linux-kernel - Re: [RFC PATCH] mm/migration: Remove anon vma locking from try_to

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Date:	Sat, 1 Dec 2012 10:50:44 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC PATCH] mm/migration: Remove anon vma locking from
 try_to_unmap() use

On Sat, Dec 1, 2012 at 10:41 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> I'll try the rwsem and see how it goes?

Yeah. That should be an easy conversion (just convert everything to
use the write-lock first, and then you can make one or two migration
places use the read version).

Side note: The mutex code tends to potentially generate slightly
faster noncontended locks than rwsems, and it does have the
MUTEX_SPIN_ON_OWNER feature that makes the contention case often
*much* better, so there are real downsides to rw-semaphores.

But for this load, it does seem like the scalability advantages of an
rwsem *might* be worth it.

Side note: in contrast, the rwlock spinning reader-writer locks are
basically never a win - the downsides just about always negate any
theoretical scalability advantage. rwsem's can work well, we already
use it for mmap_sem, for example, to allow concurrent page faults, and
it was a *big* scalabiloity win there. Although then we did the "drop
mmap_sem over IO and retry", and that might have negated many of the
advantages of the mmap_sem.

> Hm, indeed. For performance runs I typically disable lock
> debugging - which might have made me not directly notice some of
> the performance problems.

Yeah, lock debugging really tends to make anything that is close to
contended be absolutely *horribly* contended. Doubly so for the
mutexes because it disables the spinning code, but it's true in
general too.

                Linus
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