linux-kernel - Re: 4.7-rc1/s390: WARNING: CPU: 5 PID: 1 at kernel/events/core.c:8485 perf_pmu

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Message-Id: <20160608090909.GC7998@linux.vnet.ibm.com>
Date:	Wed, 8 Jun 2016 11:09:09 +0200
From:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Subject: Re: 4.7-rc1/s390: WARNING: CPU: 5 PID: 1 at
 kernel/events/core.c:8485 perf_pmu_register+0x420/0x428

On Mon, Jun 06, 2016 at 12:44:50PM +0200, Christian Borntraeger wrote:
> On 06/06/2016 12:29 PM, Peter Zijlstra wrote:
> > On Mon, Jun 06, 2016 at 11:29:36AM +0200, Hendrik Brueckner wrote:
> > 
> >>>> Looks like perf_pmu_register does not like to be called twice (once for the counter
> >>>> and once for the sampling facility).
> >>>
> >>> Twice isn't the problem per se, its trying to register two PMUs for
> >>> perf_hw_context that is the problem.
> >>>
> >>> The perf core does not expect or deal well with that.
> >>>
> >>> The perf core expects a single HW PMU in that when it schedules
> >>> hw_context events, and encounters an failure to pmu::add() (because the
> >>> hw pmu is 'full') it stops trying to add more events.
> >>
> >> On s390, there are actually two distinct measurement facilities and, thus,
> >> two HW PMUs for each.  There is the hardware counter and hardware sampling
> >> facility/PMU.
> > 
> > Can you quickly describe the cf one; or provide a link to a document
> > doing so?
> 
> http://www-01.ibm.com/support/docview.wss?uid=isg26fcd1cc32246f4c8852574ce0044734a
> 
> (or google for "The Load-Program-Parameter and the CPU-Measurement Facilities"
> or SA23-2260)

That's the right document.

> 
> > If this is a simple always running counter without interrupt you could
> > make it a 'software' PMU which can always schedule, similar to the x86
> > MSR driver (arch/x86/events/msr.c).
> 
> Its a bunch of counters for events like cache writes, cycles, instructions,
> but Hendrik can probably better answer that specific question.

The counters are organized in "counter sets".  The counter sets are started
and stopped according to the events (map to counters) that become scheduled.
Counters in the same set can be already runnning and the PMU will take care
of this.  And, finally, there is no interrupt for counters; they are used for
counting (perf stat) only.

So looking at MSR driver and some other x86 specific driver, I think making
the cf PMU a 'software' PMU is a nice option to pursue.

Thanks and kind regards,
  Hendrik