diff mbox series

[v4,7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

Message ID 1707982910-27680-8-git-send-email-mihai.carabas@oracle.com
State New
Headers
Series [v4,1/8] x86: Move ARCH_HAS_CPU_RELAX to arch |

Commit Message

Mihai Carabas Feb. 15, 2024, 7:41 a.m. UTC 
  cpu_relax on ARM64 does a simple "yield". Thus we replace it with
smp_cond_load_relaxed which basically does a "wfe".

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
---
 drivers/cpuidle/poll_state.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

Comments

Tomohiro Misono (Fujitsu) Feb. 26, 2024, 8:36 a.m. UTC | #1 
Hi,
> Subject: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed
> 
> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> smp_cond_load_relaxed which basically does a "wfe".
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
> ---
>  drivers/cpuidle/poll_state.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..1e45be906e72 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -13,6 +13,7 @@
>  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>  			       struct cpuidle_driver *drv, int index)
>  {
> +	unsigned long ret;
>  	u64 time_start;
> 
>  	time_start = local_clock_noinstr();
> @@ -26,12 +27,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> 
>  		limit = cpuidle_poll_time(drv, dev);
> 
> -		while (!need_resched()) {
> -			cpu_relax();
> -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> -				continue;
> -
> +		for (;;) {
>  			loop_count = 0;
> +
> +			ret = smp_cond_load_relaxed(&current_thread_info()->flags,
> +						    VAL & _TIF_NEED_RESCHED ||
> +						    loop_count++ >= POLL_IDLE_RELAX_COUNT);
> +
> +			if (!(ret & _TIF_NEED_RESCHED))
> +				break;

Should this be "if (ret & _TIF_NEED_RESCHED) since we want to break here
if the flag is set, or am I misunderstood?

Regards,
Tomohiro

> +
>  			if (local_clock_noinstr() - time_start > limit) {
>  				dev->poll_time_limit = true;
>  				break;
> --
> 1.8.3.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
Ankur Arora Feb. 28, 2024, 4:36 a.m. UTC | #2 
Tomohiro Misono (Fujitsu) <misono.tomohiro@fujitsu.com> writes:

> Hi,
> > Subject: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed
> > 
> > cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> > smp_cond_load_relaxed which basically does a "wfe".
> >
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
> > ---
> >  drivers/cpuidle/poll_state.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> > index 9b6d90a72601..1e45be906e72 100644
> > --- a/drivers/cpuidle/poll_state.c
> > +++ b/drivers/cpuidle/poll_state.c
> > @@ -13,6 +13,7 @@
> >  static int __cpuidle poll_idle(struct cpuidle_device *dev,
> >                              struct cpuidle_driver *drv, int index)
> >  {
> > +     unsigned long ret;
> >       u64 time_start;
> >
> >       time_start = local_clock_noinstr();
> > @@ -26,12 +27,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> >
> >               limit = cpuidle_poll_time(drv, dev);
> >
> > -             while (!need_resched()) {
> > -                     cpu_relax();
> > -                     if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> > -                             continue;
> > -
> > +             for (;;) {
> >                       loop_count = 0;
> > +
> > +                     ret = smp_cond_load_relaxed(&current_thread_info()->flags,
> > +                                                 VAL & _TIF_NEED_RESCHED ||
> > +                                                 loop_count++ >= POLL_IDLE_RELAX_COUNT);
> > +
> > +                     if (!(ret & _TIF_NEED_RESCHED))
> > +                             break;
> 
> Should this be "if (ret & _TIF_NEED_RESCHED) since we want to break here
> if the flag is set, or am I misunderstood?

Yeah, you are right. The check is inverted.

I'll be re-spinning this series. Will fix. Though, it probably makes sense
to just keep the original "while (!need_resched())" check.

Thanks for the review.

--
ankur
diff mbox series

Patch

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index 9b6d90a72601..1e45be906e72 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -13,6 +13,7 @@ 
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
 {
+	unsigned long ret;
 	u64 time_start;
 
 	time_start = local_clock_noinstr();
@@ -26,12 +27,16 @@  static int __cpuidle poll_idle(struct cpuidle_device *dev,
 
 		limit = cpuidle_poll_time(drv, dev);
 
-		while (!need_resched()) {
-			cpu_relax();
-			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-				continue;
-
+		for (;;) {
 			loop_count = 0;
+
+			ret = smp_cond_load_relaxed(&current_thread_info()->flags,
+						    VAL & _TIF_NEED_RESCHED ||
+						    loop_count++ >= POLL_IDLE_RELAX_COUNT);
+
+			if (!(ret & _TIF_NEED_RESCHED))
+				break;
+
 			if (local_clock_noinstr() - time_start > limit) {
 				dev->poll_time_limit = true;
 				break;