linux-kernel - Re: [PATCH 2.6.26-rc3 : schedule] remove unlikely macros in workqueue.c/queue_delayed_work

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Message-Id: <20080529132915.8caeb501.akpm@linux-foundation.org>
Date:	Thu, 29 May 2008 13:29:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	youquan_song@...ux.intel.com
Cc:	linux-kernel@...r.kernel.org, venkatesh.pallipadi@...el.com
Subject: Re: [PATCH 2.6.26-rc3 : schedule] remove unlikely macros in 
 workqueue.c/queue_delayed_work_on

On Thu, 29 May 2008 00:02:50 -0700 (PDT)
youquan_song@...ux.intel.com wrote:

> cpufreq_ondemand governor call queue_delayed_work_on with entry parameter
> "cpu" every sample rate(every logical cpu during 20ms).check the value of
> "cpu","cpu>=0" condition meeting rate is over than 90%.
> 
> This patch remove the unlikely macros to benefit kernel schedule and
> reader comprehension.
> 
> Signed-off-by: Youquan Song <youquan.song@...el.com>
> ---
>  workqueue.c    |    2++++++
>  1 file changed,     1 insertions(+), 1 deletions(-)
> 
> --- linux-2.6/kernel/workqueue.c	2008-05-13 10:10:11.000000000 -0400
> +++ linux-2.6-new/kernel/workqueue.c	2008-05-29 09:46:16.000000000 -0400
> @@ -230,7 +230,7 @@
> 		timer->data = (unsigned long)dwork;
> 		timer->function = delayed_work_timer_fn;
> 
> -		if (unlikely(cpu >= 0))
> +		if (cpu >= 0)
> 			add_timer_on(timer, cpu);
> 		else
> 			add_timer(timer);
> 

This is very much dependent on what workload the machine is running.

There are 189 queue_delayed_work() callsites and they all want the
unlikely() to be there.

There are six queue_delayed_work_on() callsites and they don't want the
unlikely().

Don't know what to do here.  Fortunately it doesn't matter much ;)

-mm has profile-likely-unlikely-macros.patch which can be used to
instrument these things (that feature seems to get broken regularly
though).

But the instrumentation should be performed across a broad range of
workloads.


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