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

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Date:	Sat, 26 Jul 2008 17:26:50 +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:

> > 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?
> 
> 1:            bash-3498  [01]  5459.824565:      0:140:R   +  7971:120:R
> 2:          <idle>-0     [00]  5459.824836:      0:140:R ==>  7971:120:R
> 3:            bash-3498  [01]  5459.824984:   3498:120:S ==>     0:140:R
> 4:          <idle>-0     [01]  5459.825342:      0:140:R ==>  7971:120:R
> 5:              ls-7971  [00]  5459.825380:   7971:120:R   +     3:  0:S
> 6:              ls-7971  [00]  5459.825384:   7971:120:R ==>     3:  0:R
> 7:     migration/0-3     [00]  5459.825401:      3:  0:S ==>     0:140:R
> 8:              ls-7971  [01]  5459.825565:   7971:120:R   +   598:115:S
> 
> The above was from my tutorial at OLS. I added the annotated numbers 
> at the beginning of the line. This is the sched_switch tracer.

so it's off by 60 usecs. That matters to the chronology of SMP events, 
and to the irqsoff and preemptoff tracers - if we update the clock 
incorrectly - i.e. if we modify the clock across CPUs so that a running 
CPU can see an 'involuntary' jump in local time.

that should be fixed then - and all tracers (and the scheduler) will 
improve - instead of having this special-case.

	Ingo
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