linux-kernel - Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups

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Date:   Wed, 15 Dec 2021 19:25:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...k.io>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, mgorman@...e.de,
        bristot@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-api@...r.kernel.org, x86@...nel.org,
        Paul Turner <pjt@...gle.com>, Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups

On Wed, Dec 15, 2021 at 09:56:06AM -0800, Peter Oskolkov wrote:
> On Wed, Dec 15, 2021 at 2:06 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >  /*
> > + * Enqueue tsk to it's server's runnable list and wake the server for pickup if
> > + * so desired. Notable LAZY workers will not wake the server and rely on the
> > + * server to do pickup whenever it naturally runs next.
> 
> No, I never suggested we needed per-server runnable queues: in all my
> patchsets I had a single list of idle (runnable) workers.

This is not about the idle servers..

So without the LAZY thing on, a previously blocked task hitting sys_exit
will enqueue itself on the runnable list and wake the server for pickup.

IIRC you didn't like the server waking while it was still running
another task, but instead preferred to have it pick up the newly
enqueued task when next it ran.

LAZY enables that.. *however* it does need to wake the server when it is
idle, otherwise they'll all sit there waiting for one another.