linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Date:	Thu, 13 Nov 2008 10:16:40 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

2008/11/13 Ingo Molnar <mingo@...e.hu>:
>
> * Frédéric Weisbecker <fweisbec@...il.com> wrote:
>
>> BTW I'm wondering about consistency in time capturing. When I look
>> into kernel/sched_clock.c I see this in introduction:
>>
>> "The clock: sched_clock_cpu() is monotonic per cpu, and should be
>> somewhat consistent between cpus (never more than 2 jiffies
>> difference)."
>>
>> Two Jiffies, that could result in a lot of inconsistency in the way
>> of nanosec capturing. The current task can be preempted between the
>> call time and the return time and I'm doing a
>> cpu_clock(raw_smp_processor_id) on these two times. Should I keep
>> the same processor_id for these two captures? But what would happen
>> if this cpu is shut down between these two times? One other solution
>> would be to plan time capture in usec but I would mostly lose the
>> interest of function cost measuring....
>>
>> What do you think?
>
> in practice the jitter is much lower - a couple of microseconds - up
> to a few dozen at most.
>
> But it's a possibility, and i think the best solution is something
> that Steve suggested yesterday: a /debug/tracing/trace_options flag
> that turns on global ordering for tracing timestamps. Something like:
>
>  echo global_timestamps > /debug/tracing/trace_options
>
> tracers could also change the default of this flag. The function-cost
> tracer will probably want to default to globally synchronous
> timestamps, while the preempt and irqsoff tracers want to default to
> local timestamps only.
>
> Would something like this work for you?
>
>        Ingo
>


But I guess this flag would apply on the timestamp inserted by the
ring-buffer. Unfortunately I can't use it
since I have to capture the clock for two times and not only during
insertion in the ring-buffer.
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