Messages in this thread | | | Date | Thu, 13 Nov 2008 13:54:19 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless |
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* 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".
The cmpxchg would be used to put the above one into a loop, and instead of updating the global time in a racy way:
prev_global = now_global;
We'd update it via the cmpxchg:
atomic64_t prev_global_time;
...
while (atomic64_cmpxchg(&prev_global_time, prev_global, now_global) != prev_global) { [...] }
To make sure the global time goes monotonic. (this way we also avoid a spinlock - locks are fragile for instrumentation)
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