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Message-ID: <f0f7bce7-4d98-c5d0-5e75-55f27a9a0d69@huawei.com>
Date: Sat, 18 Mar 2023 15:45:25 +0800
From: Zhang Qiao <zhangqiao22@...wei.com>
To: Vincent Guittot <vincent.guittot@...aro.org>,
<linux-kernel@...r.kernel.org>
CC: <mingo@...hat.com>, <peterz@...radead.org>,
<juri.lelli@...hat.com>, <dietmar.eggemann@....com>,
<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
<bristot@...hat.com>, <vschneid@...hat.com>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
在 2023/3/18 0:08, Vincent Guittot 写道:
> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> fixes an overflowing bug, but ignore a case that se->exec_start is reset
> after a migration.
>
> For fixing this case, we delay the reset of se->exec_start after
> placing the entity which se->exec_start to detect long sleeping task.
>
> In order to take into account a possible divergence between the clock_task
> of 2 rqs, we increase the threshold to around 104 days.
>
>
> Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>
> My last proposal was not yet correct as the exec_start was not always
> reset after migrating a task. I finally found this solution which keeps
> the long sleep detection to one place as well as the reset of se->exec_start.
>
Tested-by: Zhang Qiao <zhangqiao22@...wei.com>
I have retested it with this version, and the result is fine.
-------
Performance counter stats for 'hackbench -g 44 -f 20 --process --pipe -l 60000 -s 100' (10 runs):
80.10 +- 1.22 seconds time elapsed ( +- 1.53% )
Thanks,
Zhang Qiao.
> kernel/sched/fair.c | 57 +++++++++++++++++++++++++++++++++++----------
> 1 file changed, 45 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f499e9a74b5..421173fde351 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4648,11 +4648,33 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
> #endif
> }
>
> +static inline bool entity_is_long_sleeper(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));
> +
> + /* Happen while migrating because of clock task divergence */
> + if (sleep_time <= se->exec_start)
> + return false;
> +
> + sleep_time -= se->exec_start;
> + if (sleep_time > ((1ULL << 63) / scale_load_down(NICE_0_LOAD)))
> + return true;
> +
> + return false;
> +}
> +
> 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,
> @@ -4684,13 +4706,24 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>
> /*
> * Pull vruntime of the entity being placed to the base level of
> - * cfs_rq, to prevent boosting it if placed backwards. If the entity
> - * slept for a long time, don't even try to compare its vruntime with
> - * 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)
> + * cfs_rq, to prevent boosting it if placed backwards.
> + * However, min_vruntime can advance much faster than real time, with
> + * the extreme being when an entity with the minimal weight always runs
> + * on the cfs_rq. If the waking entity slept for a long time, 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: scale_load_down(NICE_0_LOAD) / MIN_SHARES.
> + * When placing a migrated waking entity, its exec_start has been set
> + * from a different rq. In order to take into account a possible
> + * divergence between new and prev rq's clocks task because of irq and
> + * stolen time, we take an additional margin.
> + * So, cutting off on the sleep time of
> + * 2^63 / scale_load_down(NICE_0_LOAD) ~ 104 days
> + * should be safe.
> + */
> + if (entity_is_long_sleeper(se))
> se->vruntime = vruntime;
> else
> se->vruntime = max_vruntime(se->vruntime, vruntime);
> @@ -4770,6 +4803,9 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>
> if (flags & ENQUEUE_WAKEUP)
> place_entity(cfs_rq, se, 0);
> + /* Entity has migrated, no longer consider this task hot */
> + if (flags & ENQUEUE_MIGRATED)
> + se->exec_start = 0;
>
> check_schedstat_required();
> update_stats_enqueue_fair(cfs_rq, se, flags);
> @@ -7665,9 +7701,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> /* Tell new CPU we are migrated */
> se->avg.last_update_time = 0;
>
> - /* We have migrated, no longer consider this task hot */
> - se->exec_start = 0;
> -
> update_scan_period(p, new_cpu);
> }
>
> @@ -8701,7 +8734,7 @@ static void attach_task(struct rq *rq, struct task_struct *p)
> lockdep_assert_rq_held(rq);
>
> WARN_ON_ONCE(task_rq(p) != rq);
> - activate_task(rq, p, ENQUEUE_NOCLOCK);
> + activate_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_MIGRATED);
> check_preempt_curr(rq, p, 0);
> }
>
>
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