diff mbox series

[v5,2/2] sched/fair: Check a task has a fitting cpu when updating misfit

Message ID 20240205021123.2225933-3-qyousef@layalina.io
State New
Headers
Series sched: Don't trigger misfit if affinity is restricted |

Commit Message

Qais Yousef Feb. 5, 2024, 2:11 a.m. UTC 
  If a misfit task is affined to a subset of the possible cpus, we need to
verify that one of these cpus can fit it. Otherwise the load balancer
code will continuously trigger needlessly leading the balance_interval
to increase in return and eventually end up with a situation where real
imbalances take a long time to address because of this impossible
imbalance situation.

This can happen in Android world where it's common for background tasks
to be restricted to little cores.

Similarly if we can't fit the biggest core, triggering misfit is
pointless as it is the best we can ever get on this system.

To be able to detect that; we use asym_cap_list to iterate through
capacities in the system to see if the task is able to run at a higher
capacity level based on its p->cpus_ptr. We do that when the affinity
change, a fair task is forked, or when a task switched to fair policy.
We store the max_allowed_capacity in task_struct to allow for cheap
comparison in the fast path.

Improve check_misfit_status() function to be more readable. At one
iteration of the patch it was thought we can drop one of the checks. The
current form hopefully should make it more obvious what is being checked
and why.

Test:
=====

Add

	trace_printk("balance_interval = %lu\n", interval)

in get_sd_balance_interval().

run
	if [ "$MASK" != "0" ]; then
		adb shell "taskset -a $MASK cat /dev/zero > /dev/null"
	fi
	sleep 10
	// parse ftrace buffer counting the occurrence of each valaue

Where MASK is either:

	* 0: no busy task running
	* 1: busy task is pinned to 1 cpu; handled today to not cause
	  misfit
	* f: busy task pinned to little cores, simulates busy background
	  task, demonstrates the problem to be fixed

Results:
========

Note how occurrence of balance_interval = 128 overshoots for MASK = f.

BEFORE
------

	MASK=0

		   1 balance_interval = 175
		 120 balance_interval = 128
		 846 balance_interval = 64
		  55 balance_interval = 63
		 215 balance_interval = 32
		   2 balance_interval = 31
		   2 balance_interval = 16
		   4 balance_interval = 8
		1870 balance_interval = 4
		  65 balance_interval = 2

	MASK=1

		  27 balance_interval = 175
		  37 balance_interval = 127
		 840 balance_interval = 64
		 167 balance_interval = 63
		 449 balance_interval = 32
		  84 balance_interval = 31
		 304 balance_interval = 16
		1156 balance_interval = 8
		2781 balance_interval = 4
		 428 balance_interval = 2

	MASK=f

		   1 balance_interval = 175
		1328 balance_interval = 128
		  44 balance_interval = 64
		 101 balance_interval = 63
		  25 balance_interval = 32
		   5 balance_interval = 31
		  23 balance_interval = 16
		  23 balance_interval = 8
		4306 balance_interval = 4
		 177 balance_interval = 2

AFTER
-----

Note how the high values almost disappear for all MASK values. The
system has background tasks that could trigger the problem without
simulate it even with MASK=0.

	MASK=0

		 103 balance_interval = 63
		  19 balance_interval = 31
		 194 balance_interval = 8
		4827 balance_interval = 4
		 179 balance_interval = 2

	MASK=1

		 131 balance_interval = 63
		   1 balance_interval = 31
		  87 balance_interval = 8
		3600 balance_interval = 4
		   7 balance_interval = 2

	MASK=f

		   8 balance_interval = 127
		 182 balance_interval = 63
		   3 balance_interval = 31
		   9 balance_interval = 16
		 415 balance_interval = 8
		3415 balance_interval = 4
		  21 balance_interval = 2

Signed-off-by: Qais Yousef <qyousef@layalina.io>
---
 include/linux/sched.h |  1 +
 init/init_task.c      |  1 +
 kernel/sched/fair.c   | 84 +++++++++++++++++++++++++++++++++----------
 3 files changed, 68 insertions(+), 18 deletions(-)

Comments

Hillf Danton Feb. 6, 2024, 10:55 a.m. UTC | #1 
On Mon,  5 Feb 2024 02:11:23 +0000 Qais Yousef <qyousef@layalina.io>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5092,24 +5092,36 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
>  
>  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
>  {
> +	unsigned long cpu_cap;
> +	int cpu = cpu_of(rq);
> +
>  	if (!sched_asym_cpucap_active())
>  		return;
>  
> -	if (!p || p->nr_cpus_allowed == 1) {
> -		rq->misfit_task_load = 0;
> -		return;
> -	}
> +	if (!p || p->nr_cpus_allowed == 1)
> +		goto out;
>  
> -	if (task_fits_cpu(p, cpu_of(rq))) {
> -		rq->misfit_task_load = 0;
> -		return;
> -	}
> +	cpu_cap = arch_scale_cpu_capacity(cpu);
> +
> +	/* If we can't fit the biggest CPU, that's the best we can ever get. */
> +	if (cpu_cap == rq->rd->max_cpu_capacity)
> +		goto out;
> +
> +	/* Affinity allows us to go somewhere higher? */
> +	if (cpu_cap == p->max_allowed_capacity)
> +		goto out;

Looks good.
> +
> +	if (task_fits_cpu(p, cpu))
> +		goto out;
>  
>  	/*
>  	 * Make sure that misfit_task_load will not be null even if
>  	 * task_h_load() returns 0.
>  	 */
>  	rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1);
> +	return;
> +out:
> +	rq->misfit_task_load = 0;
>  }
>  
>  #else /* CONFIG_SMP */
> @@ -8241,6 +8253,36 @@ static void task_dead_fair(struct task_struct *p)
>  	remove_entity_load_avg(&p->se);
>  }
>  
> +/*
> + * Check the max capacity the task is allowed to run at for misfit detection.
> + */
> +static void set_task_max_allowed_capacity(struct task_struct *p)
> +{
> +	struct asym_cap_data *entry;
> +
> +	if (!sched_asym_cpucap_active())
> +		return;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(entry, &asym_cap_list, link) {
> +		cpumask_t *cpumask;
> +
> +		cpumask = cpu_capacity_span(entry);
> +		if (!cpumask_intersects(p->cpus_ptr, cpumask))
> +			continue;
> +
> +		p->max_allowed_capacity = entry->capacity;
> +		break;

Given what max_allowed_capacity could mean, it is needed to find the max
capacity by iterating the asym_cap_list instead of the first capacity
determined by the allowed CPU mask.
> +	}
> +	rcu_read_unlock();
> +}

BTW is it working in case of systems with a super core?

	cpu0-3	cpu4-6	cpu7
	little	big	super
	core	core	core
Qais Yousef Feb. 6, 2024, 3:24 p.m. UTC | #2 
On 02/06/24 18:55, Hillf Danton wrote:

> > +/*
> > + * Check the max capacity the task is allowed to run at for misfit detection.
> > + */
> > +static void set_task_max_allowed_capacity(struct task_struct *p)
> > +{
> > +	struct asym_cap_data *entry;
> > +
> > +	if (!sched_asym_cpucap_active())
> > +		return;
> > +
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(entry, &asym_cap_list, link) {
> > +		cpumask_t *cpumask;
> > +
> > +		cpumask = cpu_capacity_span(entry);
> > +		if (!cpumask_intersects(p->cpus_ptr, cpumask))
> > +			continue;
> > +
> > +		p->max_allowed_capacity = entry->capacity;
> > +		break;
> 
> Given what max_allowed_capacity could mean, it is needed to find the max
> capacity by iterating the asym_cap_list instead of the first capacity
> determined by the allowed CPU mask.

I don't think we can rely on that as there is no guarantee on the position of
the biggest CPU. We would have to iterate through every CPU in the mask instead
to figure out the one with the largest capacity. And we moved to this as
Vincent wasn't keen on assuming we have few CPUs and potentially not scaling on
systems with large number of CPUs. This list should be faster as the number of
capacity level is much smaller than the number of CPUs.

> > +	}
> > +	rcu_read_unlock();
> > +}
> 
> BTW is it working in case of systems with a super core?
> 
> 	cpu0-3	cpu4-6	cpu7
> 	little	big	super
> 	core	core	core

It should. Super here should have the capacity of 1024 and everything else
scaled relative to it like any other system with 3 tiers of capacities.
Vincent Guittot Feb. 12, 2024, 5:27 p.m. UTC | #3 
On Mon, 5 Feb 2024 at 03:11, Qais Yousef <qyousef@layalina.io> wrote:
>
> If a misfit task is affined to a subset of the possible cpus, we need to
> verify that one of these cpus can fit it. Otherwise the load balancer
> code will continuously trigger needlessly leading the balance_interval
> to increase in return and eventually end up with a situation where real
> imbalances take a long time to address because of this impossible
> imbalance situation.
>
> This can happen in Android world where it's common for background tasks
> to be restricted to little cores.
>
> Similarly if we can't fit the biggest core, triggering misfit is
> pointless as it is the best we can ever get on this system.
>
> To be able to detect that; we use asym_cap_list to iterate through
> capacities in the system to see if the task is able to run at a higher
> capacity level based on its p->cpus_ptr. We do that when the affinity
> change, a fair task is forked, or when a task switched to fair policy.
> We store the max_allowed_capacity in task_struct to allow for cheap
> comparison in the fast path.
>
> Improve check_misfit_status() function to be more readable. At one
> iteration of the patch it was thought we can drop one of the checks. The
> current form hopefully should make it more obvious what is being checked
> and why.
>
> Test:
> =====
>
> Add
>
>         trace_printk("balance_interval = %lu\n", interval)
>
> in get_sd_balance_interval().
>
> run
>         if [ "$MASK" != "0" ]; then
>                 adb shell "taskset -a $MASK cat /dev/zero > /dev/null"
>         fi
>         sleep 10
>         // parse ftrace buffer counting the occurrence of each valaue
>
> Where MASK is either:
>
>         * 0: no busy task running
>         * 1: busy task is pinned to 1 cpu; handled today to not cause
>           misfit
>         * f: busy task pinned to little cores, simulates busy background
>           task, demonstrates the problem to be fixed
>
> Results:
> ========
>
> Note how occurrence of balance_interval = 128 overshoots for MASK = f.
>
> BEFORE
> ------
>
>         MASK=0
>
>                    1 balance_interval = 175
>                  120 balance_interval = 128
>                  846 balance_interval = 64
>                   55 balance_interval = 63
>                  215 balance_interval = 32
>                    2 balance_interval = 31
>                    2 balance_interval = 16
>                    4 balance_interval = 8
>                 1870 balance_interval = 4
>                   65 balance_interval = 2
>
>         MASK=1
>
>                   27 balance_interval = 175
>                   37 balance_interval = 127
>                  840 balance_interval = 64
>                  167 balance_interval = 63
>                  449 balance_interval = 32
>                   84 balance_interval = 31
>                  304 balance_interval = 16
>                 1156 balance_interval = 8
>                 2781 balance_interval = 4
>                  428 balance_interval = 2
>
>         MASK=f
>
>                    1 balance_interval = 175
>                 1328 balance_interval = 128
>                   44 balance_interval = 64
>                  101 balance_interval = 63
>                   25 balance_interval = 32
>                    5 balance_interval = 31
>                   23 balance_interval = 16
>                   23 balance_interval = 8
>                 4306 balance_interval = 4
>                  177 balance_interval = 2
>
> AFTER
> -----
>
> Note how the high values almost disappear for all MASK values. The
> system has background tasks that could trigger the problem without
> simulate it even with MASK=0.
>
>         MASK=0
>
>                  103 balance_interval = 63
>                   19 balance_interval = 31
>                  194 balance_interval = 8
>                 4827 balance_interval = 4
>                  179 balance_interval = 2
>
>         MASK=1
>
>                  131 balance_interval = 63
>                    1 balance_interval = 31
>                   87 balance_interval = 8
>                 3600 balance_interval = 4
>                    7 balance_interval = 2
>
>         MASK=f
>
>                    8 balance_interval = 127
>                  182 balance_interval = 63
>                    3 balance_interval = 31
>                    9 balance_interval = 16
>                  415 balance_interval = 8
>                 3415 balance_interval = 4
>                   21 balance_interval = 2
>
> Signed-off-by: Qais Yousef <qyousef@layalina.io>
> ---
>  include/linux/sched.h |  1 +
>  init/init_task.c      |  1 +
>  kernel/sched/fair.c   | 84 +++++++++++++++++++++++++++++++++----------
>  3 files changed, 68 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 03bfe9ab2951..1e7bf52de607 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -827,6 +827,7 @@ struct task_struct {
>  #endif
>
>         unsigned int                    policy;
> +       unsigned long                   max_allowed_capacity;
>         int                             nr_cpus_allowed;
>         const cpumask_t                 *cpus_ptr;
>         cpumask_t                       *user_cpus_ptr;
> diff --git a/init/init_task.c b/init/init_task.c
> index 5727d42149c3..01b3199d4cde 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -82,6 +82,7 @@ struct task_struct init_task
>         .cpus_ptr       = &init_task.cpus_mask,
>         .user_cpus_ptr  = NULL,
>         .cpus_mask      = CPU_MASK_ALL,
> +       .max_allowed_capacity   = SCHED_CAPACITY_SCALE,
>         .nr_cpus_allowed= NR_CPUS,
>         .mm             = NULL,
>         .active_mm      = &init_mm,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b803030c3a03..8b8035f5c8f6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5092,24 +5092,36 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
>
>  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
>  {
> +       unsigned long cpu_cap;
> +       int cpu = cpu_of(rq);
> +
>         if (!sched_asym_cpucap_active())
>                 return;
>
> -       if (!p || p->nr_cpus_allowed == 1) {
> -               rq->misfit_task_load = 0;
> -               return;
> -       }
> +       if (!p || p->nr_cpus_allowed == 1)
> +               goto out;
>
> -       if (task_fits_cpu(p, cpu_of(rq))) {
> -               rq->misfit_task_load = 0;
> -               return;
> -       }
> +       cpu_cap = arch_scale_cpu_capacity(cpu);
> +
> +       /* If we can't fit the biggest CPU, that's the best we can ever get. */
> +       if (cpu_cap == rq->rd->max_cpu_capacity)

Isn't the condition above also covered by the condition below and
becomes now useless ?

> +               goto out;
> +
> +       /* Affinity allows us to go somewhere higher? */
> +       if (cpu_cap == p->max_allowed_capacity)
> +               goto out;
> +
> +       if (task_fits_cpu(p, cpu))
> +               goto out;
>
>         /*
>          * Make sure that misfit_task_load will not be null even if
>          * task_h_load() returns 0.
>          */
>         rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1);
> +       return;
> +out:
> +       rq->misfit_task_load = 0;
>  }
>
>  #else /* CONFIG_SMP */
> @@ -8241,6 +8253,36 @@ static void task_dead_fair(struct task_struct *p)
>         remove_entity_load_avg(&p->se);
>  }
>
> +/*
> + * Check the max capacity the task is allowed to run at for misfit detection.
> + */
> +static void set_task_max_allowed_capacity(struct task_struct *p)
> +{
> +       struct asym_cap_data *entry;
> +
> +       if (!sched_asym_cpucap_active())
> +               return;
> +
> +       rcu_read_lock();
> +       list_for_each_entry_rcu(entry, &asym_cap_list, link) {
> +               cpumask_t *cpumask;
> +
> +               cpumask = cpu_capacity_span(entry);
> +               if (!cpumask_intersects(p->cpus_ptr, cpumask))
> +                       continue;
> +
> +               p->max_allowed_capacity = entry->capacity;
> +               break;
> +       }
> +       rcu_read_unlock();
> +}
> +
> +static void set_cpus_allowed_fair(struct task_struct *p, struct affinity_context *ctx)
> +{
> +       set_cpus_allowed_common(p, ctx);
> +       set_task_max_allowed_capacity(p);
> +}
> +
>  static int
>  balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
> @@ -9601,16 +9643,18 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
>                                 (arch_scale_cpu_capacity(cpu_of(rq)) * 100));
>  }
>
> -/*
> - * Check whether a rq has a misfit task and if it looks like we can actually
> - * help that task: we can migrate the task to a CPU of higher capacity, or
> - * the task's current CPU is heavily pressured.
> - */
> -static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
> +/* Check if the rq has a misfit task */
> +static inline bool check_misfit_status(struct rq *rq, struct sched_domain *sd)
>  {
> -       return rq->misfit_task_load &&
> -               (arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity ||
> -                check_cpu_capacity(rq, sd));
> +       if (!rq->misfit_task_load)
> +               return false;

I think that only the above is enough ...

> +
> +       /* Can we migrate to a CPU with higher capacity? */
> +       if (arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity)

because rq->misfit_task_load is set to 0 if
arch_scale_cpu_capacity(rq->cpu) == rq->rd->max_cpu_capacity

That would also mean that we don't need to keep and set
rd->max_cpu_capacity anymore as we remove the 2 uses of it

> +               return true;
> +
> +       /* Is the task's CPU being heavily pressured? */
> +       return check_cpu_capacity(rq, sd);

and this one has already been tested in nohz_balancer_kick() before
calling check_misfit_status()

>  }
>
>  /*
> @@ -12647,6 +12691,8 @@ static void task_fork_fair(struct task_struct *p)
>         rq_lock(rq, &rf);
>         update_rq_clock(rq);
>
> +       set_task_max_allowed_capacity(p);
> +
>         cfs_rq = task_cfs_rq(current);
>         curr = cfs_rq->curr;
>         if (curr)
> @@ -12770,6 +12816,8 @@ static void switched_to_fair(struct rq *rq, struct task_struct *p)
>  {
>         attach_task_cfs_rq(p);
>
> +       set_task_max_allowed_capacity(p);
> +
>         if (task_on_rq_queued(p)) {
>                 /*
>                  * We were most likely switched from sched_rt, so
> @@ -13154,7 +13202,7 @@ DEFINE_SCHED_CLASS(fair) = {
>         .rq_offline             = rq_offline_fair,
>
>         .task_dead              = task_dead_fair,
> -       .set_cpus_allowed       = set_cpus_allowed_common,
> +       .set_cpus_allowed       = set_cpus_allowed_fair,
>  #endif
>
>         .task_tick              = task_tick_fair,
> --
> 2.34.1
>
Qais Yousef Feb. 20, 2024, 3:59 p.m. UTC | #4 
On 02/12/24 18:27, Vincent Guittot wrote:

> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b803030c3a03..8b8035f5c8f6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5092,24 +5092,36 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
> >
> >  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> >  {
> > +       unsigned long cpu_cap;
> > +       int cpu = cpu_of(rq);
> > +
> >         if (!sched_asym_cpucap_active())
> >                 return;
> >
> > -       if (!p || p->nr_cpus_allowed == 1) {
> > -               rq->misfit_task_load = 0;
> > -               return;
> > -       }
> > +       if (!p || p->nr_cpus_allowed == 1)
> > +               goto out;
> >
> > -       if (task_fits_cpu(p, cpu_of(rq))) {
> > -               rq->misfit_task_load = 0;
> > -               return;
> > -       }
> > +       cpu_cap = arch_scale_cpu_capacity(cpu);
> > +
> > +       /* If we can't fit the biggest CPU, that's the best we can ever get. */
> > +       if (cpu_cap == rq->rd->max_cpu_capacity)
> 
> Isn't the condition above also covered by the condition below and
> becomes now useless ?

Yes, you're right. If it is allowed to run on rd->max_cpu_capacity then the
below check will cover it. If it is not allowed, then it won't be there on the
first place.

I'll drop it.

> > -/*
> > - * Check whether a rq has a misfit task and if it looks like we can actually
> > - * help that task: we can migrate the task to a CPU of higher capacity, or
> > - * the task's current CPU is heavily pressured.
> > - */
> > -static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
> > +/* Check if the rq has a misfit task */
> > +static inline bool check_misfit_status(struct rq *rq, struct sched_domain *sd)
> >  {
> > -       return rq->misfit_task_load &&
> > -               (arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity ||
> > -                check_cpu_capacity(rq, sd));
> > +       if (!rq->misfit_task_load)
> > +               return false;
> 
> I think that only the above is enough ...
> 
> > +
> > +       /* Can we migrate to a CPU with higher capacity? */
> > +       if (arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity)
> 
> because rq->misfit_task_load is set to 0 if
> arch_scale_cpu_capacity(rq->cpu) == rq->rd->max_cpu_capacity
> 
> That would also mean that we don't need to keep and set
> rd->max_cpu_capacity anymore as we remove the 2 uses of it

+1

I'll drop max_cpu_capacity as a new patch on top

> 
> > +               return true;
> > +
> > +       /* Is the task's CPU being heavily pressured? */
> > +       return check_cpu_capacity(rq, sd);
> 
> and this one has already been tested in nohz_balancer_kick() before
> calling check_misfit_status()

Yes, removed.

I realized that I wanted to also add a new patch to not double balance_interval
for misfit failures. I think you indicated that seems the right thing to do?


Thanks

--
Qais Yousef
Qais Yousef Feb. 20, 2024, 4:15 p.m. UTC | #5 
On 02/20/24 15:59, Qais Yousef wrote:

> I realized that I wanted to also add a new patch to not double balance_interval
> for misfit failures. I think you indicated that seems the right thing to do?

I think this should do it?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 70ffbb1aa15c..b12b7de495d0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11552,8 +11552,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
         * repeatedly reach this code, which would lead to balance_interval
         * skyrocketing in a short amount of time. Skip the balance_interval
         * increase logic to avoid that.
+        *
+        * Similarly misfit migration which is not necessarily an indication of
+        * the system being busy and requires lb to backoff to let it settle
+        * down.
         */
-       if (env.idle == CPU_NEWLY_IDLE)
+       if (env.idle == CPU_NEWLY_IDLE ||
+           env.migration_type == migrate_misfit)
                goto out;

        /* tune up the balancing interval */
Vincent Guittot Feb. 20, 2024, 5:37 p.m. UTC | #6 
On Tue, 20 Feb 2024 at 17:15, Qais Yousef <qyousef@layalina.io> wrote:
>
> On 02/20/24 15:59, Qais Yousef wrote:
>
> > I realized that I wanted to also add a new patch to not double balance_interval
> > for misfit failures. I think you indicated that seems the right thing to do?
>
> I think this should do it?

yes it should do it

>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 70ffbb1aa15c..b12b7de495d0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11552,8 +11552,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>          * repeatedly reach this code, which would lead to balance_interval
>          * skyrocketing in a short amount of time. Skip the balance_interval
>          * increase logic to avoid that.
> +        *
> +        * Similarly misfit migration which is not necessarily an indication of
> +        * the system being busy and requires lb to backoff to let it settle
> +        * down.
>          */
> -       if (env.idle == CPU_NEWLY_IDLE)
> +       if (env.idle == CPU_NEWLY_IDLE ||
> +           env.migration_type == migrate_misfit)
>                 goto out;
>
>         /* tune up the balancing interval */
diff mbox series

Patch

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 03bfe9ab2951..1e7bf52de607 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -827,6 +827,7 @@  struct task_struct {
 #endif
 
 	unsigned int			policy;
+	unsigned long			max_allowed_capacity;
 	int				nr_cpus_allowed;
 	const cpumask_t			*cpus_ptr;
 	cpumask_t			*user_cpus_ptr;
diff --git a/init/init_task.c b/init/init_task.c
index 5727d42149c3..01b3199d4cde 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -82,6 +82,7 @@  struct task_struct init_task
 	.cpus_ptr	= &init_task.cpus_mask,
 	.user_cpus_ptr	= NULL,
 	.cpus_mask	= CPU_MASK_ALL,
+	.max_allowed_capacity	= SCHED_CAPACITY_SCALE,
 	.nr_cpus_allowed= NR_CPUS,
 	.mm		= NULL,
 	.active_mm	= &init_mm,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b803030c3a03..8b8035f5c8f6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5092,24 +5092,36 @@  static inline int task_fits_cpu(struct task_struct *p, int cpu)
 
 static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
 {
+	unsigned long cpu_cap;
+	int cpu = cpu_of(rq);
+
 	if (!sched_asym_cpucap_active())
 		return;
 
-	if (!p || p->nr_cpus_allowed == 1) {
-		rq->misfit_task_load = 0;
-		return;
-	}
+	if (!p || p->nr_cpus_allowed == 1)
+		goto out;
 
-	if (task_fits_cpu(p, cpu_of(rq))) {
-		rq->misfit_task_load = 0;
-		return;
-	}
+	cpu_cap = arch_scale_cpu_capacity(cpu);
+
+	/* If we can't fit the biggest CPU, that's the best we can ever get. */
+	if (cpu_cap == rq->rd->max_cpu_capacity)
+		goto out;
+
+	/* Affinity allows us to go somewhere higher? */
+	if (cpu_cap == p->max_allowed_capacity)
+		goto out;
+
+	if (task_fits_cpu(p, cpu))
+		goto out;
 
 	/*
 	 * Make sure that misfit_task_load will not be null even if
 	 * task_h_load() returns 0.
 	 */
 	rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1);
+	return;
+out:
+	rq->misfit_task_load = 0;
 }
 
 #else /* CONFIG_SMP */
@@ -8241,6 +8253,36 @@  static void task_dead_fair(struct task_struct *p)
 	remove_entity_load_avg(&p->se);
 }
 
+/*
+ * Check the max capacity the task is allowed to run at for misfit detection.
+ */
+static void set_task_max_allowed_capacity(struct task_struct *p)
+{
+	struct asym_cap_data *entry;
+
+	if (!sched_asym_cpucap_active())
+		return;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(entry, &asym_cap_list, link) {
+		cpumask_t *cpumask;
+
+		cpumask = cpu_capacity_span(entry);
+		if (!cpumask_intersects(p->cpus_ptr, cpumask))
+			continue;
+
+		p->max_allowed_capacity = entry->capacity;
+		break;
+	}
+	rcu_read_unlock();
+}
+
+static void set_cpus_allowed_fair(struct task_struct *p, struct affinity_context *ctx)
+{
+	set_cpus_allowed_common(p, ctx);
+	set_task_max_allowed_capacity(p);
+}
+
 static int
 balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
@@ -9601,16 +9643,18 @@  check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
 				(arch_scale_cpu_capacity(cpu_of(rq)) * 100));
 }
 
-/*
- * Check whether a rq has a misfit task and if it looks like we can actually
- * help that task: we can migrate the task to a CPU of higher capacity, or
- * the task's current CPU is heavily pressured.
- */
-static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
+/* Check if the rq has a misfit task */
+static inline bool check_misfit_status(struct rq *rq, struct sched_domain *sd)
 {
-	return rq->misfit_task_load &&
-		(arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity ||
-		 check_cpu_capacity(rq, sd));
+	if (!rq->misfit_task_load)
+		return false;
+
+	/* Can we migrate to a CPU with higher capacity? */
+	if (arch_scale_cpu_capacity(rq->cpu) < rq->rd->max_cpu_capacity)
+		return true;
+
+	/* Is the task's CPU being heavily pressured? */
+	return check_cpu_capacity(rq, sd);
 }
 
 /*
@@ -12647,6 +12691,8 @@  static void task_fork_fair(struct task_struct *p)
 	rq_lock(rq, &rf);
 	update_rq_clock(rq);
 
+	set_task_max_allowed_capacity(p);
+
 	cfs_rq = task_cfs_rq(current);
 	curr = cfs_rq->curr;
 	if (curr)
@@ -12770,6 +12816,8 @@  static void switched_to_fair(struct rq *rq, struct task_struct *p)
 {
 	attach_task_cfs_rq(p);
 
+	set_task_max_allowed_capacity(p);
+
 	if (task_on_rq_queued(p)) {
 		/*
 		 * We were most likely switched from sched_rt, so
@@ -13154,7 +13202,7 @@  DEFINE_SCHED_CLASS(fair) = {
 	.rq_offline		= rq_offline_fair,
 
 	.task_dead		= task_dead_fair,
-	.set_cpus_allowed	= set_cpus_allowed_common,
+	.set_cpus_allowed	= set_cpus_allowed_fair,
 #endif
 
 	.task_tick		= task_tick_fair,