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

Libo Chen
2022-08-01T15:01:58Z

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.

Libo
> I've reverted this commit (14b3f2d9ee8df3b) for the time being from
> tip:sched/core.
>
> Thanks for the heads-up!
>
> Thanks,
>
> Ingo