linux-kernel - Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize

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Date:	Wed, 05 Jun 2013 09:36:26 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy
 synchronize_sched()

On Wed, 2013-06-05 at 13:51 +0200, Peter Zijlstra wrote:

> > @@ -456,9 +471,13 @@ static int __unregister_ftrace_function(
> >  	/*
> >  	 * Dynamic ops may be freed, we must make sure that all
> >  	 * callers are done before leaving this function.
> > +	 *
> > +	 * Again, normal synchronize_sched() is not good enough.
> > +	 * We need to do a hard force of sched synchronization.
> >  	 */
> >  	if (ops->flags & FTRACE_OPS_FL_DYNAMIC)
> > -		synchronize_sched();
> > +		schedule_on_each_cpu(ftrace_sync);
> > +
> >  
> >  	return 0;
> >  }
> > 
> 
> So what happens if schedule_on_each_cpu() returns -ENOMEM? :-)

Hah! I was thinking the same thing when I wrote it, but as this was just
an RFC, I wanted to hear more about this current approach before adding
more.

I was going to also add something like this:

	if (schedule_on_each_cpu(ftrace_sync) < 0) {
		/*
		 * Crap, memory pressure caused this to fail.
		 * Do a synchronize_sched() and then a msleep().
		 * The race that  fails synchronize_sched() is really small
		 * and a msleep() should clear it (but not guaranteed like
		 * a schedule_on_each_cpu() does).
		 */
		synchronize_sched();
		msleep(5);
	}

-- Steve


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