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Subject: Re: [RFC][PATCH 3/5] perf: Add hrtimer code for PMI-less hardware
 counters
From: Peter Zijlstra <peterz@infradead.org>
To: Matt Fleming <matt@console-pimps.org>
Cc: Zhang Rui <rui.zhang@intel.com>, linux-kernel@vger.kernel.org,
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Date: Mon, 30 Aug 2010 14:55:02 +0200
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On Mon, 2010-08-30 at 13:13 +0100, Matt Fleming wrote:
> Currently, it's impossible to periodically sample hardware counters that
> lack performance monitoring interrupt (PMI) support. In order to sample
> these counters we can create an event group which is backed by a
> hrtimer, thereby simulating a PMI.
> 
> When the hrtimer goes off we sample the values in the hardware
> counters. Because we obviously can't rely on the hrtimer going off at
> exactly the sample period (say, every 1000 cache misses) the values need
> to be weighted by the variable period since the last hrtimer went
> off. This is so that we can compensate for the variability in hrtimer
> period.
> 
> If perf record tries to create a sampling counter and the hardware
> doesn't support it then we'll fall back to creating an event group with
> a hrtimer.


Why is this changing kernel code?

You can create those groups in userspace..