linux-kernel - perf: some questions about perf software events

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Message-ID: <m3tyj73qev.fsf@gmail.com>
Date:	Wed, 24 Nov 2010 12:35:36 +0100
From:	Franck Bui-Huu <vagabon.xyz@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: perf: some questions about perf software events

Hello Peter,

I've still a couple of questions after looking at the software events
code, hope you don't mind.

For pure software events (ie excluding {task,cpu}-clock), does it make
sense to set a sample frequency ? I would have done something like this:

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 31515200..df27fd8 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4671,6 +4671,8 @@ static int perf_swevent_init(struct perf_event *event)

 	if (event->attr.type != PERF_TYPE_SOFTWARE)
 		return -ENOENT;
+	if (event->attr.freq)
+		return -EINVAL;

 	switch (event_id) {
 	case PERF_COUNT_SW_CPU_CLOCK:

That is for no 'contiguous' events, setting a sampling frequency doesn't
really make sense since for example you could set a frequency to 1000 HZ
for the software ALIGNMENT_FAULT event and never get any samplings or at
least getting sampling but with a totally different rate. And the
current code doesn't look to handle sample_freq anyway.

Also I'm currently not seeing any real differences between cpu-clock and
task-clock events. They both seem to count the time elapsed when the
task is running on a CPU. Am I wrong ?

Thanks
-- 
		Franck
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