Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

[Vincent Guittot]

On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>
> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
> entity being placed") fix an overflowing bug, but ignore
> a case that se->exec_start is reset after a migration.
>
> For fixing this case, we reset the vruntime of a long
> sleeping task in migrate_task_rq_fair().
>
> Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>
> v1 -> v2:
> - fix some typos and update comments
> - reformat the patch
>
> ---
>  kernel/sched/fair.c | 76 ++++++++++++++++++++++++++++++++-------------
>  1 file changed, 55 insertions(+), 21 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7a1b1f855b96..74c9918ffe76 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4648,11 +4648,45 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  #endif
>  }
>
> +static inline bool entity_is_long_sleep(struct sched_entity *se)
> +{
> +       struct cfs_rq *cfs_rq;
> +       u64 sleep_time;
> +
> +       if (se->exec_start == 0)
> +               return false;
> +
> +       cfs_rq = cfs_rq_of(se);
> +       sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> +       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
> +               return true;
> +
> +       return false;
> +}
> +
> +static inline u64 sched_sleeper_credit(struct sched_entity *se)
> +{
> +       unsigned long thresh;
> +
> +       if (se_is_idle(se))
> +               thresh = sysctl_sched_min_granularity;
> +       else
> +               thresh = sysctl_sched_latency;
> +
> +       /*
> +        * Halve their sleep time's effect, to allow
> +        * for a gentler effect of sleepers:
> +        */
> +       if (sched_feat(GENTLE_FAIR_SLEEPERS))
> +               thresh >>= 1;
> +
> +       return thresh;
> +}
> +
>  static void
>  place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>  {
>         u64 vruntime = cfs_rq->min_vruntime;
> -       u64 sleep_time;
>
>         /*
>          * The 'current' period is already promised to the current tasks,
> @@ -4664,23 +4698,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>                 vruntime += sched_vslice(cfs_rq, se);
>
>         /* sleeps up to a single latency don't count. */
> -       if (!initial) {
> -               unsigned long thresh;
> -
> -               if (se_is_idle(se))
> -                       thresh = sysctl_sched_min_granularity;
> -               else
> -                       thresh = sysctl_sched_latency;
> -
> -               /*
> -                * Halve their sleep time's effect, to allow
> -                * for a gentler effect of sleepers:
> -                */
> -               if (sched_feat(GENTLE_FAIR_SLEEPERS))
> -                       thresh >>= 1;
> -
> -               vruntime -= thresh;
> -       }
> +       if (!initial)
> +               vruntime -= sched_sleeper_credit(se);
>
>         /*
>          * Pull vruntime of the entity being placed to the base level of
> @@ -4689,8 +4708,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>          * the base as it may be too far off and the comparison may get
>          * inversed due to s64 overflow.
>          */
> -       sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> -       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
> +       if (entity_is_long_sleep(se))
>                 se->vruntime = vruntime;
>         else
>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
> @@ -7635,7 +7653,23 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>         if (READ_ONCE(p->__state) == TASK_WAKING) {
>                 struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> -               se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> +               /*
> +                * We determine whether a task sleeps for long by checking
> +                * se->exec_start, and if it is, we sanitize its vruntime at
> +                * place_entity(). However, after a migration, this detection
> +                * method fails due to se->exec_start being reset.
> +                *
> +                * For fixing this case, we add the same check here. For a task
> +                * which has slept for a long time, its vruntime should be reset
> +                * to cfs_rq->min_vruntime with a sleep credit. Because waking
> +                * task's vruntime will be added to cfs_rq->min_vruntime when
> +                * enqueue, we only need to reset the se->vruntime of waking task
> +                * to a credit here.
> +                */
> +               if (entity_is_long_sleep(se))
> +                       se->vruntime = -sched_sleeper_credit(se);
> +               else
> +                       se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
>         }
>
>         if (!task_on_rq_migrating(p)) {
> --
> 2.17.1
>