linux-kernel - Re: [PATCH 1/4] Pte drop ptep_get_and

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Date:	Wed, 18 Apr 2007 09:05:59 -0400
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@....de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Hugh Dickins <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Michel Lespinasse <walken@...are.com>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jan Beulich <jbeulich@...ell.com>
Subject: Re: [PATCH 1/4] Pte drop ptep_get_and_clear paravirt op.patch

Zachary Amsden wrote:
> Are you sure that it still wins even with these patches?  I can't see
> ptep_get_and_clear getting much faster than a pure non-emulated, and
> the stress case which wins 25-30% is fork/exit being able to drop the
> pte updates in zap_pte_range from trapping and / or going into a pte
> update queue.  That is what my patches addressed for shadow paging
> performance - the call to pte_update can be dropped - but you should
> already be getting this benefit for Xen, since there pte_update is a
> nop.  Are you sure you are unpinning the page tables and mapping them
> back as writable pages prior to address space destruction?

I'll ask Jan for more details later today, but he did this work on
xen-unstable.  The specific change is that it only bothers to update the
pte if it wasn't already not-present.  I guess that's a generally
applicable optimisation, though it wouldn't do much for native.

> Maybe the best strategy is to just re-add ptep_get_and_clear as a
> paravirt-op after the whole set of patches and before Jan's speedup
> patch? 

Yes, I'm happy with native_ptep_get_and_clear to be a direct pagetable
write for pulldown, and then use ptep_get_and_clear as a hook for
manipulating a live pagetable.  I don't see any particular need to put
Jan's patch into the pvops Xen right now, but it would be nice to have
the option.

    J

View attachment "jan.patch" of type "text/x-patch" (18525 bytes)