linux-kernel - Re: [PATCH 2/3] posix_timers: Defer per process timer stop after timers processing

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Message-ID: <20130506230332.GA1225@somewhere>
Date:	Tue, 7 May 2013 01:03:33 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Olivier Langlois <olivier@...llion01.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] posix_timers: Defer per process timer stop after
 timers processing

On Tue, Apr 30, 2013 at 01:51:58PM -0400, Olivier Langlois wrote:
> 
> > 
> > > Maybe the condition around the posix_cpu_timer_schedule() block inside
> > > cpu_timer_fire() could even be a good candidate for 'unlikely'
> > > qualifier.
> > 
> > Well, cpu_timer_fire() is probably not a fast path. So helping branch
> > prediction there probably won't have much measurable effect in practice.
> > 
> Frederic, I'm totally sure that you are right on the measurable effect.
> When I did propose the 'unlikely' qualifier, please note, that I also
> had a documentary purpose in mind.
> 
> Would you have searched the 'likely' path that does
> posix_cpu_timer_schedule() when you did modify the code if the
> 'unlikely' tag would have been present?

It's indeed sometimes a good indicator.

But here it's in the end of a batch of conditional blocks, so it sort
of already suggests itself as an unlikely event.

But if you feel the comment can be improved, don't hesitate to send a patch.



> 
> Greetings,
> Olivier
> 
> 
> 
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