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Date: Tue, 20 Nov 2012 17:22:05 -0800 (PST)
From: David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] x86/mm: Don't flush the TLB on #WP pmd fixups
In-Reply-To: <20121120120251.GA15742@gmail.com>
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To: Ingo Molnar <mingo@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Turner <pjt@google.com>, Lee Schermerhorn <Lee.Schermerhorn@hp.com>, Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>

On Tue, 20 Nov 2012, Ingo Molnar wrote:

> Subject: x86/mm: Don't flush the TLB on #WP pmd fixups
> From: Ingo Molnar <mingo@kernel.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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