linux-kernel - Re: [tip:perf/urgent] perf, x86: Catch spurious interrupts after disabling counters

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Date:	Fri, 1 Oct 2010 09:17:50 +0200
From:	Robert Richter <robert.richter@....com>
To:	Don Zickus <dzickus@...hat.com>
CC:	Stephane Eranian <eranian@...gle.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"ming.m.lin@...el.com" <ming.m.lin@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [tip:perf/urgent] perf, x86: Catch spurious interrupts after
 disabling counters

On 30.09.10 15:44:51, Don Zickus wrote:
> On Thu, Sep 30, 2010 at 11:12:46AM +0200, Robert Richter wrote:

> > As soon as you stop executing the chain, there are chances to miss an
> > nmi for other parts of the system. Where is no way to avoid this. So
> > your argument above is valid also for regular perf nmis and not only
> > for catched-spurious or back-to-back nmis.
> 
> I don't agree with that.  Most nmi handlers can do a check to see if their
> subsystem triggered an nmi or not.  Now we may not catch it in the right
> order because one handler is higher in the chain than the other, but
> ultimately the other handler will get its chance to execute because it
> fired its own nmi (which hasn't been lost).

No, as soon as a handler with higher priority detected an nmi by its
own and handled it, it returns with a stop and all subsequent handlers
get ignored without the chance to check their hardware. So, if perf
consumes an nmi because a counter triggered, there are rare cases that
other handlers may not be executed.

> Whereas the problem Stephane is describing is that the heurestics of the
> perf counters 'eats' an NMI, thus possibly starving another handler.  With
> back-to-back nmis we are at least polite, letting everyone have a chance to
> process the nmi before we indulge ourselves and 'eat' it (if it still
> around to be eaten).
> 
> However in the case of the 'catched-spurious', we selfishly 'eat' the NMI
> without really knowing if it was our to be eaten.  That was the
> difference and the concern.

But, this argument is valid. It would be better to handle
catched-spurious in the 'unknown' path to give other handlers the
chance to check their hardware.

I don't think this is a show-stopper for v2.6.36 even because the perf
handler runs with the lowest priority now. So we will have enough time
after the merge window to improve the code here.

-Robert

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

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