Messages in this thread | | | Date | Thu, 13 Nov 2008 10:23:40 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless |
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* Frédéric Weisbecker <fweisbec@gmail.com> wrote:
> 2008/11/13 Ingo Molnar <mingo@elte.hu>: > > > > * Frédéric Weisbecker <fweisbec@gmail.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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