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 10:23:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
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


* 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
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