Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

[Vincent Guittot]

On Thu, 9 Feb 2023 at 20:31, Roman Kagan <rkagan@xxxxxxxxx> wrote:
>
> From: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
>
> When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
> to the base level (around cfs_rq->min_vruntime), so that the entity
> doesn't gain extra boost when placed backwards.
>
> However, if the entity being placed wasn't executed for a long time, its
> vruntime may get too far behind (e.g. while cfs_rq was executing a
> low-weight hog), which can inverse the vruntime comparison due to s64
> overflow.  This results in the entity being placed with its original
> vruntime way forwards, so that it will effectively never get to the cpu.
>
> To prevent that, ignore the vruntime of the entity being placed if it
> didn't execute for longer than the time that can lead to an overflow.
>
> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> [rkagan: formatted, adjusted commit log, comments, cutoff value]
> Co-developed-by: Roman Kagan <rkagan@xxxxxxxxx>
> Signed-off-by: Roman Kagan <rkagan@xxxxxxxxx>

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

> ---
> v2 -> v3:
> - make cutoff less arbitrary and update comments [Vincent]
>
> v1 -> v2:
> - add Zhang Qiao's s-o-b
> - fix constant promotion on 32bit
>
>  kernel/sched/fair.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f8736991427..3baa6b7ea860 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4656,6 +4656,7 @@ 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,
> @@ -4685,8 +4686,24 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>                 vruntime -= thresh;
>         }
>
> -       /* ensure we never gain time by being placed backwards. */
> -       se->vruntime = max_vruntime(se->vruntime, vruntime);
> +       /*
> +        * Pull vruntime of the entity being placed to the base level of
> +        * cfs_rq, to prevent boosting it if placed backwards.
> +        * However, min_vruntime can advance much faster than real time, with
> +        * the exterme being when an entity with the minimal weight always runs
> +        * on the cfs_rq.  If the new entity slept for long, its vruntime
> +        * difference from min_vruntime may overflow s64 and their comparison
> +        * may get inversed, so ignore the entity's original vruntime in that
> +        * case.
> +        * The maximal vruntime speedup is given by the ratio of normal to
> +        * minimal weight: NICE_0_LOAD / MIN_SHARES, so cutting off on the
> +        * sleep time of 2^63 / NICE_0_LOAD should be safe.
> +        */
> +       sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> +       if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD)
> +               se->vruntime = vruntime;
> +       else
> +               se->vruntime = max_vruntime(se->vruntime, vruntime);
>  }
>
>  static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
> --
> 2.34.1
>
>
>
>
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