linux-kernel - Re: [PATCH -tip 04/32] sched: Core-wide rq->lock

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Message-ID: <20201126003543.GG163610@balbir-desktop>
Date:   Thu, 26 Nov 2020 11:35:43 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
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        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 04/32] sched: Core-wide rq->lock

On Tue, Nov 24, 2020 at 09:16:17AM +0100, Peter Zijlstra wrote:
> On Sun, Nov 22, 2020 at 08:11:52PM +1100, Balbir Singh wrote:
> > On Tue, Nov 17, 2020 at 06:19:34PM -0500, Joel Fernandes (Google) wrote:
> > > From: Peter Zijlstra <peterz@...radead.org>
> > > 
> > > Introduce the basic infrastructure to have a core wide rq->lock.
> > >
> > 
> > Reading through the patch, it seems like all the CPUs have to be
> > running with sched core enabled/disabled? Is it possible to have some
> > cores with core sched disabled?
> 
> Yep, patch even says so:
> 
>  + * XXX entirely possible to selectively enable cores, don't bother for now.

Yes, it does in the comments, I looked at just the changelog :)

> 
> > I don't see a strong use case for it,
> > but I am wondering if the design will fall apart if that assumption is
> > broken?
> 
> The use-case I have is not using stop-machine. That is, stopping a whole
> core at a time, instead of the whole sodding machine. It's on the todo
> list *somewhere*....
> 
>

Good to know, I guess that would need a transition of the entire core to
idle and maintenance of a mask that prevents tasks that need core
scheduling from getting off of CPUs that enable core scheduling.

Balbir Singh.