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

[Ingo Molnar <mingo@elte.hu>]

* Frederic Weisbecker <fweisbec@gmail.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.

	Ingo