linux-kernel - Re: [PATCH] x86/mm: Don't flush the TLB on #WP pmd fixups

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Message-ID: <alpine.DEB.2.00.1211201720520.6232@chino.kir.corp.google.com>
Date:	Tue, 20 Nov 2012 17:22:05 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] x86/mm: Don't flush the TLB on #WP pmd fixups

On Tue, 20 Nov 2012, Ingo Molnar wrote:

> Subject: x86/mm: Don't flush the TLB on #WP pmd fixups
> From: Ingo Molnar <mingo@...nel.org>
> Date: Tue Nov 20 14:46:34 CET 2012
> 
> If we have a write protection #PF and fix up the pmd then the
> hugetlb code [the only user of pmdp_set_access_flags], in its
> do_huge_pmd_wp_page() page fault resolution function calls
> pmdp_set_access_flags() to mark the pmd permissive again,
> and flushes the TLB.
> 
> This TLB flush is unnecessary: a flush on #PF is guaranteed on
> most (all?) x86 CPUs, and even in the worst-case we'll generate
> a spurious fault.
> 
> So remove it.
> 

This patch did not cause the 2% speedup that you reported with THP 
enabled for me:

   numa/core at ec05a2311c35:           136918.34 SPECjbb2005 bops
   numa/core at 01aa90068b12:           128315.19 SPECjbb2005 bops (-6.3%)
   numa/core at 01aa90068b12 + patch:   128184.77 SPECjbb2005 bops (-6.4%)
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