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

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Date:	Sat, 1 Dec 2012 19:41:35 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > So as a quick concept hack I wrote the patch attached below.
> > (It's not signed off, see the patch description text for the
> > reason.)
> 
> Well, it confirms that anon_vma locking is a big problem, but 
> as outlined in my other email it's completely incorrect from 
> an actual behavior standpoint.

Yeah.

> Btw, I think the anon_vma lock could be made a spinlock 
> instead of a mutex or rwsem, but that would probably take more 
> work. We *shouldn't* be doing anything that needs IO inside 
> the anon_vma lock, though, so it *should* be doable. But there 
> are probably quite a bit of allocations inside the lock, and I 
> know it covers huge areas, so a spinlock might not only be 
> hard to convert to, it quite likely has latency issues too.

I'll try the rwsem and see how it goes?

> Oh, btw, MUTEX_SPIN_ON_OWNER may well improve performance too, 
> but it gets disabled by DEBUG_MUTEXES. So some of the 
> performance impact of the vma locking may be *very* 
> kernel-config dependent.

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

Thanks,

	Ingo
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