Messages in this thread | | | Date | Thu, 13 Nov 2008 10:16:40 +0100 | From | "Frédéric Weisbecker" <> | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless |
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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. -- 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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