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Date: Thu, 13 Jun 2024 20:02:37 +0800
From: Chunxin Zang <spring.cxz@...il.com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Honglei Wang <jameshongleiwang@....com>,
mingo@...hat.com,
Peter Zijlstra <peterz@...radead.org>,
juri.lelli@...hat.com,
vincent.guittot@...aro.org,
dietmar.eggemann@....com,
rostedt@...dmis.org,
bsegall@...gle.com,
mgorman@...e.de,
bristot@...hat.com,
vschneid@...hat.com,
Mike Galbraith <efault@....de>,
K Prateek Nayak <kprateek.nayak@....com>,
linux-kernel@...r.kernel.org,
yangchen11@...iang.com,
Jerry Zhou <zhouchunhua@...iang.com>,
Chunxin Zang <zangchunxin@...iang.com>
Subject: Re: [PATCH v2] sched/fair: Reschedule the cfs_rq when current is
ineligible
> On Jun 13, 2024, at 19:54, Chen Yu <yu.c.chen@...el.com> wrote:
>
> On 2024-06-12 at 18:39:11 +0800, Chunxin Zang wrote:
>>
>>
>>> On Jun 7, 2024, at 13:07, Chen Yu <yu.c.chen@...el.com> wrote:
>>>
>>> On 2024-05-29 at 22:18:06 +0800, Chunxin Zang wrote:
>>>> I found that some tasks have been running for a long enough time and
>>>> have become illegal, but they are still not releasing the CPU. This
>>>> will increase the scheduling delay of other processes. Therefore, I
>>>> tried checking the current process in wakeup_preempt and entity_tick,
>>>> and if it is illegal, reschedule that cfs queue.
>>>>
>>>> When RUN_TO_PARITY is enabled, its behavior essentially remains
>>>> consistent with the original process. When NO_RUN_TO_PARITY is enabled,
>>>> some additional preemptions will be introduced, but not too many.
>>>>
>>>> I have pasted some test results below.
>>>> I isolated four cores for testing and ran hackbench in the background,
>>>> and observed the test results of cyclictest.
>>>>
>>>> hackbench -g 4 -l 100000000 &
>>>> cyclictest --mlockall -D 5m -q
>>>>
>>>> EEVDF PATCH EEVDF-NO_PARITY PATCH-NO_PARITY
>>>>
>>>> # Min Latencies: 00006 00006 00006 00006
>>>> LNICE(-19) # Avg Latencies: 00191 00133 00089 00066
>>>> # Max Latencies: 15442 08466 14133 07713
>>>>
>>>> # Min Latencies: 00006 00010 00006 00006
>>>> LNICE(0) # Avg Latencies: 00466 00326 00289 00257
>>>> # Max Latencies: 38917 13945 32665 17710
>>>>
>>>> # Min Latencies: 00019 00053 00010 00013
>>>> LNICE(19) # Avg Latencies: 37151 25852 18293 23035
>>>> # Max Latencies: 2688299 4643635 426196 425708
>>>>
>>>> I captured and compared the number of preempt occurrences in wakeup_preempt
>>>> to see if it introduced any additional overhead.
>>>>
>>>> Similarly, hackbench is used to stress the utilization of four cores to
>>>> 100%, and the method for capturing the number of PREEMPT occurrences is
>>>> referenced from [1].
>>>>
>>>> schedstats EEVDF PATCH EEVDF-NO_PARITY PATCH-NO_PARITY CFS(6.5)
>>>> .stats.check_preempt_count 5053054 5045388 5018589 5029585
>>>> .stats.patch_preempt_count ------- 0020495 ------- 0700670 -------
>>>> .stats.need_preempt_count 0570520 0458947 3380513 3116966 1140821
>>>>
>>>> From the above test results, there is a slight increase in the number of
>>>> preempt occurrences in wakeup_preempt. However, the results vary with each
>>>> test, and sometimes the difference is not that significant.
>>>>
>>>> [1]: https://lore.kernel.org/all/20230816134059.GC982867@hirez.programming.kicks-ass.net/T/#m52057282ceb6203318be1ce9f835363de3bef5cb
>>>>
>>>> Signed-off-by: Chunxin Zang <zangchunxin@...iang.com>
>>>> Reviewed-by: Chen Yang <yangchen11@...iang.com>
>>>>
>>>> ------
>>>> Changes in v2:
>>>> - Make the logic that determines the current process as ineligible and
>>>> triggers preemption effective only when NO_RUN_TO_PARITY is enabled.
>>>> - Update the commit message
>>>> ---
>>>> kernel/sched/fair.c | 17 +++++++++++++++++
>>>> 1 file changed, 17 insertions(+)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 03be0d1330a6..fa2c512139e5 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -745,6 +745,17 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>> return vruntime_eligible(cfs_rq, se->vruntime);
>>>> }
>>>>
>>>> +static bool check_entity_need_preempt(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>> +{
>>>> + if (sched_feat(RUN_TO_PARITY) && se->vlag != se->deadline)
>>>> + return true;
>>>
>>> If I understand correctly, here it intends to check if the current se
>>> has consumed its 1st slice after been picked at set_next_entity(), and if yes do a reschedule.
>>> check_entity_need_preempt() is added at the end of entity_tick(), which could overwrite
>>> the police to reschedule current: (entity_tick()->update_curr()->update_deadline()), only there
>>> are more than 1 runnable tasks will the current be preempted, even if it has expired the 1st
>>> requested slice.
>>>
>>
>> The purpose of the modification is to increase preemption opportunities without breaking the
>> RUN_TO_PARITY rule. However, it clearly introduces some additional preemptions, or perhaps
>> there should be a check for the eligibility of the se. Also, to avoid overwriting the scheduling
>> strategy in entity_tick, would a modification like the following be more appropriate?
>>
>
> I wonder if we can only take care of the NO_RUN_TO_PARITY case? Something like this,
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 03be0d1330a6..5e49a15bbdd3 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -745,6 +745,21 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
>> return vruntime_eligible(cfs_rq, se->vruntime);
>> }
>>
>> +static bool check_entity_need_preempt(struct cfs_rq *cfs_rq, struct sched_entity *se)
>> +{
> if (sched_feat(RUN_TO_PARITY) || cfs_rq->nr_running <= 1 ||
> !entity_eligible(cfs_rq, se))
> return false;
>
> return true;
>
> Thoughts?
>
This does indeed look better. In that case, do I need to make the changes this way and send
out a version 3?
thanks
Chunxin
> thanks,
> Chenyu
>
>> + if (cfs_rq->nr_running <= 1)
>> + return false;
>> +
>> + if (sched_feat(RUN_TO_PARITY) && se->vlag != se->deadline
>> + && !entity_eligible(cfs_rq, se))
>> + return true;
>> +
>> + if (!sched_feat(RUN_TO_PARITY) && !entity_eligible(cfs_rq, se))
>> + return true;
>> +
>> + return false;
>> +}
>> +
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