linux-kernel - Re: [PATCH] mmu notifiers #v5

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Message-ID: <20080131234101.GS7185@v2.random>
Date:	Fri, 1 Feb 2008 00:41:01 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	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>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [PATCH] mmu notifiers #v5

On Thu, Jan 31, 2008 at 03:09:55PM -0800, Christoph Lameter wrote:
> On Thu, 31 Jan 2008, Christoph Lameter wrote:
> 
> > > pagefault against the main linux page fault, given we already have all
> > > needed serialization out of the PT lock. XPMEM is forced to do that
> > 
> > pt lock cannot serialize with invalidate_range since it is split. A range 
> > requires locking for a series of ptes not only individual ones.
> 
> Hmmm.. May be okay after all. I see that you are only doing it on the pte 
> level. This means the range callbacks are taking down a max of 512 
> entries. So you have a callback for each pmd. A callback for 2M of memory?

Exactly. The point of _pages is to reduce of an order of magnitude
(512, or 1024 times) the number of needed invalidate_page calls in a
few places where it's a strightforward optimization for both KVM and
GRU. Thanks to the PT lock this remains a totally obviously safe
design and it requires zero additional locking anywhere (nor linux VM,
nor in the mmu notifier methods, nor in the KVM/GRU page fault).

Sure you can do invalidate_range_start/end for more than 2M(/4M on
32bit) max virtual ranges. But my approach that averages the fixed
mmu_lock cost already over 512(/1024) ptes will make any larger
"range" improvement not strongly measurable anymore given to do that
you have to add locking as well and _surely_ decrease the GRU
scalability with tons of threads and tons of cpus potentially making
GRU a lot slower _especially_ on your numa systems.
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