in
>> the past because jiffies is currently 760,045 !!
>> 
>> This triggers the periodic balance againwhich sets rq->next_balance back to
>> 760,143.
>> 
>> Rinse and repeat. End result is you have periodic balance every jiffies. With
>> this patch the issue goes away but we could fix it differently as you
>> mentioned, we need to pull newidle balance back but perhaps not so
>> aggressively. How about something like the untested diff I enclosed at the
>> end of this email?
>> 
>> <idle>-0   [000]  13.081781: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,045)
>> cyclictest-120   [000]  13.081806: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
>> cyclictest-120   [000]  13.081807: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,045)
>> cyclictest-120   [000]  13.082130: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
>> cyclictest-120   [000]  13.082338: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
>> cyclictest-120   [000]  13.082636: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8)
>> <idle>-0   [000]  13.082823: trigger_load_balance: time_after_eq(jiffies=760,046, rq->next_balance=760,024) = 1
>> <idle>-0   [000]  13.082823: softirq_raise: vec=7 [action=SCHED]
>> <idle>-0   [000]  13.082871: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,046)
>> trace-cmd-114   [000]  13.082876: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
>> trace-cmd-114   [000]  13.082876: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,046)
>> cyclictest-120   [000]  13.083333: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
>> cyclictest-120   [000]  13.083633: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8)
>> <idle>-0   [000]  13.083656: trigger_load_balance: time_after_eq(jiffies=760,047, rq->next_balance=760,024) = 1
>> <idle>-0   [000]  13.083656: softirq_raise: vec=7 [action=SCHED]
>> <idle>-0   [000]  13.083702: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,047)
>> cyclictest-120   [000]  13.083729: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
>> cyclictest-120   [000]  13.083730: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,047)
>> cyclictest-120   [000]  13.083960: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
>> cyclictest-120   [000]  13.084069: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
>> cyclictest-120   [000]  13.084423: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8)
>> cyclictest-120   [000]  13.084633: trigger_load_balance: time_after_eq(jiffies=760,048, rq->next_balance=760,024) = 1
>> cyclictest-120   [000]  13.084633: softirq_raise: vec=7 [action=SCHED]
>> cyclictest-120   [000]  13.084678: rebalance_domains: rq[cpu=0]->next_balance:
>> 
>>>>>> Fix by updating last_balance when a newidle load balance actually
>>>>>> happens and then updating next_balance. This is also how it is done in
>>>>>> other load balance paths.
>>>>>> 
>>>>>> Testing shows a significant drop in softirqs when running:
>>>>>> cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
>>>>>> 
>>>>>> Goes from ~6k to ~800.
>>>>> 
>>>>> Even if your figures look interesting, your patch adds regression in
>>>>> the load balance and the fairness.
>>>> 
>>>> Yes I see that now. However it does illustrate the problem IMO.
>>>> 
>>>>> We can probably do improve the current behavior for decreasing number
>>>>> of ILB but your proposal is not the right solution IMO
>>>> 
>>>> One of the problems is if you have task goes idle a lot, then the
>>>> newidle_balance mechanism triggers the periodic balance every jiffie (once
>>>> per millisecond on HZ=1000).
>>> 
>>> every msec seems quite a lot.
>> 
>> Yeah!
>> 
>>>> 
>>>> Following are some traces I collected.
>>>> 
>>>> cyclictest-123   [003]   522.650574  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157)
>>>>   <idle>-0      [003]   522.651443  trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1
>>>>   <idle>-0      [003]   522.651461  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158)
>>>> cyclictest-123   [003]   522.651494  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158)
>>>>   <idle>-0      [003]   522.652522  trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1
>>>>   <idle>-0      [003]   522.652560  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159)
>>>> cyclictest-124   [003]   522.652586  newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159)
>>>>   <idle>-0      [003]   522.654492  trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1
>>>>   <idle>-0      [003]   522.654534  rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161)
>>> 
>>> Ok, so IIUC your trace above, this happens because the tick is not
>>> stop after entering idle so it continues to fire and triggers a load
>>> balance without checking if there is a need like what is done for nohz
>>> mode
>> 
>> The tick is normally not stopped if the CPU is awakened too soon by a timer.
>> That's pretty normal AFAIK. As you can see in the traces above, cyclictest
>> keeps waking up.
>> 
>>>> things worse for power on ARM where you have uclamp stuff happening in the
>>>> load balance paths which is quite heavy when I last traced that..
>>>> 
>>>> Further, we have observed in our tracing on real device that the update of
>>>> rq->next_balance from the newidle path is itself buggy... we observed that
>>>> because newidle balance may not update rq->last_balance, it is possible that
>>>> rq->next_balance when updated by update_next_balance() will be updated to a
>>>> value that is in the past and it will be stuck there for a long time! Perhaps
>>>> we should investigate more and fix that bug separately. Vineeth could provide
>>>> more details on the "getting stuck in the past" behavior as well.
>>> 
>>> sd->last_balance reflects last time an idle/busy load_balance happened
>>> (newly idle is out of the scope for the points that I mentioned
>>> previously).  So if no load balance happens for a while, the
>>> rq->next_balance can be in the past but I don't see a problem here. It
>>> just means that a load balance hasn't happened for a while. It can
>>> even move backward if it has been set when busy but the cpu is now
>>> idle
>> 
>> Sure, but I think it should at least set it by get_sd_balance_interval() into
>> the future. Like so (untested)? Let me know what you think and thanks!
>> 
>> ---8<-----------------------
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a3318aeff9e8..0d6667d31c51 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -11314,6 +11314,30 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)
>>    return interval;
>> }
>> 
>> +/*
>> + * Update the next balance from newidle balance.
>> + * The update of next_balance from newidle balance tries to make sure that
>> + * we don't trigger periodic balance too far in the future on a now-idle
>> + * system.  This is just like update_next_balance except that since
>> + * sd->last_balance may not have been updated for a while, we're careful to
>> + * not set next_balance in the past.
>> + */
>> +static inline void
>> +update_next_balance_newidle(struct sched_domain *sd, unsigned long *next_balance)
>> +{
>> +    unsigned long interval, next;
>> +
>> +    /* used by new idle balance, so cpu_busy = 0 */
>> +    interval = get_sd_balance_interval(sd, 0);
>> +    next = sd->last_balance + interval;
>> +
>> +    next = max(next, jiffies + interval);
>> +
>> +    if (time_after(*next_balance, next)) {
>> +        *next_balance = next;
>> +    }
>> +}
>> +
>> static inline void
>> update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>> {
>> @@ -12107,7 +12131,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>>        (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>> 
>>        if (sd)
>> -            update_next_balance(sd, &next_balance);
>> +            update_next_balance_newidle(sd, &next_balance);
>>        rcu_read_unlock();
>> 
>>        goto out;
>> @@ -12124,7 +12148,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>>        int continue_balancing = 1;
>>        u64 domain_cost;
>> 
>> -        update_next_balance(sd, &next_balance);
>> +        update_next_balance_newidle(sd, &next_balance);
>> 
>>        if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>>            break;