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

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Date:	Wed, 11 Aug 2010 16:03:07 +0200
From:	Robert Richter <robert.richter@....com>
To:	Don Zickus <dzickus@...hat.com>
CC:	Frederic Weisbecker <fweisbec@...il.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 11.08.10 08:44:43, Don Zickus wrote:
> On Wed, Aug 11, 2010 at 01:10:46PM +0200, Robert Richter wrote:
> > > > +		 *
> > > > +		 * 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. Because, in between there could have been other nmis that
> > stopped the chain and thus the 'unknown' path is not executed. The
> > trick in my patch is that you *know*, which nmi you want to skip.
> 
> I guess I am confused.  The way I read your patch was that you assumed the
> next NMI would be the one you skip and if there was another NMI in between
> the handled one and the one to skip, you would not skip it (nmi count !=
> prev + 1) and it would produce an accidental unknown nmi.

That's how it works.

> I tried to change that with my patch by setting the skip flag which would
> be drained on the next unknown nmi, independent of where it is in the NMI
> backlog of NMIs.

"setting the skip flag which would be drained on the next unknown nmi"

That's what is wrong, it drops every unknown nmi, no matter when it is
detected. In between there could be 1000's of valid other nmis
handled. You even could have been returned from nmi mode. But still,
the next unknown nmi will be dropped. Your patch could accumulate also
the number of unknown nmis to skip, and then, if 'real' unknown nmis
happen, all of them will be dropped.

-Robert

> 
> Did I misread something?
> 
> Cheers,
> Don
> 

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

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