linux-kernel - Re: Mount -o sync regression in 2.6.31

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Message-ID: <20091114131804.GA5245@nowhere>
Date:	Sat, 14 Nov 2009 14:18:09 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Frans Pop <elendil@...net.nl>, Nick Piggin <npiggin@...e.de>,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Mount -o sync regression in 2.6.31

On Fri, Nov 13, 2009 at 02:47:27PM -0800, Stephen Hemminger wrote:
> Bisecting between 2.6.30 and 2.6.31-rc1 reduced it down to:
> 
> 8f184f27300f66f6dcc8296c2dae7a1fbe8429c9 is first bad commit
> commit 8f184f27300f66f6dcc8296c2dae7a1fbe8429c9
> Author: Frederic Weisbecker <fweisbec@...il.com>
> Date:   Sat May 16 06:24:36 2009 +0200
> 
>     tracing/stat: replace linked list by an rbtree for sorting
>     
>     When the stat tracing framework prepares the entries from a tracer
>     to output them to the user, it starts by computing a linear sort
>     through a linked list to give the entries ordered by relevance
>     to the user.
>     
>     This is quite ugly and causes a small latency when we begin to
>     read the file.
>     
>     This patch changes that by turning the linked list into a red-black
>     tree. Athough the whole iteration using the start and next tracer
>     callbacks while opening the file remain the same, it is now much
>     more fast and scalable.
>     
>     The rbtree guarantees O(log(n)) insertions whereas a linked
>     list with linear sorting brought us a O(n) despair. Now the
>     (visible) latency has disapeared.
>     
>     [ Impact: kill the latency while starting to read a stat tracer file ]
>     
>     Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> 
> Which seems totally out of the path related to this, and no tracing is being done!


The trace stat thing is only used for workqueue profiling,
functions profiling, branch profiling:

CONFIG_WORKQUEUE_TRACER
CONFIG_FUNCTION_PROFILER
CONFIG_PROFILE_ANNOTATED_BRANCHES
CONFIG_PROFILE_ALL_BRANCHES


And none of them seem enabled in your config.

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