linux-kernel - Re: [PATCH 1/2] sched/wait: Break up long wake list walk

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Date:   Fri, 18 Aug 2017 13:10:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        "Liang, Kan" <kan.liang@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...e.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk

On Fri, Aug 18, 2017 at 12:58 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> which is hacky, but there's a rationale for it:
>>
>>  (a) avoid the crazy long wait queues ;)
>>
>>  (b) we know that migration is *supposed* to be CPU-bound (not IO
>> bound), so yielding the CPU and retrying may just be the right thing
>> to do.
>
> So this would degenerate into a spin when the contention is with
> other CPUs?
>
> But then if we guarantee that migration has flat latency curve
> and no long tail it may be reasonable.

Honestly, right now I'd say it's more of a "poath meant purely for
testing with some weak-ass excuse for why it might not be broken".

                 Linus