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

[Zhang Qiao <zhangqiao22@huawei.com>]

在 2023/3/2 21:34, Vincent Guittot 写道:
> 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

Hi,

I don't understand when the another migration happend. Could you tell me in more detail?

I think the next migration will happend after the wakee task enqueued, but at this time
the p->__state isn't TASK_WAKING, p->__state already be changed to TASK_RUNNING at ttwu_do_wakeup().

If such a migration exists, Previous code "se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);" maybe
perform multiple times，wouldn't it go wrong in this way?

> migrate another time before being scheduled. You should create a
> helper function like below and use it in both place

Ok, I will update at next version.


Thanks,
ZhangQiao.

>
> 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.
>>>>
>>>>
>>>>
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>>>>
>>>>
>>> .
>>>
> .
>