linux-kernel - Re: [PATCH] mmu notifiers #v6

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Message-ID: <20080220210339.GA25659@sgi.com>
Date:	Wed, 20 Feb 2008 15:03:39 -0600
From:	Jack Steiner <steiner@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Nick Piggin <npiggin@...e.de>, akpm@...ux-foundation.org,
	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] mmu notifiers #v6

On Wed, Feb 20, 2008 at 11:39:42AM +0100, Andrea Arcangeli wrote:
> Given Nick's comments I ported my version of the mmu notifiers to
> latest mainline. There are no known bugs AFIK and it's obviously safe
> (nothing is allowed to schedule inside rcu_read_lock taken by
> mmu_notifier() with my patch).
> ....

I ported the GRU driver to use the latest #v6 patch and ran a series of
tests on it using our system simulator. The simulator is slow so true
stress or swapping is not possible - at least within a finite amount of
time.

Functionally, the #v6 patch seems to work for the GRU. However, I did
notice two significant differences that make the #v6 performance worse for
the GRU than Christoph's patch.  I think one difference is easily fixable
but the other is more difficult:

	- the location of the mmu_notifier_release() callout is at a
	  different place in the 2 patches. Christoph has the callout
	  BEFORE the call to unmap_vmas() whereas you have it AFTER. The
	  net result is that the GRU does a LOT of 1-page TLB flushes
	  during process teardown.  These flushes are not done with
	  Christops's patch.

	- the range callouts in Christoph's patch benefit the GRU because
	  multiple TLB entries can be flushed with a single GRU
	  instruction (the GRU hardware supports a range flush using a
	  vaddr & length).  The #v6 patch does a TLB flush for each page in
	  the range.  Flushing on the GRU is slow so being able to flush
	  multiple pages with a single request is a benefit.

Seems like the latter difference could be significant for other users
of mmu notifiers.


--- jack
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