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

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Date:	Sat, 26 Jul 2008 14:40:58 +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:

> The current ftrace clock uses the sched_clock.c code. This code tries 
> to handle cases where the TSC is out of sync between different CPUs. 
> Unfortunately, even with insync TSCs, due to drifts between the CPU 
> clock and the GTOD clock, we might get some inaccuracy in a single CPU 
> trace.
> 
> Some tracers (irqsoff, preemptoff, preempirqsoff) only care about a 
> trace on a single CPU. This patch changes the ftrace_now (the clock 
> reader) from a function call to a function variable. On initialization 
> of a tracer, the tracer will be allowed to choose which type of clock 
> to use.
> 
> Now the irqsoff, preemptoff and preemptirqs off tracers can have 
> accurate traces with the local CPU clock without affecting the tracers 
> that want the modified clock that tries to keep the different CPU 
> clock reads in sync.

this is not a good idea. We want to fix cpu_clock(), not work around any 
deficiencies it might have.

	Ingo
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