linux-kernel - perf: rdpmc and PERF_EVENT_IOC

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Message-ID: <alpine.DEB.2.20.1608170047500.13865@macbook-air>
Date:	Wed, 17 Aug 2016 00:53:01 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	linux-kernel@...r.kernel.org
cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: perf: rdpmc and PERF_EVENT_IOC_RESET

Hello

so using rdpmc() and the mmap page to do fast perf_event reads seems to 
interact poorly with the PERF_EVENT_IOC_RESET ioctl.

>From what I can tell, on reset event->count is set to zero, but 
event->hw.prev_count is not, so the userpg->offset field ends up negative 
and weird things happen.

Shout reset just not be called if you are using the rdpmc() interface?

Vince