Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
From: Zhang Qiao
Date: Thu Mar 02 2023 - 09:29:40 EST
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?
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.
>>>>
>>>>
>>>>
>>>> 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
>>>>
>>>>
>>>>
>>> .
>>>
> .
>