linux-kernel - Re: [RFC PATCH] sched/fair: Introduce SIS_PAIR to wakeup task on local idle core first

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Date:   Fri, 19 May 2023 13:15:20 +0200
From:   Mike Galbraith <efault@....de>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tim Chen <tim.c.chen@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Abel Wu <wuyun.abel@...edance.com>,
        Yicong Yang <yangyicong@...ilicon.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Len Brown <len.brown@...el.com>,
        Chen Yu <yu.chen.surf@...il.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Aaron Lu <aaron.lu@...el.com>, Barry Song <baohua@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Introduce SIS_PAIR to wakeup task on
 local idle core first

On Thu, 2023-05-18 at 11:41 +0800, Chen Yu wrote:
> On 2023-05-17 at 21:52:21 +0200, Mike Galbraith wrote:
> >
> > That said, I don't like the waker/wakee have met heuristic much either,
> > because tasks waking one another before can just as well mean they met
> > at a sleeping lock, it does not necessarily imply latency bound IPC.
> >
> Yes, for a sleeping lock case, it does not matter whether it is woken up
> on sibling idle, or an idle CPU on another half-busy core. But for the
> pair sharing data, it could bring benefit.

That reply keeps bouncing about in my head, annoying me enough that I'm
going to reply to it so I can finally stop thinking about pipe ping-
pong and the risks of big socket only issue mitigation patches.

The object that inspired SIS_CURRENT, which then morphed into SIS_PAIR
is in effect a mutex.  The numbers derived from operation of that mutex
are not really relevant to IPC or context switches for that matter
(says me;), they're all about memory access cost deltas.

	-Mike