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

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Date:	Mon, 17 Nov 2008 19:38:29 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun field

Ingo Molnar a écrit :
> * 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.
> 
> 	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....

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