linux-kernel - Re: ftrace performance impact with different configuration

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Message-ID: <2631878.q7bjHk7YBM@laptop>
Date:	Sat, 31 Dec 2011 00:31:55 +0100
From:	Philippe Rétornaz <philippe.retornaz@...l.ch>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-arm-kernel@...ts.infradead.org, Rabin Vincent <rabin@....in>,
	leiwen@...vell.com, Lei Wen <adrian.wenl@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ftrace performance impact with different configuration

Le vendredi 30 décembre 2011 17:25:41 Steven Rostedt a écrit :
> On Fri, 2011-12-30 at 14:07 +0100, Philippe Rétornaz wrote:
> > Sorry about being a bit naive, but why it is not possible to do it in
> > two
> > steps ?
> > call stop_machine to put the jmp which skip the call to mcount
> > Then wait until all tasks hits schedule() (synchronize_sched() ?)
> (...)
> There's no way to safely modify two instructions that depend on each
> other in a preemptible kernel, with the exception of waiting for all
> CPUs to hit idle (which may never happen on a busy system). And even
> that may be racy.
> 

Thanks for the detailed explanation, I got it now. I guess the only way would 
be to use the freezer api like freeze_processes() is doing. Looks like it does 
not worth the pain.

Thanks ! 

Philippe

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