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Message-ID: <Yeac5Y5Fzu/jaUf0@FVFF7649Q05P>
Date: Tue, 18 Jan 2022 10:56:37 +0000
From: Vincent Donnefort <vincent.donnefort@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: peterz@...radead.org, mingo@...hat.com,
linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
Valentin.Schneider@....com, Morten.Rasmussen@....com,
Chris.Redpath@....com, qperret@...gle.com, Lukasz.Luba@....com
Subject: Re: [PATCH v2 2/7] sched/fair: Decay task PELT values during
migration
On Mon, Jan 17, 2022 at 06:31:25PM +0100, Vincent Guittot wrote:
> On Wed, 12 Jan 2022 at 17:14, Vincent Donnefort
> <vincent.donnefort@....com> wrote:
> >
> > Before being migrated to a new CPU, a task sees its PELT values
> > synchronized with rq last_update_time. Once done, that same task will also
> > have its sched_avg last_update_time reset. This means the time between
> > the migration and the last clock update (B) will not be accounted for in
> > util_avg and a discontinuity will appear. This issue is amplified by the
> > PELT clock scaling. If the clock hasn't been updated while the CPU is
> > idle, clock_pelt will not be aligned with clock_task and that time (A)
> > will be also lost.
> >
> > ---------|----- A -----|-----------|------- B -----|>
> > clock_pelt clock_task clock now
> >
> > This is especially problematic for asymmetric CPU capacity systems which
> > need stable util_avg signals for task placement and energy estimation.
> >
> > Ideally, this problem would be solved by updating the runqueue clocks
> > before the migration. But that would require taking the runqueue lock
> > which is quite expensive [1]. Instead estimate the missing time and update
> > the task util_avg with that value:
> >
> > A + B = clock_task - clock_pelt + sched_clock_cpu() - clock
> >
> > Neither clock_task, clock_pelt nor clock can be accessed without the
> > runqueue lock. The new runqueue clock_pelt_lag is therefore created and
> > encode those three values.
> >
> > clock_pelt_lag = clock - clock_task + clock_pelt
> >
> > And we can then write the missing time as follow:
> >
> > A + B = sched_clock_cpu() - clock_pelt_lag
> >
> > The B. part of the missing time is however an estimation that doesn't take
> > into account IRQ and Paravirt time.
> >
> > Now we have an estimation for A + B, we can create an estimator for the
> > PELT value at the time of the migration. We need for this purpose to
> > inject last_update_time which is a combination of both clock_pelt and
> > lost_idle_time. The latter is a time value which is completely lost form a
> > PELT point of view and must be ignored. And finally, we can write:
> >
> > rq_clock_pelt_estimator() = last_update_time + A + B
> > = last_update_time +
> > sched_clock_cpu() - clock_pelt_lag
> >
> > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/
> >
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 06cf7620839a..11c6aeef4583 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -618,6 +618,12 @@ struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf)
> > }
> > }
> >
> > +static void update_rq_clock_pelt_lag(struct rq *rq)
> > +{
> > + u64_u32_store(rq->clock_pelt_lag,
> > + rq->clock - rq->clock_task + rq->clock_pelt);
>
> This has several shortfalls:
> - have a look at cfs_rq_clock_pelt() and rq_clock_pelt(). What you
> name clock_pelt in your commit message and is used to update PELT and
> saved in se->avg.last_update_time is : rq->clock_pelt -
> rq->lost_idle_time - cfs_rq->throttled_clock_task_time
That's why, the PELT "lag" is added onto se->avg.last_update_time. (see the last
paragraph of the commit message) The estimator is just a time delta, that is
added on top of the entity's last_update_time. I don't see any problem with the
lost_idle_time here.
> - you are doing this whatever the state of the cpu : idle or not. But
> the clock cycles are not accounted for in the same way in both cases.
If the CPU is idle and clock_pelt == clock_task, the component A of the
estimator would be 0 and we only would account for how outdated is the rq's
clock, i.e. component B.
> - (B) doesn't seem to be accurate as you skip irq and steal time
> accounting and you don't apply any scale invariance if the cpu is not
> idle
The missing irq and paravirt time is the reason why it is called "estimator".
But maybe there's a chance of improving this part with a lockless version of
rq->prev_irq_time and rq->prev_steal_time_rq?
> - IIUC your explanation in the commit message above, the (A) period
> seems to be a problem only when idle but you apply it unconditionally.
If the CPU is idle (and clock_pelt == clock_task), only the B part would be
worth something:
A + B = [clock_task - clock_pelt] + [sched_clock_cpu() - clock]
A B
> If cpu is idle you can assume that clock_pelt should be equal to
> clock_task but you can't if cpu is not idle otherwise your sync will
> be inaccurate and defeat the primary goal of this patch. If your
> problem with clock_pelt is that the pending idle time is not accounted
> for when entering idle but only at the next update (update blocked
> load or wakeup of a thread). This patch below should fix this and
> remove your A.
That would help slightly the current situation, but this part is already
covered by the estimator.
>
>
> diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
> index e06071bf3472..855877be4dd8 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -114,6 +114,7 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq)
> {
> u32 divider = ((LOAD_AVG_MAX - 1024) << SCHED_CAPACITY_SHIFT)
> - LOAD_AVG_MAX;
> u32 util_sum = rq->cfs.avg.util_sum;
> + u64 now = rq_clock_task(rq);
> util_sum += rq->avg_rt.util_sum;
> util_sum += rq->avg_dl.util_sum;
>
> @@ -127,7 +128,10 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq)
> * rq's clock_task.
> */
> if (util_sum >= divider)
> - rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt;
> + rq->lost_idle_time += now - rq->clock_pelt;
> +
> + /* The rq is idle, we can sync to clock_task */
> + rq->clock_pelt = now;
> }
>
> static inline u64 rq_clock_pelt(struct rq *rq)
>
> ---
>
>
> > +}
> > +
> > /*
> > * RQ-clock updating methods:
> > */
> > @@ -674,6 +680,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
> > update_irq_load_avg(rq, irq_delta + steal);
> > #endif
> > update_rq_clock_pelt(rq, delta);
> > + update_rq_clock_pelt_lag(rq);
> > }
> >
> > void update_rq_clock(struct rq *rq)
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 99ea9540ece4..046d5397eb8a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6852,6 +6852,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >
> > static void detach_entity_cfs_rq(struct sched_entity *se);
> >
> > +static u64 rq_clock_pelt_estimator(struct rq *rq, u64 last_update_time)
> > +{
> > + u64 pelt_lag = sched_clock_cpu(cpu_of(rq)) -
> > + u64_u32_load(rq->clock_pelt_lag);
>
> Have you evaluated the impact of calling sched_clock_cpu(cpu_of(rq))
> for a remote cpu ? especially with a huge number of migration and
> concurrent access from several cpus
I have not, but I will have a look.
>
> > +
> > + return last_update_time + pelt_lag;
> > +}
> > +
> > /*
> > * Called immediately before a task is migrated to a new CPU; task_cpu(p) and
> > * cfs_rq_of(p) references at time of call are still valid and identify the
> > @@ -6859,6 +6867,9 @@ static void detach_entity_cfs_rq(struct sched_entity *se);
> > */
> > static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > {
> > + struct sched_entity *se = &p->se;
> > + struct rq *rq = task_rq(p);
> > +
> > /*
> > * As blocked tasks retain absolute vruntime the migration needs to
> > * deal with this by subtracting the old and adding the new
> > @@ -6866,7 +6877,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > * the task on the new runqueue.
> > */
> > if (READ_ONCE(p->__state) == TASK_WAKING) {
> > - struct sched_entity *se = &p->se;
> > struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >
> > se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> > @@ -6877,26 +6887,32 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old'
> > * rq->lock and can modify state directly.
> > */
> > - lockdep_assert_rq_held(task_rq(p));
> > - detach_entity_cfs_rq(&p->se);
> > + lockdep_assert_rq_held(rq);
> > + detach_entity_cfs_rq(se);
> >
> > } else {
> > + u64 now;
> > +
> > + remove_entity_load_avg(se);
> > +
> > /*
> > - * We are supposed to update the task to "current" time, then
> > - * its up to date and ready to go to new CPU/cfs_rq. But we
> > - * have difficulty in getting what current time is, so simply
> > - * throw away the out-of-date time. This will result in the
> > - * wakee task is less decayed, but giving the wakee more load
> > - * sounds not bad.
> > + * Here, the task's PELT values have been updated according to
> > + * the current rq's clock. But if that clock hasn't been
> > + * updated in a while, a substantial idle time will be missed,
> > + * leading to an inflation after wake-up on the new rq.
> > + *
> > + * Estimate the PELT clock lag, and update sched_avg to ensure
> > + * PELT continuity after migration.
> > */
> > - remove_entity_load_avg(&p->se);
> > + now = rq_clock_pelt_estimator(rq, se->avg.last_update_time);
> > + __update_load_avg_blocked_se(now, se);
> > }
> >
> > /* Tell new CPU we are migrated */
> > - p->se.avg.last_update_time = 0;
> > + se->avg.last_update_time = 0;
> >
> > /* We have migrated, no longer consider this task hot */
> > - p->se.exec_start = 0;
> > + se->exec_start = 0;
> >
> > update_scan_period(p, new_cpu);
> > }
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index f1a445efdc63..fdf2a9e54c0e 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1027,8 +1027,13 @@ struct rq {
> > /* Ensure that all clocks are in the same cache line */
> > u64 clock_task ____cacheline_aligned;
> > u64 clock_pelt;
> > + u64 clock_pelt_lag;
> > unsigned long lost_idle_time;
> >
> > +#ifndef CONFIG_64BIT
> > + u64 clock_pelt_lag_copy;
> > +#endif
> > +
> > atomic_t nr_iowait;
> >
> > #ifdef CONFIG_SCHED_DEBUG
> > --
> > 2.25.1
> >
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