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 10:27:07 +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:
>
>> 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.
>
> i think the clock should be a property of the tracer, not of the ring
> buffer. Hence if a tracer has the option set, it will get coherent
> timestamps - including ringbuffer insertion timestamps.
>
>        Ingo
>



If so that would be suitable. And for his purpose, the ring-buffer
would propose a function for time snapshot
adapted to the current flags and that rely on sched_clock?
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