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 13:54:19 +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:

> 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)

	Ingo
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