linux-kernel - Re: [RFC PATCH v3 10/16] sched: Core-wide rq->lock

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Date:   Fri, 31 May 2019 11:23:48 -0400
From:   Vineeth Pillai <vpillai@...italocean.com>
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>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 10/16] sched: Core-wide rq->lock

>
> I'm confused, how doesn't this break the invariant above?
>
> That is, all CPUs must at all times agree on the value of rq_lockp(),
> and I'm not seeing how that is true with the above changes.
>
While fixing the crash in cpu online/offline, I was focusing on
maintaining the invariance
of all online cpus to agree on the value of rq_lockp(). Would it be
safe to assume that
rq and rq_lock would be used only after a cpu is onlined(sched:active)?.

To maintain the strict invariance, the sibling should also disable
core scheduling, but
we need to empty the rbtree before disabling it. I am trying to see
how to empty the
rbtree safely in the offline context.

Thanks,
Vineeth