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Date: Tue, 23 Sep 2008 06:53:21 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Masami Hiramatsu <mhiramat@redhat.com>
cc: Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: Unified tracing buffer
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On Tue, 23 Sep 2008, Masami Hiramatsu wrote:

> Steven Rostedt wrote:
> > But, with that, with a global atomic counter, and the following trace:
> > 
> > cpu 0: trace_point_a
> > cpu 1: trace_point_c
> > cpu 0: trace_point_b
> > cpu 1: trace_point_d
> > 
> > 
> > Could the event a really come after event d, even though we already hit
> > event b?
> 
> yes, if event c is an interrupt event :-).
> 
>  cpu 0       cpu 1
>             hit event d
> hit event a
> log event a
>             irq event c
>             log event c

heh, This is assuming that event c is in an IRQ handler.

Since I control where event c is, I can prevent that. I'm talking about
the CPU doing something funny that would have c come after d.

But I didn't specify exactly what the events were, so I'll accept that 
explanation ;-)

-- Steve

> hit event b
> log event b
>             log event d
> 
> so, I think if we really need to order events, we have to stop
> irq right after hitting an event.
> 
> Anyway, in most case, I think it works, but as accurate as
> synchronized-TSC if hardware supports it.