Subject: Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Mike Waychison <mikew@google.com>
Cc: Nick Piggin <npiggin@suse.de>, Ying Han <yinghan@google.com>,
       Ingo Molnar <mingo@elte.hu>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, akpm <akpm@linux-foundation.org>,
       David Rientjes <rientjes@google.com>, Rohit Seth <rohitseth@google.com>,
       Hugh Dickins <hugh@veritas.com>, "H. Peter Anvin" <hpa@zytor.com>,
       edwintorok@gmail.com
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Date: Thu, 27 Nov 2008 11:00:07 +0100
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On Thu, 2008-11-27 at 01:28 -0800, Mike Waychison wrote:

> Correct.  I don't recall the numbers from the pathelogical cases we were 
> seeing, but iirc, it was on the order of 10s of seconds, likely 
> exascerbated by slower than usual disks.  I've been digging through my 
> inbox to find numbers without much success -- we've been using a variant 
> of this patch since 2.6.11.

> We generally try to avoid such things, but sometimes it a) can't be 
> easily avoided (third party libraries for instance) and b) when it hits 
> us, it affects the overall health of the machine/cluster (the monitoring 
> daemons get blocked, which isn't very healthy).

If its only monitoring, there might be another solution. If you can keep
the required data in a separate (approximate) copy so that you don't
need mmap_sem at all to show them.

If your mmap_sem is so contended your latencies are unacceptable, adding
more users to it - even statistics gathering, just isn't going to cure
the situation.

Furthermore, /proc code usually isn't written with performance in mind,
so its usually simple and robust code. Adding it to a 'hot'-path like
you're doing doesn't seem advisable.

Also, releasing and re-acquiring mmap_sem can significantly add to the
cacheline bouncing that thing already has.

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