linux-kernel - Re: [PATCH 0/4] perf_event: introduce 'perf timer' to analyze timer's behavior

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Message-ID: <20091216074607.GB32713@elte.hu>
Date:	Wed, 16 Dec 2009 08:46:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] perf_event: introduce 'perf timer' to analyze
	timer's behavior


* Xiao Guangrong <xiaoguangrong@...fujitsu.com> wrote:

> 
> 
> Ingo Molnar wrote:
> 
> >>>
> >> Yeah, it has different unit and looks ugly :-(
> >>
> >> but, the problem is we can't get HZ in userspace now, i'll export HZ by proc 
> >> or other way and rectify 'perf timer' output in my next work
> > 
> > We could export HZ as part of the ftrace event format file, in 
> > /debug/tracing/events/timers/ or so. Not sure what the best way would be to do 
> > it (without causing runtime overhead in event processing), but we should 
> > reduce our reliance on /proc and export such instrumentation information via a 
> > single, coherent interface.
> 
> Hi ingo,
> 
> Um, if we export this value in tracepoint format, it should add for every 
> timer's tracepoint.

Not if it's a special timer tracepoint 'HZ' dedicated for this. (We might even 
get events through it on dynticks kernels, or on dynamic-sched-tick systems 
[in the future] when the scheduler HZ value gets modified - or so.)

> > There's a couple of other 'system settings' values that would be useful to 
> > expose in such a way - for example the number of online CPUs. (in fact 
> > that could be exposed as a set of online/offline events plus a way to read 
> > the current count as well)
> 
> How about export those values in debugfs/tracing/*? perf tracepoint self 
> depend on this filesystem.

No - in the long run most of those APIs will go away, and what remains are the 
ftrace event format descriptors in essence. (Which we then might move to 
/proc/events/ or so.)

	Ingo
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