Message-ID: <CAKfTPtCMA+H7HNvi=N6EZhhn6JnLQ2Mf5fDWQ8FK5Hh8G-ffTg@mail.gmail.com>
Date:   Fri, 29 Apr 2022 15:23:15 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     Vincent Donnefort <vincent.donnefort@....com>,
        peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com, qperret@...gle.com
Subject: Re: [PATCH v7 2/7] sched/fair: Decay task PELT values during wakeup migration

On Fri, 29 Apr 2022 at 14:51, Tao Zhou <tao.zhou@...ux.dev> wrote:
>
> On Fri, Apr 29, 2022 at 10:20:00AM +0200, Vincent Guittot wrote:
>
> > Le vendredi 29 avril 2022 à 01:22:19 (+0800), Tao Zhou a écrit :
> > > Hi Vincent,
> > >
> > > On Thu, Apr 28, 2022 at 03:38:39PM +0200, Vincent Guittot wrote:
> > >
> > > > On Wed, 27 Apr 2022 at 19:37, Tao Zhou <tao.zhou@...ux.dev> wrote:
> > > > >
> >
> > [..]
> >
> > > > > > +                     /* sync clock_pelt_idle with last update */
> > > > > > +                     if (cfs_rq->nr_running == 0)
> > > > > > +                             update_idle_cfs_rq_clock_pelt(cfs_rq);
> > > > >
> > > > > I think that if cfs_rq->nr_running == 0 then use cfs rq pelt_idle to update
> > > > > idle cfs rq.
> > > >
> > > > update_blocked_averages() updates all cfs rq to be aligned with now so
> > > > we don't need to calculate an estimated now. update_rq_clock(rq) is
> > > > called 1st to update the rq->clock and childs
> > > >
> > > > With only need to save when happened the last update which is done in
> > > > update_rq_clock_pelt(rq) for rq->clock_pelt and with
> > > > update_idle_cfs_rq_clock_pelt(cfs) for the cfs_rq_clock_pelt
> > >
> > > I missed this.
> >
> > I ended up with something a bit different:
> >
> > ---
> >  kernel/sched/fair.c  | 133 ++++++++++++++++++++++++++++++++++---------
> >  kernel/sched/pelt.h  |  66 ++++++++++++++++++---
> >  kernel/sched/sched.h |  10 ++++
> >  3 files changed, 174 insertions(+), 35 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index abd1feeec0c2..63e4cf225292 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3335,27 +3335,12 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> >       if (cfs_rq->load.weight)
> >               return false;
> >
> > -     if (cfs_rq->avg.load_sum)
> > -             return false;
> > -
> > -     if (cfs_rq->avg.util_sum)
> > -             return false;
> > -
> > -     if (cfs_rq->avg.runnable_sum)
> > +     if (load_avg_is_decayed(&cfs_rq->avg))
> >               return false;
> >
> >       if (child_cfs_rq_on_list(cfs_rq))
> >               return false;
> >
> > -     /*
> > -      * _avg must be null when _sum are null because _avg = _sum / divider
> > -      * Make sure that rounding and/or propagation of PELT values never
> > -      * break this.
> > -      */
> > -     SCHED_WARN_ON(cfs_rq->avg.load_avg ||
> > -                   cfs_rq->avg.util_avg ||
> > -                   cfs_rq->avg.runnable_avg);
> > -
> >       return true;
> >  }
> >
> > @@ -3694,6 +3679,88 @@ 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, throttled = 0;
> > +
> > +     /* utilization is already fully decayed */
> > +     if (load_avg_is_decayed(&se->avg))
> > +             return;
> > +
> > +     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;
> > +
> > +     /*
> > +      * Estimated "now" is:
> > +      *   last_update_time: last update of the cfs_lock_pelt +
> > +      *   cfs_idle_lag: rq_clock_pelt delta bewteen last cfs update and last rq update +
>
> s/bewteen/between/
>
> > +      *   rq_idle_lag: rq_clock delta between last rq update and now
> > +      *
> > +      * with
> > +      *
> > +      * last_update_time == cfs_clock_pelt()
> > +      *                  == rq_clock_pelt() - cfs->throttled_clock_pelt_time
> > +      *
> > +      * cfs_idle_lag: rq_clock_pelt() when rq is idle - rq_clock_pelt() when cfs is idle
> > +      *
> > +      * rq_idle_lag : sched_clock_cpu() - rq_clock() when rq is idle
> > +      *
> > +      * In fact, rq_clock_pelt() that is used for last_update_time and when
> > +      * cfs is idle are the same because their last update happens atthe
>
> s/atthe/at the/
>
> > +      * same time.
> > +      *
> > +      * We can optimize "now" to be:
> > +      *   rq_clock_pelt when rq is idle - cfs->throttled_clock_pelt_time when cfs is idle +
> > +      *   sched_clock_cpu() - rq_clock() when rq is idle
> > +      *
> > +      * when rq is idle
> > +      *   rq_clock_pelt() is saved in rq->clock_pelt_idle
> > +      *   rq_clock()  is saved in rq->enter idle
> > +      *
> > +      * when cfs is idle
> > +      *    cfs->throttled_clock_pelt_time is saved in cfs_rq->throttled_pelt_idle
> > +      *
> > +      * When !CFS_BANDWIDTH, cfs->throttled_clock_pelt_time is null
> > +      */
> > +
> > +#ifdef CONFIG_CFS_BANDWIDTH
> > +     throttled = u64_u32_load(cfs_rq->throttled_pelt_idle);
> > +     /* The clock has been stopped for throttling */
> > +     if (throttled == U64_MAX)
> > +             return;
> > +#endif
> > +
> > +     now = u64_u32_load(rq->clock_pelt_idle);
> > +     now -= throttled;
> > +
> > +     /* An update happened while computing lag */
> > +     if (now < cfs_rq_last_update_time(cfs_rq))
> > +             return;
> > +
> > +     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 +4496,9 @@ 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 +7016,8 @@ 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 +7025,6 @@ 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 +7036,29 @@ 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);
> >  }
> > @@ -8149,6 +8224,10 @@ 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 4ff2ed4f8fa1..4143c6dc64dc 100644
> > --- a/kernel/sched/pelt.h
> > +++ b/kernel/sched/pelt.h
> > @@ -37,6 +37,29 @@ update_irq_load_avg(struct rq *rq, u64 running)
> >  }
> >  #endif
> >
> > +static inline bool load_avg_is_decayed(struct sched_avg *sa)
> > +{
> > +     if (sa->load_sum)
> > +             return false;
> > +
> > +     if (sa->util_sum)
> > +             return false;
> > +
> > +     if (sa->runnable_sum)
> > +             return false;
> > +
> > +     /*
> > +      * _avg must be null when _sum are null because _avg = _sum / divider
> > +      * Make sure that rounding and/or propagation of PELT values never
> > +      * break this.
> > +      */
> > +     SCHED_WARN_ON(sa->load_avg ||
> > +                   sa->util_avg ||
> > +                   sa->runnable_avg);
> > +
> > +     return true;
> > +}
> > +
> >  #define PELT_MIN_DIVIDER     (LOAD_AVG_MAX - 1024)
> >
> >  static inline u32 get_pelt_divider(struct sched_avg *avg)
> > @@ -61,6 +84,23 @@ 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 rq is idle, we can sync to clock_task */
> > +static inline void _update_idle_rq_clock_pelt(struct rq *rq)
> > +{
> > +     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));
> > +}
> > +
> >  /*
> >   * The clock_pelt scales the time to reflect the effective amount of
> >   * computation done during the running delta time but then sync back to
> > @@ -76,8 +116,7 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
> >  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);
> > +             _update_idle_rq_clock_pelt(rq);
> >               return;
> >       }
> >
> > @@ -130,17 +169,26 @@ 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;
> > +
> > +     _update_idle_rq_clock_pelt(rq);
> >  }
> >
> > -static inline u64 rq_clock_pelt(struct rq *rq)
> > +#ifdef CONFIG_CFS_BANDWIDTH
> > +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
> >  {
> > -     lockdep_assert_rq_held(rq);
> > -     assert_clock_updated(rq);
> > -
> > -     return rq->clock_pelt - rq->lost_idle_time;
> > +     /*
> > +      * Make sure that pending update of rq->clock_pelt_idle and
> > +      * rq->enter_idle are visible during update_blocked_average() before
> > +      * updating cfs_rq->throttled_pelt_idle.
> > +      */
> > +     smp_wmb();
>
> No smp_rmb() at read side and I feel it is in migrate_se_pelt_lag()
> before this line：
>     ‘now = u64_u32_load(rq->clock_pelt_idle);’
> The load of cfs_rq->throttled_pelt_idle should be valid before the load
> of rq->clock_pelt_idle and rq->enter_idle. This is paired with the write
> side in update_idle_cfs_rq_clock_pelt().

Yes

>
> > +     if (unlikely(cfs_rq->throttle_count))
> > +             u64_u32_store(cfs_rq->throttled_pelt_idle, U64_MAX);
> > +     else
> > +             u64_u32_store(cfs_rq->throttled_pelt_idle,
> > +                           cfs_rq->throttled_clock_pelt_time);
> >  }
> >
> > -#ifdef CONFIG_CFS_BANDWIDTH
> >  /* 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 +198,7 @@ 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 +253,7 @@ 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..ea9365e1a24e 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                     throttled_pelt_idle;
> > +#ifndef CONFIG_64BIT
> > +     u64                     throttled_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;
> >
> > --
> > 2.17.1

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