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SubjectRe: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless
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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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