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

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Date:   Wed, 3 Aug 2022 12:37:55 -0700
From:   Libo Chen <libo.chen@...cle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     K Prateek Nayak <kprateek.nayak@....com>, peterz@...radead.org,
        vincent.guittot@...aro.org, mgorman@...e.de,
        tim.c.chen@...ux.intel.com, 21cnbao@...il.com,
        dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context



On 8/3/22 02:18, Ingo Molnar wrote:
> * Libo Chen <libo.chen@...cle.com> wrote:
>
>>
>> On 8/1/22 06:26, Ingo Molnar wrote:
>>> * K Prateek Nayak <kprateek.nayak@....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@...e.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