linux-kernel - Re: Problem with perf hardware counters grouping

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Message-ID: <alpine.DEB.2.00.1109061538270.24588@cl320.eecs.utk.edu>
Date:	Tue, 6 Sep 2011 15:43:09 -0400
From:	Vince Weaver <vweaver1@...s.utk.edu>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Hommey <mh@...ndium.org>, <linux-kernel@...r.kernel.org>,
	<dzickus@...hat.com>
Subject: Re: Problem with perf hardware counters grouping

On Thu, 1 Sep 2011, Peter Zijlstra wrote:
> 
> Both those have 4 generic hardware counters, but x86 defaults to
> enabling the NMI watchdog which takes one, leaving you with 3 (try: echo
> 0 > /proc/sys/kernel/nmi_watchdog). If you had looked at your dmesg
> output you'd have found lines like:
> 
>   NMI watchdog enabled, takes one hw-pmu counter.
> 
> The code can only check if the group as a whole could possibly fit on a
> PMU, which is where your failure on >4 comes from.
> 
> What happens with your >3 case is that while the group is valid and
> could fit on the PMU, it won't fit at runtime because the NMI watchdog
> is taking one and won't budge (cpu-pinned counter have precedence over
> any other kind), effectively starving your group of pmu runtime.
> 
> Also, we should fix that return to say -EINVAL or so.

So any hope of a fix on this?  

As mentioned this is a serious problem for PAPI and I am trying to find a 
good way to enable a workaround in a way that doesn't punish people who 
have the watchdog disabled.

Is there a "stable" API method of determining if the nmi_watchdog is 
present and stealing a perf-counter?  

If I find a "1" in /proc/sys/kernel/nmi_watchdog can I assume a counter is 
being stolen?

Vince

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