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Date: Sat, 1 Dec 2012 19:41:35 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>
Subject: Re: [RFC PATCH] mm/migration: Remove anon vma locking from
 try_to_unmap() use
Message-ID: <20121201184135.GA32449@gmail.com>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar <mingo@kernel.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