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SubjectRe: [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
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