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

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Date:   Wed, 23 Aug 2017 14:49:31 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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..

The "wait_on_page_migration()" helper works well in the overnight testing.

Thanks,
Kan

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