linux-kernel - [PERF_EVENT 3.11] Group Multiplexing not Working Properly

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Date:	Tue, 24 Jun 2014 18:36:20 +0100
From:	Luís Taniça <matallui@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, eranian@...gle.com
Subject: [PERF_EVENT 3.11] Group Multiplexing not Working Properly

Hi,

I am trying to configure 8 events in a machine which supports only 4 (so, using multiplexing). I tried different approaches:

	1) Configure 8 events independently (works as expected)
	2) Configure 2 event-sets of 4 events each in counting mode (works as expected)
	3) Configure 2 event-sets of 4 events each in sampling mode (not working right)

In case 3) I also tried different approaches:

	3.a) 1 ring-buffer for each event-set and a callback for the POLL signal
		- only the first ring-buffer was detecting samples
		- the second ring-buffer was empty

	3.b) 1 ring-buffer shared between event-sets (using PERF_EVENT_IOC_SET_OUTPUT)
		- both event-sets were writing to the ring-buffer
		- 1st event-set only writes 2 samples while 2nd event-set writes like 20+ samples
		- samples from 1st event-set report time_enabled=time_running while samples from 2nd don’t

Moreover, in both cases the wakeup_events parameter is not working.
I believe there must be some programming bugs in perf_event while profiling in sampling mode with multiple event-sets (groups).

Regards,
matallui


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