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

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Message-ID: <CA+55aFwavpFfKNW9NVgNhLggqhii-guc5aX1X5fxrPK+==id0g@mail.gmail.com>
Date:   Tue, 22 Aug 2017 13:42:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        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>, Andi Kleen <ak@...ux.intel.com>,
        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 Tue, Aug 22, 2017 at 12:55 PM, Liang, Kan <kan.liang@...el.com> wrote:
>
>> So I propose testing the attached trivial patch.
>
> It doesn’t work.
> The call stack is the same.

So I would have expected the stack trace to be the same, and I would
even expect the CPU usage to be fairly similar, because you'd see
repeating from the callers (taking the fault again if the page is -
once again - being migrated).

But I was hoping that the wait queues would be shorter because the
loop for the retry would be bigger.

Oh well.

I'm slightly out of ideas. Apparently the yield() worked ok (apart
from not catching all cases), and maybe we could do a version that
waits on the page bit in the non-contended case, but yields under
contention?

IOW, maybe this is the best we can do for now? Introducing that
"wait_on_page_migration()" helper might allow us to tweak this a bit
as people come up with better ideas..

And then add Tim's patch for the general worst-case just in case?

             Linus

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