Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

Vincent Guittot
2024-08-22T10:20:50Z

On Thu, 22 Aug 2024 at 11:53, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Thu, 22 Aug 2024 at 11:22, Luis Machado <luis.machado@arm.com> wrote:
> >
> > On 8/22/24 09:19, Vincent Guittot wrote:
> > > Hi,
> > >
> > > On Wed, 21 Aug 2024 at 15:34, Hongyan Xia <hongyan.xia2@arm.com> wrote:
> > >>
> > >> Hi Peter,
> > >>
> > >> Sorry for bombarding this thread in the last couple of days. I'm seeing
> > >> several issues in the latest tip/sched/core after these patches landed.
> > >>
> > >> What I'm now seeing seems to be an unbalanced call of util_est. First, I applied
> > >
> > > I also see a remaining util_est for idle rq because of an unbalance
> > > call of util_est_enqueue|dequeue
> > >
> >
> > I can confirm issues with the utilization values and frequencies being driven
> > seemingly incorrectly, in particular for little cores.
> >
> > What I'm seeing with the stock series is high utilization values for some tasks
> > and little cores having their frequencies maxed out for extended periods of
> > time. Sometimes for 5+ or 10+ seconds, which is excessive as the cores are mostly
> > idle. But whenever certain tasks get scheduled there, they have a very high util
> > level and so the frequency is kept at max.
> >
> > As a consequence this drives up power usage.
> >
> > I gave Hongyan's draft fix a try and observed a much more reasonable behavior for
> > the util numbers and frequencies being used for the little cores. With his fix,
> > I can also see lower energy use for my specific benchmark.
>
> The main problem is that the util_est of a delayed dequeued task
> remains on the rq and keeps the rq utilization high and as a result
> the frequency higher than needed.
>
> The below seems to works for me and keep sync the enqueue/dequeue of
> uti_test with the enqueue/dequeue of the task as if de dequeue was not
> delayed
>
> Another interest is that we will not try to migrate a delayed dequeue
> sleeping task that doesn't actually impact the current load of the cpu
> and as a result will not help in the load balance. I haven't yet fully
> checked what would happen with hotplug

And there is the case of a delayed dequeue task that gets its affinity changed

>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fea057b311f6..0970bcdc889a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6944,11 +6944,6 @@ enqueue_task_fair(struct rq *rq, struct
> task_struct *p, int flags)
> int rq_h_nr_running = rq->cfs.h_nr_running;
> u64 slice = 0;
>
> - if (flags & ENQUEUE_DELAYED) {
> - requeue_delayed_entity(se);
> - return;
> - }
> -
> /*
> * The code below (indirectly) updates schedutil which looks at
> * the cfs_rq utilization to select a frequency.
> @@ -6957,6 +6952,11 @@ enqueue_task_fair(struct rq *rq, struct
> task_struct *p, int flags)
> */
> util_est_enqueue(&rq->cfs, p);
>
> + if (flags & ENQUEUE_DELAYED) {
> + requeue_delayed_entity(se);
> + return;
> + }
> +
> /*
> * If in_iowait is set, the code below may not trigger any cpufreq
> * utilization updates, so do it here explicitly with the IOWAIT flag
> @@ -9276,6 +9276,8 @@ int can_migrate_task(struct task_struct *p,
> struct lb_env *env)
>
> lockdep_assert_rq_held(env->src_rq);
>
> + if (p->se.sched_delayed)
> + return 0;
> /*
> * We do not migrate tasks that are:
> * 1) throttled_lb_pair, or
>
> >
> >
> > >> the following diff to warn against util_est != 0 when no tasks are on
> > >> the queue:
> > >>
> > >> https://lore.kernel.org/all/752ae417c02b9277ca3ec18893747c54dd5f277f.1724245193.git.hongyan.xia2@arm.com/
> > >>
> > >> Then, I'm reliably seeing warnings on my Juno board during boot in
> > >> latest tip/sched/core.
> > >>
> > >> If I do the same thing to util_est just like what you did in this uclamp
> > >> patch, like this:
> > >
> > > I think that the solution is simpler than your proposal and we just
> > > need to always call util_est_enqueue() before the
> > > requeue_delayed_entity
> > >
> > > @@ -6970,11 +6970,6 @@ enqueue_task_fair(struct rq *rq, struct
> > > task_struct *p, int flags)
> > > int rq_h_nr_running = rq->cfs.h_nr_running;
> > > u64 slice = 0;
> > >
> > > - if (flags & ENQUEUE_DELAYED) {
> > > - requeue_delayed_entity(se);
> > > - return;
> > > - }
> > > -
> > > /*
> > > * The code below (indirectly) updates schedutil which looks at
> > > * the cfs_rq utilization to select a frequency.
> > > @@ -6983,6 +6978,11 @@ enqueue_task_fair(struct rq *rq, struct
> > > task_struct *p, int flags)
> > > */
> > > util_est_enqueue(&rq->cfs, p);
> > >
> > > + if (flags & ENQUEUE_DELAYED) {
> > > + requeue_delayed_entity(se);
> > > + return;
> > > + }
> > > +
> > > /*
> > > * If in_iowait is set, the code below may not trigger any cpufreq
> > > * utilization updates, so do it here explicitly with the IOWAIT flag
> > >
> > >
> > >>
> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > >> index 574ef19df64b..58aac42c99e5 100644
> > >> --- a/kernel/sched/fair.c
> > >> +++ b/kernel/sched/fair.c
> > >> @@ -6946,7 +6946,7 @@ enqueue_task_fair(struct rq *rq, struct
> > >> task_struct *p, int flags)
> > >>
> > >> if (flags & ENQUEUE_DELAYED) {
> > >> requeue_delayed_entity(se);
> > >> - return;
> > >> + goto util_est;
> > >> }
> > >>
> > >> /*
> > >> @@ -6955,7 +6955,6 @@ enqueue_task_fair(struct rq *rq, struct
> > >> task_struct *p, int flags)
> > >> * Let's add the task's estimated utilization to the cfs_rq's
> > >> * estimated utilization, before we update schedutil.
> > >> */
> > >> - util_est_enqueue(&rq->cfs, p);
> > >>
> > >> /*
> > >> * If in_iowait is set, the code below may not trigger any cpufreq
> > >> @@ -7050,6 +7049,9 @@ enqueue_task_fair(struct rq *rq, struct
> > >> task_struct *p, int flags)
> > >> assert_list_leaf_cfs_rq(rq);
> > >>
> > >> hrtick_update(rq);
> > >> +util_est:
> > >> + if (!p->se.sched_delayed)
> > >> + util_est_enqueue(&rq->cfs, p);
> > >> }
> > >>
> > >> static void set_next_buddy(struct sched_entity *se);
> > >> @@ -7173,7 +7175,8 @@ static int dequeue_entities(struct rq *rq, struct
> > >> sched_entity *se, int flags)
> > >> */
> > >> static bool dequeue_task_fair(struct rq *rq, struct task_struct *p,
> > >> int flags)
> > >> {
> > >> - util_est_dequeue(&rq->cfs, p);
> > >> + if (!p->se.sched_delayed)
> > >> + util_est_dequeue(&rq->cfs, p);
> > >>
> > >> if (dequeue_entities(rq, &p->se, flags) < 0) {
> > >> if (!rq->cfs.h_nr_running)
> > >>
> > >> which is basically enqueuing util_est after enqueue_task_fair(),
> > >> dequeuing util_est before dequeue_task_fair() and double check
> > >> p->se.delayed_dequeue, then the unbalanced issue seems to go away.
> > >>
> > >> Hopefully this helps you in finding where the problem could be.
> > >>
> > >> Hongyan
> > >>
> > >> On 27/07/2024 11:27, Peter Zijlstra wrote:
> > >>> Delayed dequeue has tasks sit around on the runqueue that are not
> > >>> actually runnable -- specifically, they will be dequeued the moment
> > >>> they get picked.
> > >>>
> > >>> One side-effect is that such a task can get migrated, which leads to a
> > >>> 'nested' dequeue_task() scenario that messes up uclamp if we don't
> > >>> take care.
> > >>>
> > >>> Notably, dequeue_task(DEQUEUE_SLEEP) can 'fail' and keep the task on
> > >>> the runqueue. This however will have removed the task from uclamp --
> > >>> per uclamp_rq_dec() in dequeue_task(). So far so good.
> > >>>
> > >>> However, if at that point the task gets migrated -- or nice adjusted
> > >>> or any of a myriad of operations that does a dequeue-enqueue cycle --
> > >>> we'll pass through dequeue_task()/enqueue_task() again. Without
> > >>> modification this will lead to a double decrement for uclamp, which is
> > >>> wrong.
> > >>>
> > >>> Reported-by: Luis Machado <luis.machado@arm.com>
> > >>> Reported-by: Hongyan Xia <hongyan.xia2@arm.com>
> > >>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > >>> ---
> > >>> kernel/sched/core.c | 16 +++++++++++++++-
> > >>> 1 file changed, 15 insertions(+), 1 deletion(-)
> > >>>
> > >>> --- a/kernel/sched/core.c
> > >>> +++ b/kernel/sched/core.c
> > >>> @@ -1676,6 +1676,9 @@ static inline void uclamp_rq_inc(struct
> > >>> if (unlikely(!p->sched_class->uclamp_enabled))
> > >>> return;
> > >>>
> > >>> + if (p->se.sched_delayed)
> > >>> + return;
> > >>> +
> > >>> for_each_clamp_id(clamp_id)
> > >>> uclamp_rq_inc_id(rq, p, clamp_id);
> > >>>
> > >>> @@ -1700,6 +1703,9 @@ static inline void uclamp_rq_dec(struct
> > >>> if (unlikely(!p->sched_class->uclamp_enabled))
> > >>> return;
> > >>>
> > >>> + if (p->se.sched_delayed)
> > >>> + return;
> > >>> +
> > >>> for_each_clamp_id(clamp_id)
> > >>> uclamp_rq_dec_id(rq, p, clamp_id);
> > >>> }
> > >>> @@ -1979,8 +1985,12 @@ void enqueue_task(struct rq *rq, struct
> > >>> psi_enqueue(p, (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED));
> > >>> }
> > >>>
> > >>> - uclamp_rq_inc(rq, p);
> > >>> p->sched_class->enqueue_task(rq, p, flags);
> > >>> + /*
> > >>> + * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
> > >>> + * ->sched_delayed.
> > >>> + */
> > >>> + uclamp_rq_inc(rq, p);
> > >>>
> > >>> if (sched_core_enabled(rq))
> > >>> sched_core_enqueue(rq, p);
> > >>> @@ -2002,6 +2012,10 @@ inline bool dequeue_task(struct rq *rq,
> > >>> psi_dequeue(p, flags & DEQUEUE_SLEEP);
> > >>> }
> > >>>
> > >>> + /*
> > >>> + * Must be before ->dequeue_task() because ->dequeue_task() can 'fail'
> > >>> + * and mark the task ->sched_delayed.
> > >>> + */
> > >>> uclamp_rq_dec(rq, p);
> > >>> return p->sched_class->dequeue_task(rq, p, flags);
> > >>> }
> > >>>
> > >>>
> >