linux-kernel - Re: Unified tracing buffer

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Message-ID: <alpine.DEB.1.10.0809230651070.26011@gandalf.stny.rr.com>
Date:	Tue, 23 Sep 2008 06:53:21 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
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



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.
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