Messages in this thread | | | Date | Thu, 13 Nov 2008 13:36:17 +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>: > yeah - and it would be all means by a global entity in the beginning - > i.e. we'd just generalize the code around ring_buffer_time_stamp() to > listen to the "globally coherent" flag, and allow it to be used for > callgraph cost measurement code too. > > If the "globally coherent" flag is set, then the implementation would > be something like: > > A simple "last global timestamp" value combined with a "last local > timestamp" value, and the global timestamp is only ever moved forward. > It is updated via cmpxchg loop. This gives coherency and a monotonic > clock. The local timestamp would be taken from cpu_clock(cpu), and a > global timestamp would be constructed out of it. Or something like > that. > > Would that work? [ Would you be interested in sending patches? :-) ] > > Ingo >
Ok, so correct me if I'm wrong. Global timestamp would be captured by using sched_clock(). That's what is done currently in ring_buffer_time_stamp() And the global timestamp would be combination of a last global timestamp and a relative position from now to this last at each insertion in the ring-buffer (or tracing time capture). Am I right? I don't really understand why you want to update with a cmpxchg loop...
And the local timestamp would be built through cpu_clock() with absolute values at each captures? Because we can't consider relative values since this is loosing sense between cpu. Unless we have per_cpu relative values.....
Then, depending of the current_tracer, ring_buffer_time_stamp would act as a wrapper: if (current_tracer->time_flag == TIME_GLOBAL_COHERENT) return ring_buffer_global_timestamp(); else return ring_buffer_local_timestamp();
Or more efficient with a function pointer set at the same we change the time flag for the current tracer.
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