Date:   Tue, 14 Mar 2023 14:39:49 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Zhang Qiao <zhangqiao22@...wei.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, rkagan@...zon.de,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On Tue, 14 Mar 2023 at 14:38, Zhang Qiao <zhangqiao22@...wei.com> wrote:
>
>
>
> 在 2023/3/14 21:26, Vincent Guittot 写道:
> > On Tue, 14 Mar 2023 at 12:03, Zhang Qiao <zhangqiao22@...wei.com> wrote:
> >>
> >>
> >>
> >> 在 2023/3/13 22:23, Vincent Guittot 写道:
> >>> On Sat, 11 Mar 2023 at 10:57, Zhang Qiao <zhangqiao22@...wei.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> 在 2023/3/10 22:29, Vincent Guittot 写道:
> >>>>> Le jeudi 09 mars 2023 à 16:14:38 (+0100), Vincent Guittot a écrit :
> >>>>>> On Thu, 9 Mar 2023 at 15:37, Peter Zijlstra <peterz@...radead.org> wrote:
> >>>>>>>
> >>>>>>> On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote:
> >>>>>>>> On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote:
> >>>>>>>>
> >>>>>>>>> Then, even if we don't clear exec_start before migrating and keep
> >>>>>>>>> current value to be used in place_entity on the new cpu, we can't
> >>>>>>>>> compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT
> >>>>>>>>
> >>>>>>>> Blergh -- indeed, irq and steal time can skew them between CPUs :/
> >>>>>>>> I suppose we can fudge that... wait_start (which is basically what we're
> >>>>>>>> making it do) also does that IIRC.
> >>>>>>>>
> >>>>>>>> I really dislike having this placement muck spreadout like proposed.
> >>>>>>>
> >>>>>>> Also, I think we might be over-engineering this, we don't care about
> >>>>>>> accuracy at all, all we really care about is 'long-time'.
> >>>>>>
> >>>>>> you mean taking the patch 1/2 that you mentioned here to add a
> >>>>>> migrated field:
> >>>>>> https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com/T/#ma5637eb8010f3f4a4abff778af8db705429d003b
> >>>>>>
> >>>>>> And assume that the divergence between the rq_clock_task() can be ignored ?
> >>>>>>
> >>>>>> That could probably work but we need to replace the (60LL *
> >>>>>> NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence
> >>>>>> would not be unrealistic.
> >>>>>> and a comment to explain why it's acceptable
> >>>>>
> >>>>> Zhang,
> >>>>>
> >>>>> Could you try the patch below ?
> >>>>> This is a rebase/merge/update of:
> >>>>> -patch 1/2 above and
> >>>>> -https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/
> >>>>
> >>>>
> >>>> I applyed and tested this patch, and it make hackbench slower.
> >>>> According to my previous test results. The good result is 82.1(s).
> >>>> But the result of this patch is 108.725(s).
> >>>
> >>> By "the result of this patch is 108.725(s)",  you mean the result of
> >>> https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/
> >>> alone, don't you ?
> >>
> >> No, with your patch, the test results is 108.725(s),
> >
> > Ok
> >
> >>
> >> git diff:
> >>
> >> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >> index 63d242164b1a..93a3909ae4c4 100644
> >> --- a/include/linux/sched.h
> >> +++ b/include/linux/sched.h
> >> @@ -550,6 +550,7 @@ struct sched_entity {
> >>         struct rb_node                  run_node;
> >>         struct list_head                group_node;
> >>         unsigned int                    on_rq;
> >> +       unsigned int                    migrated;
> >>
> >>         u64                             exec_start;
> >>         u64                             sum_exec_runtime;
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index ff4dbbae3b10..e60defc39f6e 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >>         /*
> >>          * We are starting a new run period:
> >>          */
> >> +       se->migrated = 0;
> >>         se->exec_start = rq_clock_task(rq_of(cfs_rq));
> >>  }
> >>
> >> @@ -4690,9 +4691,9 @@ 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 ((s64)sleep_time > (1ULL << 63) / scale_load_down(NICE_0_LOAD) / 2) {
> >>                 se->vruntime = vruntime;
> >> -       else
> >> +       } else
> >>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
> >>  }
> >>
> >> @@ -7658,8 +7659,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> >>         se->avg.last_update_time = 0;
> >>
> >>         /* We have migrated, no longer consider this task hot */
> >> -       se->exec_start = 0;
> >> -
> >> +       se->migrated = 1;
> >>         update_scan_period(p, new_cpu);
> >>  }
> >>
> >> @@ -8343,6 +8343,8 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
> >>
> >>         if (sysctl_sched_migration_cost == 0)
> >>                 return 0;
> >> +       if (p->se.migrated)
> >> +               return 0;
> >>
> >>         delta = rq_clock_task(env->src_rq) - p->se.exec_start;
> >>
> >>
> >>
> >>>
> >>>>
> >>>>
> >>>>> 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
> >>>
> >>> Ok, it looks like we are back to normal figures
> >
> > What do those results refer to then ?
>
> Quote from this email (https://lore.kernel.org/lkml/1cd19d3f-18c4-92f9-257a-378cc18cfbc7@huawei.com/).

ok.

Then, there is something wrong in my patch. Let me look at it more deeply

>
> >
> >
> >>>
> >>>>>
> >>>>> -------------------------------------------------
> >>>>>
> >>>>> The proposal accepts a divergence of up to 52 days between the 2 rqs.
> >>>>>
> >>>>> If this work, we will prepare a proper patch
> >>>>>
> >>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >>>>> index 63d242164b1a..cb8af0a137f7 100644
> >>>>> --- a/include/linux/sched.h
> >>>>> +++ b/include/linux/sched.h
> >>>>> @@ -550,6 +550,7 @@ struct sched_entity {
> >>>>>         struct rb_node                  run_node;
> >>>>>         struct list_head                group_node;
> >>>>>         unsigned int                    on_rq;
> >>>>> +       unsigned int                    migrated;
> >>>>>
> >>>>>         u64                             exec_start;
> >>>>>         u64                             sum_exec_runtime;
> >>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>>> index 7a1b1f855b96..36acd9598b40 100644
> >>>>> --- a/kernel/sched/fair.c
> >>>>> +++ b/kernel/sched/fair.c
> >>>>> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >>>>>         /*
> >>>>>          * We are starting a new run period:
> >>>>>          */
> >>>>> +       se->migrated = 0;
> >>>>>         se->exec_start = rq_clock_task(rq_of(cfs_rq));
> >>>>>  }
> >>>>>
> >>>>> @@ -4684,13 +4685,23 @@ 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.
> >>>>> +        * 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
> >>>>
> >>>> why not is `scale_load_down(NICE_0_LOAD) / MIN_SHARES` here ?
> >>>
> >>> yes, you are right.
> >>>
> >>>>
> >>>>
> >>>>> +        * sleep time of 2^63 / NICE_0_LOAD should be safe.
> >>>>> +        * 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
> >>>>
> >>>> This divergence might be larger, it cause `sleep_time` maybe negative.
> >>>
> >>> AFAICT, we are safe with ((1ULL << 63) / scale_load_down(NICE_0_LOAD)
> >>> / 2) as long as the divergence between the 2 rqs clocks task is lower
> >>> than 2^52nsec. Do you expect a divergence higher than 2^52 nsec
> >>> (around 52 days)?
> >>>
> >>> We can probably keep using (1ULL << 63) / scale_load_down(NICE_0_LOAD)
> >>> which is already half the max value if needed.
> >>>
> >>> the fact that sleep_time can be negative is not a problem as
> >>> s64)sleep_time > will take care of this.
> >>
> >> In my opinion, when comparing signed with unsigned, the compiler converts the signed value to unsigned.
> >> So, if sleep_time < 0, "(s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2" will be true.
> >>
> >>>
> >>>>
> >>>>> +        * stolen time, we take an additional margin.
> >>>>>          */
> >>>>>         sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> >>>>> -       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
> >>>>> +       if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2)>                 se->vruntime = vruntime;
> >>>>>         else
> >>>>>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
> >>>>> @@ -7658,7 +7669,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> >>>>>         se->avg.last_update_time = 0;
> >>>>>
> >>>>>         /* We have migrated, no longer consider this task hot */
> >>>>> -       se->exec_start = 0;
> >>>>> +       se->migrated = 1;
> >>>>>
> >>>>>         update_scan_period(p, new_cpu);
> >>>>>  }
> >>>>> @@ -8344,6 +8355,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
> >>>>>         if (sysctl_sched_migration_cost == 0)
> >>>>>                 return 0;
> >>>>>
> >>>>> +       if (p->se.migrated)
> >>>>> +               return 0;
> >>>>> +
> >>>>>         delta = rq_clock_task(env->src_rq) - p->se.exec_start;
> >>>>>
> >>>>>         return delta < (s64)sysctl_sched_migration_cost;
> >>>>>
> >>>>>
> >>>>>
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>>
> >>>>> .
> >>>>>
> >>> .
> >>>
> > .
> >

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