Re: [RFC PATCH 3/4] sched/fair: delay update_tg_load_avg() for cfs_rq's removed load

[Aaron Lu <aaron.lu@intel.com>]

On Thu, Jul 20, 2023 at 03:10:30PM +0200, Vincent Guittot wrote:
> On Wed, 19 Jul 2023 at 15:29, Aaron Lu <aaron.lu@intel.com> wrote:
> >
> > On Wed, Jul 19, 2023 at 11:47:06AM +0200, Vincent Guittot wrote:
> > > On Wed, 19 Jul 2023 at 10:01, Aaron Lu <aaron.lu@intel.com> wrote:
> > > >
> > > > On Wed, Jul 19, 2023 at 01:18:26PM +0800, Aaron Lu wrote:
> > > > > Alternatively, I can remove some callsites of update_tg_load_avg() like
> > > > > you suggested below and only call update_tg_load_avg() when cfs_rq is
> > > > > decayed(really just decayed, not when it detected it has removed load
> > > > > pending or load propagated from its children). This way it would give us
> > > > > similar result as above(roughly once per ms).
> > > >
> > > > Something like this: (I think this is better since it removed those
> > > > unnecessary calls to update_tg_load_avg(), although it is inline but
> > > > still)
> > > >
> > > >
> > > > From bc749aaefa6bed36aa946921a4006b3dddb69b77 Mon Sep 17 00:00:00 2001
> > > > From: Aaron Lu <aaron.lu@intel.com>
> > > > Date: Wed, 19 Jul 2023 13:54:48 +0800
> > > > Subject: [PATCH] sched/fair: only update_tg_load_avg() when cfs_rq decayed
> > > >
> > > > ---
> > > >  kernel/sched/fair.c | 22 +++++++---------------
> > > >  1 file changed, 7 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index a80a73909dc2..7d5b7352b8b5 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -3913,16 +3913,16 @@ static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum
> > > >  }
> > > >
> > > >  /* Update task and its cfs_rq load average */
> > > > -static inline int propagate_entity_load_avg(struct sched_entity *se)
> > > > +static inline void propagate_entity_load_avg(struct sched_entity *se)
> > > >  {
> > > >         struct cfs_rq *cfs_rq, *gcfs_rq;
> > > >
> > > >         if (entity_is_task(se))
> > > > -               return 0;
> > > > +               return;
> > > >
> > > >         gcfs_rq = group_cfs_rq(se);
> > > >         if (!gcfs_rq->propagate)
> > > > -               return 0;
> > > > +               return;
> > > >
> > > >         gcfs_rq->propagate = 0;
> > > >
> > > > @@ -3936,8 +3936,6 @@ static inline int propagate_entity_load_avg(struct sched_entity *se)
> > > >
> > > >         trace_pelt_cfs_tp(cfs_rq);
> > > >         trace_pelt_se_tp(se);
> > > > -
> > > > -       return 1;
> > > >  }
> > > >
> > > >  /*
> > > > @@ -3974,9 +3972,8 @@ static inline bool skip_blocked_update(struct sched_entity *se)
> > > >
> > > >  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq) {}
> > > >
> > > > -static inline int propagate_entity_load_avg(struct sched_entity *se)
> > > > +static inline void propagate_entity_load_avg(struct sched_entity *se)
> > > >  {
> > > > -       return 0;
> > > >  }
> > > >
> > > >  static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum) {}
> > > > @@ -4086,7 +4083,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> > > >  {
> > > >         unsigned long removed_load = 0, removed_util = 0, removed_runnable = 0;
> > > >         struct sched_avg *sa = &cfs_rq->avg;
> > > > -       int decayed = 0;
> > > > +       int decayed;
> > > >
> > > >         if (cfs_rq->removed.nr) {
> > > >                 unsigned long r;
> > > > @@ -4134,11 +4131,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> > > >                  */
> > > >                 add_tg_cfs_propagate(cfs_rq,
> > > >                         -(long)(removed_runnable * divider) >> SCHED_CAPACITY_SHIFT);
> > > > -
> > > > -               decayed = 1;
> > > >         }
> > > >
> > > > -       decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
> > > > +       decayed = __update_load_avg_cfs_rq(now, cfs_rq);
> > > >         u64_u32_store_copy(sa->last_update_time,
> > > >                            cfs_rq->last_update_time_copy,
> > > >                            sa->last_update_time);
> > > > @@ -4252,7 +4247,7 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> > > >                 __update_load_avg_se(now, cfs_rq, se);
> > > >
> > > >         decayed  = update_cfs_rq_load_avg(now, cfs_rq);
> > > > -       decayed |= propagate_entity_load_avg(se);
> > > > +       propagate_entity_load_avg(se);
> > >
> > > but then you  also skip the call to cfs_rq_util_change()
> >
> > Ah right, I missed that, thanks for catching this.
> >
> > Updated:
> >
> > From 09a649f8111cfca656b7b735da975ef607b00956 Mon Sep 17 00:00:00 2001
> > From: Aaron Lu <aaron.lu@intel.com>
> > Date: Wed, 19 Jul 2023 13:54:48 +0800
> > Subject: [PATCH] sched/fair: only update_tg_load_avg() when cfs_rq decayed
> >
> > ---
> >  kernel/sched/fair.c | 17 ++++++-----------
> >  1 file changed, 6 insertions(+), 11 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index a80a73909dc2..8d4b9e0a19b6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4086,7 +4086,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> >  {
> >         unsigned long removed_load = 0, removed_util = 0, removed_runnable = 0;
> >         struct sched_avg *sa = &cfs_rq->avg;
> > -       int decayed = 0;
> > +       int decayed;
> >
> >         if (cfs_rq->removed.nr) {
> >                 unsigned long r;
> > @@ -4134,11 +4134,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> >                  */
> >                 add_tg_cfs_propagate(cfs_rq,
> >                         -(long)(removed_runnable * divider) >> SCHED_CAPACITY_SHIFT);
> > -
> > -               decayed = 1;
> 
> We need this to propagate the change in other place like cpufreq

Ah, I just made the same mistake again, sorry.

So there are three cases for a cfs_rq: load decayed, load removed and
load propagated. For all three cases, cfs_rq_util_change() needs to be
called and only for decayed, update_tg_load_avg() needs to be called.

I'll update the patch accordingly.

> >         }
> >
> > -       decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
> > +       decayed = __update_load_avg_cfs_rq(now, cfs_rq);
> >         u64_u32_store_copy(sa->last_update_time,
> >                            cfs_rq->last_update_time_copy,
> >                            sa->last_update_time);
> > @@ -4242,7 +4240,7 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> >  static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >  {
> >         u64 now = cfs_rq_clock_pelt(cfs_rq);
> > -       int decayed;
> > +       int decayed, propagated;
> >
> >         /*
> >          * Track task load average for carrying it to new CPU after migrated, and
> > @@ -4252,7 +4250,7 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> >                 __update_load_avg_se(now, cfs_rq, se);
> >
> >         decayed  = update_cfs_rq_load_avg(now, cfs_rq);
> > -       decayed |= propagate_entity_load_avg(se);
> > +       propagated = propagate_entity_load_avg(se);
> >
> >         if (!se->avg.last_update_time && (flags & DO_ATTACH)) {
> >
> > @@ -4264,19 +4262,16 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> >                  * IOW we're enqueueing a task on a new CPU.
> >                  */
> >                 attach_entity_load_avg(cfs_rq, se);
> > -               update_tg_load_avg(cfs_rq);
> > -
> >         } else if (flags & DO_DETACH) {
> >                 /*
> >                  * DO_DETACH means we're here from dequeue_entity()
> >                  * and we are migrating task out of the CPU.
> >                  */
> >                 detach_entity_load_avg(cfs_rq, se);
> > -               update_tg_load_avg(cfs_rq);
> > -       } else if (decayed) {
> > +       } else if (decayed || propagated) {
> >                 cfs_rq_util_change(cfs_rq, 0);
> >
> > -               if (flags & UPDATE_TG)
> > +               if (decayed && (flags & UPDATE_TG))
> 
> It would be simpler and more readable to clear UPDATE_TG or not set it
> from the beginning

Do you mean something like this?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8d4b9e0a19b6..084d63371355 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4249,7 +4249,7 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
 	if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD))
 		__update_load_avg_se(now, cfs_rq, se);
 
-	decayed  = update_cfs_rq_load_avg(now, cfs_rq);
+	decayed  = update_cfs_rq_load_avg(now, cfs_rq) && (flags & UPDATE_TG);
 	propagated = propagate_entity_load_avg(se);
 
 	if (!se->avg.last_update_time && (flags & DO_ATTACH)) {
@@ -4271,7 +4271,7 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
 	} else if (decayed || propagated) {
 		cfs_rq_util_change(cfs_rq, 0);
 
-		if (decayed && (flags & UPDATE_TG))
+		if (decayed)
 			update_tg_load_avg(cfs_rq);
 	}
 }

> IIUC, you rely on the fact that a decay happens every 1024 us of the
> cfs_rq_clock_pelt() which is scaled by frequency and cpu compute
> capacity. So you can end up with a cfs_rq_clock_pelt() that is far
> slower than real clock and the 1ms can easily be extended to dozens of
> ms

Thanks for the info. I'm not familiar with this clock scale part and will
need to take a closer look.

As you know, the intent is to make the unbound update to tg->load_avg
become bound for those wakeup migration heavy workloads and the way this
change does to achieve it is to remove the update to tg->load_avg on
attach and detach path, just leave the update on load decay path. And if
the current implementation makes load decay longer than 1024us, that
shouldn't be a problem for this change. I don't see an immediate problem
if update to tg->load_avg happens less often than once per ms but please
let me know if I missed something, thanks.

> 
> >                         update_tg_load_avg(cfs_rq);
> >         }
> >  }
> > --
> > 2.41.0
> >
> >
> > > >
> > > >         if (!se->avg.last_update_time && (flags & DO_ATTACH)) {
> > > >
> > > > @@ -4264,15 +4259,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> > > >                  * IOW we're enqueueing a task on a new CPU.
> > > >                  */
> > > >                 attach_entity_load_avg(cfs_rq, se);
> > > > -               update_tg_load_avg(cfs_rq);
> > > > -
> > > >         } else if (flags & DO_DETACH) {
> > > >                 /*
> > > >                  * DO_DETACH means we're here from dequeue_entity()
> > > >                  * and we are migrating task out of the CPU.
> > > >                  */
> > > >                 detach_entity_load_avg(cfs_rq, se);
> > > > -               update_tg_load_avg(cfs_rq);
> > > >         } else if (decayed) {
> > > >                 cfs_rq_util_change(cfs_rq, 0);
> > > >
> > > > --
> > > > 2.41.0
> > > >