linux-kernel - Re: [PATCH 1/2] ftrace: single CPU tracers use CPU clock

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Date:	Sat, 26 Jul 2008 15:21:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/2] ftrace: single CPU tracers use CPU clock


* Steven Rostedt <rostedt@...dmis.org> wrote:

> > this is not a good idea. We want to fix cpu_clock(), not work around 
> > any deficiencies it might have.
> 
> cpu_clock currently is "sched_clock" when the 
> CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is not set. Which is done via 
> configuration, and I noticed is set on my boxes with a stable TSC??

define 'stable TSC' ;-)

> Perhaps we need to make cpu_clock change dynamically when an unstable 
> sched_clock is detected.
> 
> Even in this case, forcing the tracers that are single CPU to use a 
> clock source that modifies itself to try to look stable across CPUs 
> still seems wrong to me.  The goal of looking stable across CPUs will 
> always be at odds with the irqsoff tracer that does not care about 
> other CPUS but cares tremendously about accurate latencies.

other tracers care too - for example to have the right chronology of 
trace events. The scheduler cares too. What kind of worst-case cross-CPU 
effects have you observed?

	Ingo
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