Re: [PATCH v6 2/7] sched/fair: Decay task PELT values during wakeup migration

From: Vincent Guittot
Date: Tue Apr 26 2022 - 12:41:25 EST

Hi Vincent,

Le mardi 26 avril 2022 à 10:35:01 (+0100), Vincent Donnefort a écrit :
> 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
>
> sched_clock_cpu() is a costly functinon. Limit the usage to the case where
> the source CPU is idle as we know this is when the clock is having the
> biggest risk of being outdated.
>
> Neither clock_task, clock_pelt nor clock can be accessed without the
> runqueue lock. We then need to store those values in a timestamp variable
> which can be accessed during the migration. rq's enter_idle will give the
> wall-clock time when the rq went idle. We have then:
>
> B = sched_clock_cpu() - rq->enter_idle.
>
> Then, to catch-up the PELT clock scaling (A), two cases:
>
> * !CFS_BANDWIDTH: We can simply use clock_task(). This value is stored
> in rq's clock_pelt_idle, before the rq enters idle. The estimated time
> is then:
>
> rq->clock_pelt_idle + sched_clock_cpu() - rq->enter_idle.
>
> * CFS_BANDWIDTH: We can't catch-up with clock_task because of the
> throttled_clock_task_time offset. cfs_rq's clock_pelt_idle is then
> giving the PELT clock when the cfs_rq becomes idle. This gives:
>
> A = rq->clock_pelt_idle - cfs_rq->clock_pelt_idle
>
> And gives the following estimated time:
>
> cfs_rq->last_update_time +
> rq->clock_pelt_idle - cfs_rq->clock_pelt_idle + (A)
> sched_clock_cpu() - rq->enter_idle (B)
>
> The (B) part of the missing time is however an estimation that doesn't
> take into account IRQ and Paravirt time.
>
> [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@xxxxxxx/
>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index abd1feeec0c2..1256e2c0e2e2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3694,6 +3694,48 @@ static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum
>
> #endif /* CONFIG_FAIR_GROUP_SCHED */
>
> +#ifdef CONFIG_NO_HZ_COMMON
> +static inline void migrate_se_pelt_lag(struct sched_entity *se)
> +{
> + struct cfs_rq *cfs_rq;
> + struct rq *rq;
> + bool is_idle;
> + u64 now;
> +
> + cfs_rq = cfs_rq_of(se);
> + rq = rq_of(cfs_rq);
> +
> + rcu_read_lock();
> + is_idle = is_idle_task(rcu_dereference(rq->curr));
> + rcu_read_unlock();
> +
> + /*
> + * The lag estimation comes with a cost we don't want to pay all the
> + * time. Hence, limiting to the case where the source CPU is idle and
> + * we know we are at the greatest risk to have an outdated clock.
> + */
> + if (!is_idle)
> + return;
> +
> +#ifdef CONFIG_CFS_BANDWIDTH
> + now = u64_u32_load(cfs_rq->clock_pelt_idle);
> + /* The clock has been stopped for throttling */
> + if (now == U64_MAX)
> + return;
> +
> + now += cfs_rq_last_update_time(cfs_rq);
> + now -= u64_u32_load(rq->clock_pelt_idle);

This is a bit difficult to read and follow.

we should have
/* start at the last cfs pelt update time */
now = cfs_rq_last_update_time(cfs_rq);
/* Add delta between cfs rq becoming idle and rq becoming idle */
now += u64_u32_load(cfs_rq->clock_pelt_idle); - u64_u32_load(rq->clock_pelt_idle);

But IIUC, you started with u64_u32_load(cfs_rq->clock_pelt_idle) to dectect throttled case

All this would deserve a comment explaining what we are computing here at least


> +#else
> + now = u64_u32_load(rq->clock_pelt_idle);
> +#endif
> + now += sched_clock_cpu(cpu_of(rq)) - u64_u32_load(rq->enter_idle);
> +
> + __update_load_avg_blocked_se(now, se);
> +}
> +#else
> +static void migrate_se_pelt_lag(struct sched_entity *se) {}
> +#endif
> +
> /**
> * update_cfs_rq_load_avg - update the cfs_rq's load/util averages
> * @now: current time, as per cfs_rq_clock_pelt()
> @@ -4429,6 +4471,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> */
> if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
> update_min_vruntime(cfs_rq);
> +
> + if (cfs_rq->nr_running == 0)
> + update_idle_cfs_rq_clock_pelt(cfs_rq);
> }
>
> /*
> @@ -6946,6 +6991,8 @@ 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;
> +
> /*
> * As blocked tasks retain absolute vruntime the migration needs to
> * deal with this by subtracting the old and adding the new
> @@ -6953,7 +7000,6 @@ 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);
> @@ -6965,25 +7011,29 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> * rq->lock and can modify state directly.
> */
> lockdep_assert_rq_held(task_rq(p));
> - detach_entity_cfs_rq(&p->se);
> + detach_entity_cfs_rq(se);
>
> } else {
> + 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 missing time from the cfs_rq last_update_time
> + * and update sched_avg to improve the PELT continuity after
> + * migration.
> */
> - remove_entity_load_avg(&p->se);
> + migrate_se_pelt_lag(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/pelt.h b/kernel/sched/pelt.h
> index 4ff2ed4f8fa1..0380f750adbe 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -103,6 +103,14 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
> rq->clock_pelt += delta;
> }
>
> +static inline u64 rq_clock_pelt(struct rq *rq)
> +{
> + lockdep_assert_rq_held(rq);
> + assert_clock_updated(rq);
> +
> + return rq->clock_pelt - rq->lost_idle_time;
> +}
> +
> /*
> * When rq becomes idle, we have to check if it has lost idle time
> * because it was fully busy. A rq is fully used when the /Sum util_sum
> @@ -130,17 +138,24 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq)
> */
> if (util_sum >= divider)
> rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt;
> -}
>
> -static inline u64 rq_clock_pelt(struct rq *rq)
> -{
> - lockdep_assert_rq_held(rq);
> - assert_clock_updated(rq);
> + /* The rq is idle, we can sync with clock_task */
> + rq->clock_pelt = rq_clock_task(rq);
>
> - return rq->clock_pelt - rq->lost_idle_time;
> + u64_u32_store(rq->enter_idle, rq_clock(rq));
> + u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq));

rq and cfs rq clock pelt can also be updated during ILB/update_blocked_load.
So you also need this:

---
kernel/sched/fair.c | 4 ++++
kernel/sched/pelt.h | 18 ++++++++++--------
2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fa79d31a36bd..3c1d243079a3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8269,6 +8269,10 @@ static bool __update_blocked_fair(struct rq *rq, bool *done)
if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
update_tg_load_avg(cfs_rq);

+ /* sync clock_pelt_idle with last update */
+ if (cfs_rq->nr_running == 0)
+ update_idle_cfs_rq_clock_pelt(cfs_rq);
+
if (cfs_rq == &rq->cfs)
decayed = true;
}
diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
index 0380f750adbe..6a4dbff8fdd3 100644
--- a/kernel/sched/pelt.h
+++ b/kernel/sched/pelt.h
@@ -61,6 +61,14 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
WRITE_ONCE(avg->util_est.enqueued, enqueued);
}

+static inline u64 rq_clock_pelt(struct rq *rq)
+{
+ lockdep_assert_rq_held(rq);
+ assert_clock_updated(rq);
+
+ return rq->clock_pelt - rq->lost_idle_time;
+}
+
/*
* The clock_pelt scales the time to reflect the effective amount of
* computation done during the running delta time but then sync back to
@@ -78,6 +86,8 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
if (unlikely(is_idle_task(rq->curr))) {
/* The rq is idle, we can sync to clock_task */
rq->clock_pelt = rq_clock_task(rq);
+ u64_u32_store(rq->enter_idle, rq_clock(rq));
+ u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq));
return;
}

@@ -103,14 +113,6 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
rq->clock_pelt += delta;
}

-static inline u64 rq_clock_pelt(struct rq *rq)
-{
- lockdep_assert_rq_held(rq);
- assert_clock_updated(rq);
-
- return rq->clock_pelt - rq->lost_idle_time;
-}
-
/*
* When rq becomes idle, we have to check if it has lost idle time
* because it was fully busy. A rq is fully used when the /Sum util_sum
--



> }
>
> #ifdef CONFIG_CFS_BANDWIDTH
> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
> +{
> + if (unlikely(cfs_rq->throttle_count))
> + u64_u32_store(cfs_rq->clock_pelt_idle, U64_MAX);
> + else
> + u64_u32_store(cfs_rq->clock_pelt_idle,
> + rq_clock_pelt(rq_of(cfs_rq)));
> +}
> +
> /* rq->task_clock normalized against any time this cfs_rq has spent throttled */
> static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
> {
> @@ -150,6 +165,7 @@ static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
> return rq_clock_pelt(rq_of(cfs_rq)) - cfs_rq->throttled_clock_pelt_time;
> }
> #else
> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { }
> static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
> {
> return rq_clock_pelt(rq_of(cfs_rq));
> @@ -204,6 +220,7 @@ update_rq_clock_pelt(struct rq *rq, s64 delta) { }
> static inline void
> update_idle_rq_clock_pelt(struct rq *rq) { }
>
> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { }
> #endif
>
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e2cf6e48b165..07014e8cbae2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -641,6 +641,10 @@ struct cfs_rq {
> int runtime_enabled;
> s64 runtime_remaining;
>
> + u64 clock_pelt_idle;
> +#ifndef CONFIG_64BIT
> + u64 clock_pelt_idle_copy;
> +#endif
> u64 throttled_clock;
> u64 throttled_clock_pelt;
> u64 throttled_clock_pelt_time;
> @@ -1013,6 +1017,12 @@ struct rq {
> u64 clock_task ____cacheline_aligned;
> u64 clock_pelt;
> unsigned long lost_idle_time;
> + u64 clock_pelt_idle;
> + u64 enter_idle;
> +#ifndef CONFIG_64BIT
> + u64 clock_pelt_idle_copy;
> + u64 enter_idle_copy;
> +#endif
>
> atomic_t nr_iowait;
>
> --
> 2.25.1
>