linux-kernel - Re: [PATCH 3/4] perf, nmi: Move LVT un-masking into irq handlers

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Date:	Mon, 25 Apr 2011 09:39:54 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	George Spelvin <linux@...izon.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/4] perf, nmi: Move LVT un-masking into irq handlers

On Fri, Apr 22, 2011 at 10:26:36AM +0200, Ingo Molnar wrote:
> 
> * Don Zickus <dzickus@...hat.com> wrote:
> 
> > --- a/arch/x86/kernel/cpu/perf_event.c
> > +++ b/arch/x86/kernel/cpu/perf_event.c
> > @@ -1284,6 +1284,9 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
> >  
> >  	cpuc = &__get_cpu_var(cpu_hw_events);
> >  
> > +	/* chipsets have their own quirks when to unmask */
> > +	apic_write(APIC_LVTPC, APIC_DM_NMI);
> > +
> 
> What sense does this comment make in this place?
> 
> Yes, chipsets have their own quirks - but the generic handler is not one of 
> them. So a more appropriate comment would be to point out why we want to unmask 
> there - before PMU handling or after it, etc.

Yup.  I rework that.

> 
> Like the P4 quirk is documented a bit better:
> 
> > +        /*
> > +	 * P4 quirks:
> > +	 * - An overflown perfctr will assert its interrupt
> > +	 *   until the OVF flag in its CCCR is cleared.
> > +	 * - LVTPC is masked on interrupt and must be
> > +	 *   unmasked by the LVTPC handler.
> > +	 */
> > +	apic_write(APIC_LVTPC, APIC_DM_NMI);
> 
> (btw., there's whitespace damage above as well.)
> 
> Furthermore, the P4 comment should *explain* the quirk coherently, not just 
> list random facts. What happens, why, where, and why do we unmask the LVTPC in 
> that spot.

Yeah, I copied that from the old watchdog code.  I'll revise that too and
repost.

Cheers,
Don
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