linux-kernel - Re: [RFC 00/21] perf tools: Add toggling events support

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Date:	Thu, 26 Sep 2013 09:03:07 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Don Zickus <dzickus@...hat.com>,
	Andi Kleen <ak@...ux.jf.intel.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC 00/21] perf tools: Add toggling events support


* Andi Kleen <ak@...ux.intel.com> wrote:

> On Wed, Sep 25, 2013 at 02:50:26PM +0200, Jiri Olsa wrote:
> > hi,
> > sending *RFC* for toggling events support.
> > 
> > Adding perf interface that allows to create toggle events, which can
> > enable or disable another event. Whenever the toggle event is triggered
> > (has overflow), it toggles another event state and either starts or
> > stops it.
> > 
> > The goal is to be able to create toggling tracepoint events to enable and
> > disable HW counters, but the interface is generic enough to be used for
> > any kind of event. 
> 
> Haven't read the patches, but frequent full event switch in/out seems 
> very expensive.  If someone puts that switch on a common function it 
> would likely disturb things quite a bit.
>
> It would be better to keep counting and just do RDPMC on the switch 
> points, and then subtract for counting. For sampling could need a MSR 
> write to enable/disable. Still somewhat expensive, but nowhere near as 
> bad as a full switch.

This is essentially an optimized event switch and should probably be done 
on a higher level so that other instances of event/context switching 
benefit as well.

Thanks,

	Ingo
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