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

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Date:   Wed, 3 Aug 2022 11:18:27 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Libo Chen <libo.chen@...cle.com>
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


* 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?

Thanks,

	Ingo