Messages in this thread | | | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless | From | Peter Zijlstra <> | Date | Thu, 13 Nov 2008 13:59:41 +0100 |
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On Thu, 2008-11-13 at 13:54 +0100, Ingo Molnar wrote: > * Frédéric Weisbecker <fweisbec@gmail.com> wrote: > > > 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... > > the cmpxchg loop would be needed to ensure timestamp monotonicity: > every new "global time" is cmpxchg-ed with the "previous global time" > (and is first monotonicity checked). > > "prev_global_time" also acts as a global serializer: it ensures that > events are timestamped in a monotonic and ordered way. > > i.e. something like this (pseudocode, without the cmpxchg): > > u64 prev_global_time; > > DEFINE_PER_CPU(prev_local_time); > > u64 global_time() > { > u64 now, delta, now_global; > > prev_global = prev_global_time; > now = sched_clock(); > delta = now - per_cpu(prev_local_time, this_cpu); > per_cpu(prev_local_time, this_cpu) = now; > > now_global = prev_global + delta; > prev_global = now_global; > > return now_global; > } > > note how we build "global time" out of "local time".
This goes down shit-creek real fast if the TSC goes funny and jumps fwd or something.
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