From: Zhang Qiao <zhangqiao22@huawei.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Roman Kagan <rkagan@amazon.de>,
Peter Zijlstra <peterz@infradead.org>,
<linux-kernel@vger.kernel.org>,
Valentin Schneider <vschneid@redhat.com>,
Ben Segall <bsegall@google.com>, Waiman Long <longman@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mel Gorman <mgorman@suse.de>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
Date: Fri, 3 Mar 2023 14:51:44 +0800 [thread overview]
Message-ID: <5b8bb880-de5a-dd99-4168-89d1281e8348@huawei.com> (raw)
In-Reply-To: <CAKfTPtC=Y-ihdiO3ctrdsLmhMNraf6c-Ft2059T=z38kwzdJ=Q@mail.gmail.com>
在 2023/3/2 22:55, Vincent Guittot 写道:
> On Thu, 2 Mar 2023 at 15:29, Zhang Qiao <zhangqiao22@huawei.com> wrote:
>>
>>
>>
>> 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?
>
> 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
>>>>>> Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
>>>>>> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
>>>>>> Sitz: Berlin
>>>>>> Ust-ID: DE 289 237 879
>>>>>>
>>>>>>
>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>
next prev parent reply other threads:[~2023-03-03 6:51 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-09 19:31 [PATCH v3] sched/fair: sanitize vruntime of entity being placed Roman Kagan
2023-02-21 9:38 ` Vincent Guittot
2023-02-21 16:57 ` Roman Kagan
2023-02-21 17:26 ` Vincent Guittot
2023-02-27 8:42 ` Roman Kagan
2023-02-27 14:37 ` Vincent Guittot
2023-02-27 17:00 ` Dietmar Eggemann
2023-02-27 17:15 ` Vincent Guittot
2023-03-02 9:36 ` Zhang Qiao
2023-03-02 13:34 ` Vincent Guittot
2023-03-02 14:29 ` Zhang Qiao
2023-03-02 14:55 ` Vincent Guittot
2023-03-03 6:51 ` Zhang Qiao [this message]
2023-03-03 8:32 ` Vincent Guittot
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