lkml.org 
[lkml]   [2022]   [Jun]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle
From
On 2022/6/1 18:58, Valentin Schneider wrote:
> On 01/06/22 13:54, Tianchen Ding wrote:
>> On 2022/5/31 23:56, Valentin Schneider wrote:
>>
>>> Thanks!
>>>
>>> So I'm thinking we could first make that into
>>>
>>> if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
>>>
>>> Then building on this, we can generalize using the wakelist to any remote
>>> idle CPU (which on paper isn't as much as a clear win as just WF_ON_CPU,
>>> depending on how deeply idle the CPU is...)
>>>
>>> We need the cpu != this_cpu check, as that's currently served by the
>>> WF_ON_CPU check (AFAIU we can only observe p->on_cpu in there for remote
>>> tasks).
>>>
>>> ---
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index 66c4e5922fe1..60038743f2f1 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -3830,13 +3830,20 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
>>> if (!cpus_share_cache(smp_processor_id(), cpu))
>>> return true;
>>>
>>> + if (cpu == smp_processor_id())
>>> + return false;
>>> +
>>> /*
>>> * If the task is descheduling and the only running task on the
>>> * CPU then use the wakelist to offload the task activation to
>>> * the soon-to-be-idle CPU as the current CPU is likely busy.
>>> * nr_running is checked to avoid unnecessary task stacking.
>>> + *
>>> + * Note that we can only get here with (wakee) p->on_rq=0,
>>> + * p->on_cpu can be whatever, we've done the dequeue, so
>>> + * the wakee has been accounted out of ->nr_running
>>> */
>>> - if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <= 1)
>>> + if (!cpu_rq(cpu)->nr_running)
>>> return true;
>>>
>>> return false;
>>
>> Hi Valentin. I've done a simple unixbench test (Pipe-based Context
>> Switching) on my x86 machine with full threads (104).
>>
>> old patch1 patch1+patch2
>> score 7825.4 7500(more)-8000 9061.6
>>
>> patch1: use !cpu_rq(cpu)->nr_running instead of cpu_rq(cpu)->nr_running <= 1
>> patch2: ignore WF_ON_CPU check
>>
>> The score of patch1 is not stable. I've tested for many times and the
>> score is floating between about 7500-8000 (more at 7500).
>>
>> patch1 means more strict limit on using wakelist. But it may cause
>> performance regression.
>>
>> It seems that, using wakelist properly can help improve wakeup
>> performance, but using it too much may cause more IPIs. It's a trade-off
>> about how strict the ttwu_queue_cond() is.
>>
>> Anyhow, I think patch2 should be a pure improvement. What's your idea?
>
> Thanks for separately testing these two.
>
> I take it the results for patch1 are noticeably more swingy than the
> baseline? (FWIW boxplots are usually a nice way to summarize those sort of
> results).
>

Hmm... I'm not familiar with this...
T want to say that I'm not sure about the performance impact about
patch1. While from the view of logic, patch1 should be correct.

> WF_ON_CPU && nr_running == 1 means the wakee is scheduling out *and* there
> is another task queued, I'm guessing that's relatively common in your
> unixbench scenario...
>
> Either way, I think we want to keep the two changes separate for the sake
> of testing and bisecting.

Yes. I'll split the patch to 2 parts. One for logic fix and another for
performance improvement.

\
 
 \ /
  Last update: 2022-06-01 14:03    [W:0.063 / U:0.496 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site