linux-kernel - Re: [PATCH] perf, x86: catch spurious interrupts after disabling counters

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Date:	Fri, 24 Sep 2010 12:03:45 +0200
From:	Robert Richter <robert.richter@....com>
To:	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"gorcunov@...il.com" <gorcunov@...il.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	"ming.m.lin@...el.com" <ming.m.lin@...el.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"eranian@...gle.com" <eranian@...gle.com>
Subject: Re: [PATCH] perf, x86: catch spurious interrupts after disabling
 counters

On 23.09.10 23:18:34, Don Zickus wrote:

> > I was able to duplicate the problem and can confirm this patch fixes the
> > issue for me.  I tried poking around (similar to things Robert probably
> > did) and had no luck.  Something just doesn't make sense, but I guess for
> > now this patch is good enough for me. :-)
> 
> Ah ha!  I figured out what the problem was, need to disable the pmu while
> processing the nmi. :-)   Finally something simple in this crazy unknown
> NMI spree.
> 
> Oh yeah and trace_printk is now my new favorite tool!
> 
> From: Don Zickus <dzickus@...hat.com>
> Date: Thu, 23 Sep 2010 22:52:09 -0400
> Subject: [PATCH] x86, perf: disable pmu from counting when processing irq
> 
> On certain AMD and Intel machines, the pmu was left enabled
> while the counters were reset during handling of the NMI.
> After the counters are reset, code continues to process an
> overflow.  During this time another counter overflow interrupt
> could happen because the counter is still ticking.  This leads to
> an unknown NMI.

I don't like the approach of disabling all counters in the nmi
handler. First, it stops counting and thus may falsify
measurement. Second, it introduces much overhead doing a rd-/wrmsrl()
for each counter.

Does your patch below solve something that my patch doesn't?

Btw, Ingo, my patch should be applied to tip/perf/urgent as it fixes
the regression you discovered on AMD systems.

Thanks,

-Robert

> 
> static int x86_pmu_handle_irq(struct pt_regs *regs)
> {
> 
> <snip..>
> 
>         for (idx = 0; idx < x86_pmu.num_counters; idx++) {
>                 if (!test_bit(idx, cpuc->active_mask))
>                         continue;
> 
> <snip..>
> 
> counter reset--> if (!x86_perf_event_set_period(event))
>                         continue;
> 
> still ticking--> if (perf_event_overflow(event, 1, &data, regs))
> <boom overflow>
> stopped here -->        x86_pmu_stop(event);
> 
> The way to solve this is to disable the pmu while processing the
> overflows and re-enable when done.
> 
> Signed-off-by: Don Zickus <dzickus@...hat.com>
> ---
>  arch/x86/kernel/cpu/perf_event.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 48c6d8d..d4fe95d 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1158,6 +1158,8 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
>  
>  	cpuc = &__get_cpu_var(cpu_hw_events);
>  
> +	x86_pmu_disable_all();
> +
>  	for (idx = 0; idx < x86_pmu.num_counters; idx++) {
>  		if (!test_bit(idx, cpuc->active_mask))
>  			continue;
> @@ -1182,6 +1184,8 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
>  			x86_pmu_stop(event, 0);
>  	}
>  
> +	x86_pmu_enable_all(0);
> +
>  	if (handled)
>  		inc_irq_stat(apic_perf_irqs);
>  
> -- 
> 1.7.2.3
> 
> 

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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