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

[Zhang Qiao]

在 2023/3/2 22:55, Vincent Guittot 写道:
> On Thu, 2 Mar 2023 at 15:29, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>
>>
>>
>> 在 2023/3/2 21:34, Vincent Guittot 写道:
>>> On Thu, 2 Mar 2023 at 10:36, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> 在 2023/2/27 22:37, Vincent Guittot 写道:
>>>>> On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@xxxxxxxxx> 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@xxxxxxxxx> 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?
> 
> se->exec_start is update when the task becomes current.
> 
> You can have the sequence:
> 
> task TA runs on CPU0
>     TA's se->exec_start = xxxx
> TA is put back into the rb tree waiting for next slice while another
> task is running
> CPU1 pulls TA which migrates on CPU1
>     migrate_task_rq_fair() w/ TA's se->exec_start == xxxx
>         TA's se->exec_start = 0
> TA is put into the rb tree of CPU1 waiting to run on CPU1
> CPU2 pulls TA which migrates on CPU2
>     migrate_task_rq_fair() w/ TA's se->exec_start == 0
>         TA's se->exec_start = 0
Hi, Vincent,

yes, you're right, such sequence does exist. But at this point, p->__state != TASK_WAKING.

I have a question, Whether there is case that is "p->se.exec_start == 0 && p->__state == TASK_WAKING" ?
I analyzed the code and concluded that this case isn't existed, is it right?

Thanks.
ZhangQiao.

> 
>>
>> 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?
> 
> the vruntime have been updated when enqueued but not exec_start
> 
>>
>>> 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.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Amazon Development Center Germany GmbH
>>>>>> Krausenstr. 38
>>>>>> 10117 Berlin
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>>>>>> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
>>>>>> Sitz: Berlin
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>>>>>>
>>>>>>
>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>