linux-kernel - Re: [PATCH updated v2] sched/fair: core wide cfs task priority comparison

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Message-ID: <20200508090925.GV5298@hirez.programming.kicks-ass.net>
Date:   Fri, 8 May 2020 11:09:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Aaron Lu <aaron.lwe@...il.com>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        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>,
        Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH updated v2] sched/fair: core wide cfs task priority
 comparison

On Fri, May 08, 2020 at 04:44:19PM +0800, Aaron Lu wrote:
> On Wed, May 06, 2020 at 04:35:06PM +0200, Peter Zijlstra wrote:

> > Aside from this being way to complicated for what it does -- you
> > could've saved the min_vruntime for each rq and compared them with
> > subtraction -- it is also terminally broken afaict.
> >
> > Consider any infeasible weight scenario. Take for instance two tasks,
> > each bound to their respective sibling, one with weight 1 and one with
> > weight 2. Then the lower weight task will run ahead of the higher weight
> > task without bound.
> 
> I don't follow how this could happen. Even the lower weight task runs
> first, after some time, the higher weight task will get its turn and
> from then on, the higher weight task will get more chance to run(due to
> its higher weight and thus, slower accumulation of vruntime).

That seems to assume they're mutually exclusive. In that case, as I
argued, we only have a single runqueue and then yes it works. But if
they're not exclusive, and can run concurrently, it comes apart.