linux-kernel - Re: Unified tracing buffer

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Date:	Mon, 22 Sep 2008 19:26:36 -0700
From:	"Darren Hart" <darren@...art.com>
To:	"Mathieu Desnoyers" <compudj@...stal.dyndns.org>
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

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,

-- 
Darren Hart
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