linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Date:	Wed, 14 May 2008 10:57:08 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Robin Holt <holt@....com>, Nick Piggin <npiggin@...e.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrea Arcangeli <andrea@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Anthony Liguori <aliguori@...ibm.com>,
	Chris Wright <chrisw@...hat.com>,
	Marcelo Tosatti <marcelo@...ck.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Wed, 14 May 2008, Linus Torvalds wrote:

> One thing to realize is that most of the time (read: pretty much *always*) 
> when we have the problem of wanting to sleep inside a spinlock, the 
> solution is actually to just move the sleeping to outside the lock, and 
> then have something else that serializes things.

The problem is that the code in rmap.c try_to_umap() and friends loops 
over reverse maps after taking a spinlock. The mm_struct is only known 
after the rmap has been acccessed. This means *inside* the spinlock.

That is why I tried to convert the locks to scan the revese maps to 
semaphores. If that is done then one can indeed do the callouts outside of 
atomic contexts.

> Can it be done? I don't know. But I do know that I'm unlikely to accept a 
> noticeable slowdown in some very core code for a case that affects about 
> 0.00001% of the population. In other words, I think you *have* to do it.

With larger number of processor semaphores make a lot of sense since the 
holdoff times on spinlocks will increase. If we go to sleep then the 
processor can do something useful instead of hogging a cacheline.

A rw lock there can also increase concurrency during reclaim espcially if 
the anon_vma chains and the number of address spaces mapping a page is 
high.

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