Re: [PATCH] sched/fair: no sync wakeup from interrupt context

Libo Chen
2022-08-03T19:38:34Z

On 8/3/22 02:18, Ingo Molnar wrote:
> * Libo Chen <libo.chen@oracle.com> wrote:
>
>>
>> On 8/1/22 06:26, Ingo Molnar wrote:
>>> * K Prateek Nayak <kprateek.nayak@amd.com> wrote:
>>>
>>>> Hello Libo and Peter,
>>>>
>>>> tl;dr
>>>>
>>>> - We observed a major regression with tbench when testing the latest tip
>>>> sched/core at:
>>>> commit 14b3f2d9ee8d "sched/fair: Disallow sync wakeup from interrupt context"
>>>> Reason for the regression are the fewer affine wakeups that leaves the
>>>> client farther away from the data it needs to consume next primed in the
>>>> waker's LLC.
>>>> Such regressions can be expected from tasks that use sockets to communicate
>>>> significant amount of data especially on system with multiple LLCs.
>>>>
>>>> - Other benchmarks have a comparable behavior to the tip at previous commit
>>>> commit : 91caa5ae2424 "sched/core: Fix the bug that task won't enqueue
>>>> into core tree when update cookie"
>>>>
>>>> I'll leave more details below.
>>> Mel Gorman also warned about this negative side-effect in:
>>>
>>> Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context
>>> Date: Fri, 15 Jul 2022 11:07:38 +0100
>>> Message-ID: <20220715100738.GD3493@suse.de>
>>>
>>> https://urldefense.com/v3/__https://lore.kernel.org/all/20220715100738.GD3493@suse.de/__;!!ACWV5N9M2RV99hQ!PQsIeuK0UwII-A0xS-B3plepNniNeyw14OJowT1cYL-tnuN99MkWfg9C8P60tVFFrnxj0NEanUmEkA$
>> ?? Mel was talking about a completely different thing, I brought up a
>> different patch that I wanted to revert and Mel thought it would hurt other
>> workloads which don't benefit from pulling but
>> as you can see, tbench somehow benefits from it, at least according to one
>> metric from one workload.
> Yeah - but nevertheless the discussion with Mel was open-ended AFAICS, and
> the 'major tbench regression' report by K Prateek Nayak above still stands
> and needs to be investigated/understood, right?
Oh yes, I have no issue with holding the patch back until the regression
is fully understood. I was just a little confused on your reference to
Mel's comments. Anyway, I will post my investigation soon.

Libo
> Thanks,
>
> Ingo