Re: [PATCH v2 2/7] sched/fair: Decay task PELT values during migration

[Vincent Guittot]

On Wed, 12 Jan 2022 at 17:14, Vincent Donnefort
<vincent.donnefort@xxxxxxx> 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@xxxxxxx/
>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
>
> 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
- 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.
- (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
- 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 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.


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

> +
> +       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
>