linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Date:	Thu, 13 Nov 2008 13:36:17 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

2008/11/13 Ingo Molnar <mingo@...e.hu>:
> yeah - and it would be all means by a global entity in the beginning -
> i.e. we'd just generalize the code around ring_buffer_time_stamp() to
> listen to the "globally coherent" flag, and allow it to be used for
> callgraph cost measurement code too.
>
> If the "globally coherent" flag is set, then the implementation would
> be something like:
>
> A simple "last global timestamp" value combined with a "last local
> timestamp" value, and the global timestamp is only ever moved forward.
> It is updated via cmpxchg loop. This gives coherency and a monotonic
> clock. The local timestamp would be taken from cpu_clock(cpu), and a
> global timestamp would be constructed out of it. Or something like
> that.
>
> Would that work? [ Would you be interested in sending patches? :-) ]
>
>        Ingo
>


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...

And the local timestamp would be built through cpu_clock() with
absolute values at each captures? Because we can't consider
relative values since this is loosing sense between cpu. Unless we
have per_cpu relative values.....

Then, depending of the current_tracer, ring_buffer_time_stamp would
act as a wrapper:
if (current_tracer->time_flag == TIME_GLOBAL_COHERENT)
    return ring_buffer_global_timestamp();
else
    return ring_buffer_local_timestamp();

Or more efficient with a function pointer set at the same we change
the time flag for the current tracer.
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