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    SubjectRe: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
    On Thu, 2 Mar 2023 at 10:36, Zhang Qiao <zhangqiao22@huawei.com> wrote:
    >
    >
    >
    > 在 2023/2/27 22:37, Vincent Guittot 写道:
    > > On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@amazon.de> wrote:
    > >>
    > >> On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote:
    > >>> On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@amazon.de> wrote:
    > >>>> What scares me, though, is that I've got a message from the test robot
    > >>>> that this commit drammatically affected hackbench results, see the quote
    > >>>> below. I expected the commit not to affect any benchmarks.
    > >>>>
    > >>>> Any idea what could have caused this change?
    > >>>
    > >>> Hmm, It's most probably because se->exec_start is reset after a
    > >>> migration and the condition becomes true for newly migrated task
    > >>> whereas its vruntime should be after min_vruntime.
    > >>>
    > >>> We have missed this condition
    > >>
    > >> Makes sense to me.
    > >>
    > >> But what would then be the reliable way to detect a sched_entity which
    > >> has slept for long and risks overflowing in .vruntime comparison?
    > >
    > > For now I don't have a better idea than adding the same check in
    > > migrate_task_rq_fair()
    >
    > Hi, Vincent,
    > I fixed this condition as you said, and the test results are as follows.
    >
    > testcase: hackbench -g 44 -f 20 --process --pipe -l 60000 -s 100
    > version1: v6.2
    > version2: v6.2 + commit 829c1651e9c4
    > version3: v6.2 + commit 829c1651e9c4 + this patch
    >
    > -------------------------------------------------
    > version1 version2 version3
    > test1 81.0 118.1 82.1
    > test2 82.1 116.9 80.3
    > test3 83.2 103.9 83.3
    > avg(s) 82.1 113.0 81.9
    >
    > -------------------------------------------------
    > After deal with the task migration case, the hackbench result has restored.
    >
    > The patch as follow, how does this look?
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index ff4dbbae3b10..3a88d20fd29e 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -4648,6 +4648,26 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
    > #endif
    > }
    >
    > +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)
    > {
    > @@ -4664,23 +4684,8 @@ 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
    > @@ -4690,7 +4695,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
    > * 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 (se->exec_start != 0 && (s64)sleep_time > 60LL * NSEC_PER_SEC)
    > se->vruntime = vruntime;
    > else
    > se->vruntime = max_vruntime(se->vruntime, vruntime);
    > @@ -7634,8 +7639,12 @@ 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);
    > + u64 sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
    >
    > - se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
    > + if ((s64)sleep_time > 60LL * NSEC_PER_SEC)

    You also need to test (se->exec_start !=0) here because the task might
    migrate another time before being scheduled. You should create a
    helper function like below and use it in both place

    static inline bool entity_long_sleep(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;
    }


    > + se->vruntime = -sched_sleeper_credit(se);
    > + else
    > + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
    > }
    >
    > if (!task_on_rq_migrating(p)) {
    >
    >
    >
    > Thanks.
    > Zhang Qiao.
    >
    > >
    > >>
    > >> Thanks,
    > >> Roman.
    > >>
    > >>
    > >>
    > >> Amazon Development Center Germany GmbH
    > >> Krausenstr. 38
    > >> 10117 Berlin
    > >> Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
    > >> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
    > >> Sitz: Berlin
    > >> Ust-ID: DE 289 237 879
    > >>
    > >>
    > >>
    > > .
    > >

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