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

[Ingo Molnar <mingo@elte.hu>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> Ingo Molnar a écrit :
> > * 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
> 
> 
> Ok I will try with 50. If there are still a lot and often missing 
> traces with this depth, perhaps should we consider a hybrid solution 
> between ret stack and trampolines? We could use the normal ret stack 
> on struct info for most common cases and the trampoline when we are 
> exceeding the depth....

dunno, trampolines make me feel uneasy.

Could you set it to some really large value (200) and add a "max depth 
seen" variable perhaps, and see the maximum depth?

	Ingo