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Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function
 return tracer lockless
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Ingo Molnar <mingo@elte.hu>
Cc: =?ISO-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
       Steven Rostedt <rostedt@goodmis.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>
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Date: Thu, 13 Nov 2008 13:59:41 +0100
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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.