Message-ID: <544F9302.4010001@redhat.com>
Date: Tue, 28 Oct 2014 08:58:42 -0400
From: Rik van Riel <riel@redhat.com>
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To: Andi Kleen <andi@firstfloor.org>, Alex Thorlton <athorlton@sgi.com>
CC: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Bob Liu <lliubbo@gmail.com>, David Rientjes <rientjes@google.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vladimir Davydov <vdavydov@parallels.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/4] Convert khugepaged to a task_work function
References: <1414032567-109765-1-git-send-email-athorlton@sgi.com> <87lho0pf4l.fsf@tassilo.jf.intel.com>
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On 10/28/2014 08:12 AM, Andi Kleen wrote:
> Alex Thorlton <athorlton@sgi.com> writes:
> 
>> Last week, while discussing possible fixes for some unexpected/unwanted behavior
>> from khugepaged (see: https://lkml.org/lkml/2014/10/8/515) several people
>> mentioned possibly changing changing khugepaged to work as a task_work function
>> instead of a kernel thread.  This will give us finer grained control over the
>> page collapse scans, eliminate some unnecessary scans since tasks that are
>> relatively inactive will not be scanned often, and eliminate the unwanted
>> behavior described in the email thread I mentioned.
> 
> With your change, what would happen in a single threaded case?
> 
> Previously one core would scan and another would run the workload.
> With your change both scanning and running would be on the same
> core.
> 
> Would seem like a step backwards to me.

It's not just scanning, either.

Memory compaction can spend a lot of time waiting on
locks. Not consuming CPU or anything, but just waiting.

I am not convinced that moving all that waiting to task
context is a good idea.

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