Messages in this thread | | | Date | Thu, 13 Nov 2008 14:02:00 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless |
| |
* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 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.
yes, it was simplified pseudocode: instead of raw sched_clock() it should use a more reliable source like cpu_clock(this_cpu).
the important bit is how to build a global clock out of the local clock.
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/
| |