linux-kernel - Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun field

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Date:	Tue, 18 Nov 2008 15:48:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun
	field


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

> 
> On Mon, 17 Nov 2008, Ingo Molnar wrote:
> 
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > Impact: help to find the better depth of trace
> > > 
> > > We decided to arbitrary define the depth of function return trace as 
> > > "20". Perhaps this is not enough. To help finding an optimal depth, 
> > > we measure now the overrun: the number of functions that have been 
> > > missed for the current thread. By default this is not displayed, we 
> > > have to do set a particular flag on the return tracer: echo overrun 
> > > > /debug/tracing/trace_options And the overrun will be printed on 
> > > the right.
> > > 
> > > As the trace shows below, the current 20 depth is not enough.
> > > 
> > > update_wall_time+0x37f/0x8c0 -> update_xtime_cache (345 ns) (Overruns: 2838)
> > > update_wall_time+0x384/0x8c0 -> clocksource_get_next (1141 ns) (Overruns: 2838)
> > > do_timer+0x23/0x100 -> update_wall_time (3882 ns) (Overruns: 2838)
> > 
> > hm, interesting. Have you tried to figure out what a practical depth 
> > limit would be?
> > 
> > With lockdep we made the experience that function call stacks can be 
> > very deep - if we count IRQ contexts too it can be up to 100 in the 
> > extreme cases. (but at that stage kernel stack limits start hitting 
> > us)
> > 
> > I'd say 50 would be needed.
> 
> I was just looking at the stack tracer, and it pretty much gives us 
> the answer ;-) I'm hitting on max traces around 55, but some of 
> those are asm calls. We could do 50 or 60?  We probably want to make 
> sure that the two do not come close to hitting. That is, the bottom 
> of the stack to overwrite the saved return addresses.

does the stack tracer properly nest across IRQ entry boundaries 
already on x86? We used to have problems in that area.

	Ingo
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