linux-kernel - Re: [RFC][PATCH 05/10] mm: Remove likely() from grab_cache_page_write

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Message-ID: <20101207065659.GA3307@amd>
Date:	Tue, 7 Dec 2010 17:56:59 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC][PATCH 05/10] mm: Remove likely() from
  grab_cache_page_write_begin()

On Mon, Dec 06, 2010 at 09:24:10PM -0500, Steven Rostedt wrote:
> [ Resending to Nick's real email address ]
> 
> From: Steven Rostedt <srostedt@...hat.com>
> 
> Running the annotated branch profiler on a box doing average work
> (firefox, evolution, xchat, distcc farm), the likely() used in
> grab_cache_page_write_begin() was incorrect most of the time:
> 
>  correct incorrect  %        Function                  File              Line
>  ------- ---------  -        --------                  ----              ----
>  1924262 71332401  97 grab_cache_page_write_begin    filemap.c           2206
> 
> Adding a trace_printk() and running the function tracer limited to
> just this function I can see:
> 
>         gconfd-2-2696  [000]  4467.268935: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=7
>         gconfd-2-2696  [000]  4467.268946: grab_cache_page_write_begin <-ext3_write_begin
>         gconfd-2-2696  [000]  4467.268947: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=8
>         gconfd-2-2696  [000]  4467.268959: grab_cache_page_write_begin <-ext3_write_begin
>         gconfd-2-2696  [000]  4467.268960: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=9
>         gconfd-2-2696  [000]  4467.268972: grab_cache_page_write_begin <-ext3_write_begin
>         gconfd-2-2696  [000]  4467.268973: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=10
>         gconfd-2-2696  [000]  4467.268991: grab_cache_page_write_begin <-ext3_write_begin
>         gconfd-2-2696  [000]  4467.268992: grab_cache_page_write_begin: page=          (null) mapping=ffff8800676a9460 index=11
>         gconfd-2-2696  [000]  4467.269005: grab_cache_page_write_begin <-ext3_write_begin
> 
> Which shows that a lot of calls from ext3_write_begin will result in
> the page returned by "find_lock_page" will be NULL.
> 
> Cc: Nick Piggin <npiggin@...nel.dk>

Looks good, 

Acked-by: Nick Piggin <npiggin@...nel.dk>
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