linux-kernel - Re: [PATCH] perf, nmi: fix unknown NMI warning

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Message-ID: <20140215095843.GJ14089@laptop.programming.kicks-ass.net>
Date:	Sat, 15 Feb 2014 10:58:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...nel.org, eranian@...gle.com, linux-kernel@...r.kernel.org,
	Markus Metzger <markus.t.metzger@...el.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf, nmi: fix unknown NMI warning

On Fri, Feb 14, 2014 at 04:44:08PM -0800, Andi Kleen wrote:
> 
> Make intel_pmu_handle_irq() take the full exit path when returning early.

This reminds me of the late-ack stuff;

The way I understand interrupts to work is that when you raise the
interrupt it gets latched, when you ACK you drop the latch. Then when it
gets re-raised while its still in progress, it gets latched again and
the irq-enable at the end of the running handler will get it to trigger
again.

So by late-ACK-ing the PMI we can miss PMIs that happen between enabling
the PMU and ACKing the PMI.

We should either re-check the overflow mask after the ACK or do the ACK
while the PMU is disabled.
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