linux-kernel - Re: perf_counters issue with enable_on

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Date:	Tue, 25 Aug 2009 08:31:54 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	eranian@...il.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: perf_counters issue with enable_on_exec

stephane eranian writes:

> As you indicated the issue is with the timing information and I think
> it is not related to enable_on_exec. It is more related to the fact
> that to enable a group with a single ioctl() you enable ALL BUT the
> leader. But that means that the time_enabled for the !leader is
> ticking. Thus scaling won't be as expected yet it is correct
> given what happens internally.

No, time_enabled shouldn't be increasing for a counter that's in a
group whose leader is disabled.

I'll try out your test program today.

> I think there needs to be a distinction between 'enabled immediately
> but cannot run because group is not totally enabled' and 'cannot run
> because the group has been multiplexed out yet all could be dispatched
> because all events were dispatched'. In the former, it seems you don't
> want time_enabled to tick, while in the latter you do. In other words,
> time_enabled ticks for each event if the group is 'dispatch-able' (or
> runnable in your terminology) otherwise it does not. time_enabled reflects
> the fact that the group could run but did not have access to the PMU
> resource because of contention with other groups.

Right.  That is exactly what the code is supposed to do.  If it
doesn't there's a bug.

Paul.
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