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

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Message-ID: <6e8b81de-e985-9222-29c5-594c6849c351@linux.intel.com>
Date:   Wed, 23 Aug 2017 08:58:37 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     "Liang, Kan" <kan.liang@...el.com>,
        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>,
        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 08/23/2017 07:49 AM, Liang, Kan wrote:
>> 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.
> 


Linus,

Will you still consider the original patch as a fail safe mechanism?

Thanks.

Tim