Re: ftrace performance impact with different configuration

From: Philippe Rétornaz
Date: Fri Dec 30 2011 - 18:32:29 EST

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 !


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