linux-kernel - Re: Unified tracing buffer

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Message-ID: <33307c790809231402o24c93820tcfbf80d116c729c3@mail.gmail.com>
Date:	Tue, 23 Sep 2008 14:02:08 -0700
From:	"Martin Bligh" <mbligh@...gle.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Masami Hiramatsu" <mhiramat@...hat.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Mathieu Desnoyers" <compudj@...stal.dyndns.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Steven Rostedt" <rostedt@...dmis.org>, darren@...art.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

>> I think the tolerance is about 500 cycles. If that's not sufficient, I guess
>> we'll have to either live with some slight misordering (which people have
>> pointed out is kind of inevitable anyway) on these broken machines?
>> It was sufficient for what we were using it for, but maybe not for everyone.
>
> Well, I dont care about the trace reordering at all. I care about user
> space visible time going backwards issues observed via the
> gettimeofday vsyscall. 500 cycles should be fine, I doubt that we can
> migrate in less than that :)

Right, that's what we were interested in.

> I guess you try this only for machines where the TSC runs with
> constant frequency, right ?

We don't do DVFS on the 'broken' TSC machines, just halt.
But yes, it's selective
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