linux-kernel - Re: post-3.18 performance regression in TLB flushing code

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Message-ID: <5491D0D2.5070103@sr71.net>
Date:	Wed, 17 Dec 2014 10:52:02 -0800
From:	Dave Hansen <dave@...1.net>
To:	Will Deacon <will.deacon@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Michal Simek <monstr@...str.eu>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: post-3.18 performance regression in TLB flushing code

On 12/17/2014 08:53 AM, Will Deacon wrote:
> On Wed, Dec 17, 2014 at 04:28:23PM +0000, Linus Torvalds wrote:
>> On Wed, Dec 17, 2014 at 2:08 AM, Will Deacon <will.deacon@....com> wrote:
>> So why not just this trivial patch, to make the logic be the same it
>> used to be (just using "end > 0" instead of the old "need_flush")?
> 
> Looks fine to me... Dave?

First of all, this is quite observable when testing single-threaded on a
desktop.  This is a mildly crusty Sandybridge CPU from 2011.  I made 3
runs with a single thread: ./brk1_processes -s 30 -t 1

	   fb7332a9fed : 4323385
	   fb7332a9fed^: 4503736
fb7332a9fed+Linus's fix: 4516761

These things are also a little bit noisy, so we're well within the
margin of error with Linus's fix.

This also holds up on the large system.
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