linux-kernel - Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling.

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Date:   Fri, 23 Oct 2020 13:59:45 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and
 scheduling.

On Fri, Oct 23, 2020 at 05:05:44PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 19, 2020 at 09:43:16PM -0400, Joel Fernandes (Google) wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> > Instead of only selecting a local task, select a task for all SMT
> > siblings for every reschedule on the core (irrespective which logical
> > CPU does the reschedule).
> 
> This:
> 
> > 
> > During a CPU hotplug event, schedule would be called with the hotplugged
> > CPU not in the cpumask. So use for_each_cpu(_wrap)_or to include the
> > current cpu in the task pick loop.
> > 
> > There are multiple loops in pick_next_task that iterate over CPUs in
> > smt_mask. During a hotplug event, sibling could be removed from the
> > smt_mask while pick_next_task is running. So we cannot trust the mask
> > across the different loops. This can confuse the logic. Add a retry logic
> > if smt_mask changes between the loops.
> 
> isn't entirely accurate anymore, is it?

Yes you are right, we need to delete this bit from the changelog. :-(. I'll
go do that.

thanks,

 - Joel