linux-kernel - Re: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters

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Message-ID: <20090926183246.GA4141@in.ibm.com>
Date:	Sun, 27 Sep 2009 00:02:46 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>, peterz@...radead.org
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH] perf_core: provide a kernel-internal interface to
	get to performance counters

On Sat, Sep 26, 2009 at 12:03:28PM -0400, Frank Ch. Eigler wrote:
> 
> Arjan van de Ven <arjan@...radead.org> writes:
> 
> > [...]
> > There are reasons for kernel code to ask for, and use, performance counters.
> > For example, in CPU freq governors this tends to be a good idea, but there
> > are other examples possible as well of course.
> > 
> > This patch adds the needed bits to do enable this functionality; they have been
> > tested in an experimental cpufreq driver that I'm working on, and the changes
> > are all that I needed to access counters properly.
> > [...]
> 
> For what it's worth, this sort of thing also looks useful from
> systemtap's point of view.

Wouldn't SystemTap be another user that desires support for multiple/all CPU
perf-counters (apart from hw-breakpoints as a potential user)? As Arjan pointed
out, perf's present design would support only a per-CPU or per-task counter;
not both.

Thanks,
K.Prasad

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