linux-kernel - Re: [RFC PATCH] drivers: char: Add a dynamic clock for the trace clock

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Date:	Wed, 11 Dec 2013 20:49:00 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sonny Rao <sonnyrao@...omium.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Olof Johansson <olof@...om.net>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, pawel.moll@....com
Subject: Re: [RFC PATCH] drivers: char: Add a dynamic clock for the trace
 clock

On Wed, 11 Dec 2013 17:17:30 -0800
Sonny Rao <sonnyrao@...omium.org> wrote:

> On Wed, Dec 11, 2013 at 11:30 AM, Stephane Eranian <eranian@...gle.com> wrote:
> > Sonny,
> >
> > Your patch has a couple of problems for me:
> >  - requires CONFIG_TRACING
> >
> > You should directly invoke getrawmonotonic()
> >  and inline the code from trace_clock_getres().
> >
> > That's how I managed to compile your kernel module on my system.
> 
> You need the changes in kernel/trace/trace.c which is why it's
> dependent on CONFIG_TRACING.
> If we put those functions elsewhere we could remove that dependency,
> but it sounds like people want to just fix the clock that perf uses so
> that it's exportable and not handle this with something like this
> patch, which is better.

I have no issue moving the trace_clock.c code into lib/ and we can add
a CONFIG_TRACE_CLOCK option that can be set by perf and ftrace.

> 
> Also, we should ensure that perf and ftrace are guaranteed to use the
> same clock, I think it just happens to be the same right now.

ftrace has several clocks that it uses:

o local - basically sched_clock()
o global - something like hpet that is monotonic across CPUs but slower
o counter - a simple atomic counter (no time associated to it)
o uptime - jiffy counter
o perf  - trace_clock, which is what perf uses
o x86_tsc - the raw tsc counter.

# cat /sys/kernel/debug/trace_clock
[local] global counter uptime perf x86-tsc

-- Steve
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