linux-kernel - Re: [RFC PATCH v3 00/16] Core scheduling v3

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Date:   Tue, 13 Aug 2019 10:24:37 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.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>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <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>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3

On 2019/8/12 23:38, Vineeth Remanan Pillai wrote:
>> I have two other small changes that I think are worth sending out.
>>
>> The first simplify logic in pick_task() and the 2nd avoid task pick all
>> over again when max is preempted. I also refined the previous hack patch to
>> make schedule always happen only for root cfs rq. Please see below for
>> details, thanks.
>>
> I see a potential issue here. With the simplification in pick_task,
> you might introduce a livelock where the match logic spins for ever.
> But you avoid that with the patch 2, by removing the loop if a pick
> preempts max. The potential problem is that, you miss a case where
> the newly picked task might have a match in the sibling on which max
> was selected before. By selecting idle, you ignore the potential match.

Oh that's right, I missed this.

> As of now, the potential match check does not really work because,
> sched_core_find will always return the same task and we do not check
> the whole core_tree for a next match. This is in my TODO list to have
> sched_core_find to return the best next match, if match was preempted.
> But its a bit complex and needs more thought.

Sounds worth to do :-)