Re: [PATCH v2] sched/fair: Reschedule the cfs_rq when current is ineligible

[Chunxin Zang]

> On Jun 13, 2024, at 19:54, Chen Yu <yu.c.chen@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m52057282ceb6203318be1ce9f835363de3bef5cb
>>>> 
>>>> Signed-off-by: Chunxin Zang <zangchunxin@xxxxxxxxxxx>
>>>> Reviewed-by: Chen Yang <yangchen11@xxxxxxxxxxx>
>>>> 
>>>> ------
>>>> 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;
>> +}
>> +