linux-kernel - Re: Unified tracing buffer

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Message-ID: <20080923023115.GE24937@Krystal>
Date:	Mon, 22 Sep 2008 22:31:15 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Darren Hart <darren@...art.com>
Cc:	Roland Dreier <rdreier@...co.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Martin Bligh <mbligh@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

* Darren Hart (darren@...art.com) wrote:
> On Mon, Sep 22, 2008 at 7:02 PM, Mathieu Desnoyers <compudj@...stal.dyndns.org>
> > So only in the specific case of instrumentation of things like locking,
> > where it is possible to insure that instrumentation is synchronized with
> > the instrumented operation, does it make a difference to choose the TSC
> > (which implies a slight delta between the TSCs due to cache line delays
> > at synchronization and delay due to TSCs drifts caused by temperature)
> > over an atomic increment.
> >
> 
> Hrm, i think that overlooks the other reason to use a time based counter over
> an atomic increment: you might care about time.  Perhaps one might be less
> concerned with actual order tightly grouped events and more concerned with the
> actual time delta between more temporally distant events.  In that case, using
> a clocksource would still be valuable. Although admitedtly the caller could
> embed that in their payload, but since we seem to agree we need some kind of
> counter, the time-based counter appears to be the most flexible.
> 
> Thanks,
> 

See my answer to Linus for a proposal on how to do both :)

Mathieu

> -- 
> Darren Hart
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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