linux-kernel - Re: [RFC][PATCH 3/5] perf: Add hrtimer code for PMI-less hardware counters

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Date:	Mon, 30 Aug 2010 16:12:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Zhang Rui <rui.zhang@...el.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Robert Richter <robert.richter@....com>,
	Lin Ming <ming.m.lin@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Len Brown <lenb@...nel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [RFC][PATCH 3/5] perf: Add hrtimer code for PMI-less hardware
 counters

On Mon, 2010-08-30 at 14:27 +0100, Matt Fleming wrote:

> I'm lost. Is it possible to do this patch entirely in userspace? How do
> we periodically sample the counters if it's not being done in the
> kernel?

perf_event_attr leader = {
 .type = PERF_TYPE_SOFTWARE,
 .config = PERF_COUNT_SW_TASK_CLOCK,
 .sample_period = xxx,
 .sample_type = PERF_SAMPLE_READ|...,
 .read_format = PERF_FORMAT_GROUP,
};

perf_event_attr counter = {
 .type = PERF_TYPE_HARDWARE,
 .config = PERF_COUNT_HW_CPU_CYCLES,
};
 
leader_fd = sys_perf_event_open(&leader, pid, cpu, 0, 0);
counter_fd = sys_perf_event_open(&counter, pid, cpu, leader_fd, 0);

Which gives you a group of 2 events, one software timer that samples,
one hardware counter that only counts.
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