linux-kernel - Re: Unified tracing buffer

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Date:	Tue, 23 Sep 2008 00:10:32 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland Dreier <rdreier@...co.com>,
	Martin Bligh <mbligh@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	darren@...art.com, "Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

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
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.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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