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

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Message-ID: <20200421025131.GA227300@aaronlu-desktop>
Date:   Tue, 21 Apr 2020 10:51:31 +0800
From:   Aaron Lu <aaron.lwe@...il.com>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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] sched/fair: core wide cfs task priority
 comparison

On Mon, Apr 20, 2020 at 06:26:34PM -0400, Vineeth Remanan Pillai wrote:
> On Mon, Apr 20, 2020 at 4:08 AM Aaron Lu <aaron.lwe@...il.com> wrote:
> >
> > On Fri, Apr 17, 2020 at 05:40:45PM +0800, Aaron Lu wrote:
> 
> > The adjust is only needed when core scheduling is enabled while I
> > mistakenly called it on both enable and disable. And I come to think
> > normalize is a better name than adjust.
> >
> I guess we would also need to update the min_vruntime of the sibling
> to match the rq->core->min_vruntime on coresched disable. Otherwise
> a new enqueue on root cfs of the sibling would inherit the very old
> min_vruntime before coresched enable and thus would starve all the
> already queued tasks until the newly enqueued se's vruntime catches up.

Yes this is a concern but AFAICS, there is no problem. Consider:
- when there is no queued task across the disable boundary, the stale
  min_vruntime doesn't matter as you said;
- when there are queued tasks across the disable boundary, the newly
  queued task will normalize its vruntime against the sibling_cfs_rq's
  min_vruntime, if the min_vruntime is stale and problem would occur.
  But my reading of the code made me think this min_vruntime should
  have already been updated by update_curr() in enqueue_entity() before
  being used by this newly enqueued task and update_curr() would bring
  the stale min_vruntime to the smallest vruntime of the queued ones so
  again, no problem should occur.

I have done a simple test locally before sending the patch out and didn't
find any problem but maybe I failed to hit the race window. Let me know
if I misunderstood something.
 
> Other than that, I think the patch looks good. We haven't tested it
> yet. Will do a round of testing and let you know soon.

Thanks.