Messages in this thread | | | Date | Thu, 13 Nov 2008 10:27:07 +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: > >> 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 >
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? -- 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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