linux-kernel - Re: [PATCH] perf, x86: try to handle unknown nmis with running perfctrs

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Date:	Fri, 13 Aug 2010 06:37:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Robert Richter <robert.richter@....com>
Cc:	Don Zickus <dzickus@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Lin Ming <ming.m.lin@...el.com>, Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] perf, x86: try to handle unknown nmis with running
	perfctrs

On Wed, Aug 11, 2010 at 01:10:46PM +0200, Robert Richter wrote:
> On 10.08.10 22:44:55, Frederic Weisbecker wrote:
> > On Tue, Aug 10, 2010 at 04:48:56PM -0400, Don Zickus wrote:
> > > @@ -1200,7 +1200,7 @@ void perf_events_lapic_init(void)
> > >  	apic_write(APIC_LVTPC, APIC_DM_NMI);
> > >  }
> > >  
> > > -static DEFINE_PER_CPU(unsigned int, perfctr_handled);
> > > +static DEFINE_PER_CPU(unsigned int, perfctr_skip);
> 
> Yes, using perfctr_skip is better to understand ...
> 
> > > @@ -1229,14 +1228,11 @@ perf_event_nmi_handler(struct notifier_block *self,
> > >  		 * was handling a perfctr. Otherwise we pass it and
> > >  		 * let the kernel handle the unknown nmi.
> > >  		 *
> > > -		 * Note: this could be improved if we drop unknown
> > > -		 * NMIs only if we handled more than one perfctr in
> > > -		 * the previous NMI.
> > >  		 */
> > > -		this_nmi = percpu_read(irq_stat.__nmi_count);
> > > -		prev_nmi = __get_cpu_var(perfctr_handled);
> > > -		if (this_nmi == prev_nmi + 1)
> > > +		if (__get_cpu_var(perfctr_skip)){
> > > +			__get_cpu_var(perfctr_skip) -=1;
> > >  			return NOTIFY_STOP;
> > > +		}
> > >  		return NOTIFY_DONE;
> > >  	default:
> > >  		return NOTIFY_DONE;
> > > @@ -1246,11 +1242,21 @@ perf_event_nmi_handler(struct notifier_block *self,
> > >  
> > >  	apic_write(APIC_LVTPC, APIC_DM_NMI);
> > >  
> > > -	if (!x86_pmu.handle_irq(regs))
> > > +	handled = x86_pmu.handle_irq(regs);
> > > +	if (!handled)
> > > +		/* not our NMI */
> > >  		return NOTIFY_DONE;
> > > -
> > > -	/* handled */
> > > -	__get_cpu_var(perfctr_handled) = percpu_read(irq_stat.__nmi_count);
> > > +	else if (handled > 1)
> > > +		/*
> > > +		 * More than one perfctr triggered.  This could have
> > > +		 * caused a second NMI that we must now skip because
> > > +		 * we have already handled it.  Remember it.
> > > +		 *
> > > +		 * NOTE: We have no way of knowing if a second NMI was
> > > +		 * actually triggered, so we may accidentally skip a valid
> > > +		 * unknown nmi later.
> > > +		 */
> > > +		__get_cpu_var(perfctr_skip) +=1;
> 
> ... but this will not work. You have to mark the *absolute* nmi number
> here. If you only raise a flag, the next unknown nmi will be dropped,
> every.



Isn't it what we want? Only the next unknown nmi gets dropped.




> Because, in between there could have been other nmis that
> stopped the chain and thus the 'unknown' path is not executed.



I'm not sure what you mean here. Are you thinking about a third
NMI source that triggers while we are still handling the first
NMI in the back to back sequence?



> The trick in my patch is that you *know*, which nmi you want to skip.


Well with the flag you also know which nmi you want to skip.

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