linux-kernel - Re: BUG: Function graph tracer hang

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Message-ID: <20090429104006.GE2373@elte.hu>
Date:	Wed, 29 Apr 2009 12:40:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: BUG: Function graph tracer hang


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Tue, 28 Apr 2009, Frederic Weisbecker wrote:
> 
> > 
> > Note that the branch profiler does that:
> > 
> > ______f.miss_hit[______r]++;
> > 
> > Which is a read + write on the cacheline.
> > If each "if" are profiled in the timer interrupt, we can
> > have the cachelines doing a ping-pong of dirtifying since the above
> > variable is shared.
> > 
> > Then the timer interrupt becomes slower. The function graph tracer itself makes
> > it slower.
> > Moreover it is traced itself. So not only the "if" in code are traced, but also
> > each "if" processed by the function graph tracer on function calls and returns.
> > 
> > Which means a fair amount of cacheline dirtifying.
> > 
> > Then if the timer interrupt is slowed, and we have a lot of them (1000 Hz),
> > the system spends all of its time inside it.
> > 
> > At least we need the branch tracing to be done per cpu, I guess.
> 
> This can be done by basically reimplementing what percpu does. 
> This is because the data is saved off in its own section at every 
> if statement. We could copy that section per cpu and add code to 
> the incrementors to add only to their own CPU buffers.

Why not just make these variables regular percpu constructs?

> This is low in my priority of things to do, but I'll at least add 
> it to my "to do" list.

I think we should mark the branch tracer as CONFIG_BROKEN - there's 
been too many problems with it. Thoughts?

	Ingo
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