linux-kernel - Re: [PATCH] mmu notifiers #v7

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Date:	Thu, 28 Feb 2008 17:17:33 -0600
From:	Jack Steiner <steiner@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Christoph Lameter <clameter@....com>,
	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
Subject: Re: [PATCH] mmu notifiers #v7

> > The release should be called much earlier to allow the driver to release 
> > all resources in one go. This way each vma must be processed individually. 
> > For our gobs of memory this method may create a scaling problem on exit().
> 
> Good point, it has to be called earlier for GRU, but it's not a
> performance issue. GRU doesn't pin the pages so it should make the
> global invalidate in ->release _before_ unmap_vmas. Linux can't fault
> in the ptes anymore because mm_users is zero so there's no need of a
> ->release_begin/end, the _begin is enough.
> 

I disagree. The location of the callout IS a performance issue. In simple
comparisons of the 2 patches (Christoph's vs. Andrea's), Andrea's has a 7X
increase in the number of TLB purges being issued to the GRU. TLB flushing
is slow and can impact the performance of of tasks using the GRU.

--- jack
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