linux-kernel - [PATCH 3/3] alpha: Implement HW performance events on the EV67 and later CPUs.

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Date:	Tue, 27 Jul 2010 22:29:38 +1200
From:	Michael Cree <mcree@...on.net.nz>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Michael Cree <mcree@...on.net.nz>, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-alpha@...r.kernel.org
Subject: [PATCH 3/3] alpha: Implement HW performance events on the EV67 and later CPUs.

On 04/05/10 19:40, Peter Zijlstra wrote:
> On Sat, 2010-05-01 at 11:55 +1200, Michael Cree wrote:
>> +/* I wonder what this is for ??? */
>> +void set_perf_event_pending(void)
>> +{
>> +}
> 
> If the PMU Interrupt comes in as an NMI we cannot take locks and such
> from its handler. So what we do is queue that work to be ran later. The
> kernel has a fallback to run this stuff from the timer hardirq, but if
> the PMI is IRQ context (like ARM) you can simply call
> perf_event_do_pending() at its tail.
> 
> Alternatively you can self-IPI like x86 does and run
> perf_event_do_pending() from there.

OK, I couldn't see how to self-IPI on the Apha (there is no obvious way in
the Alpha arch code, and the Alpha HW Ref. Man. only describes such a
feature being supported by the PALcode for OpenVMS which we are obviously
not running) and the PMI on Alpha is at such a high priority there is no
chance that the lock can be freed while in the PMI handler, so I have put
the call to perf_event_do_pending() in the timer interrupt, but unlike the
PowerPC code I don't know any tricks to force the timer interrupt to happen
early when there is pending work, so there is a potential maximum delay of
1ms from pending work being notified to calling perf_event_do_pending().  Is
this delay acceptable?

I also fix a nasty, but heretofore hidden, bug that could completely lock up
a machine with continuous PMIs due to a throttled PMC being accidently
re-enabled by the other PMC. It only showed up when I set the NMI flag true
in the call to perf_event_overflow() and had two events counting
simultaneously with a very short sample period.

I also see the x86 and Sparc code in 2.6.35 have reimplemented
the call to hw_perf_group_sched_in() in terms of new functions start_txn(),
cancel_txn() and commit_txn().  Is this change necessary to get the Alpha
code accepted for 2.6.36?  If so it would be helpful if I could have a brief
description of what each function does.

The updated patch follows this message.

Cheers
Michael.

Michael Cree (1):
  alpha: Implement HW performance events on the EV67 and later CPUs.

 arch/alpha/include/asm/perf_event.h |    8 +-
 arch/alpha/kernel/Makefile          |    1 +
 arch/alpha/kernel/irq_alpha.c       |    3 +
 arch/alpha/kernel/perf_event.c      |  842 +++++++++++++++++++++++++++++++++++
 arch/alpha/kernel/time.c            |   26 ++
 5 files changed, 879 insertions(+), 1 deletions(-)
 create mode 100644 arch/alpha/kernel/perf_event.c

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