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

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Message-ID: <20081113094027.GK25479@elte.hu>
Date:	Thu, 13 Nov 2008 10:40:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
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

* Frédéric Weisbecker <fweisbec@...il.com> wrote:

> If so that would be suitable. And for his purpose, the ring-buffer 
> would propose a function for time snapshot adapted to the current 
> flags and that rely on sched_clock?

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
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