linux-kernel - Re: Perhaps a side effect regarding NMI returns

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Message-ID: <CA+55aFzgM55hXTs4griX5e9=v_O+=ue+7Rj0PTD=M7hFYpyULQ@mail.gmail.com>
Date:	Tue, 29 Nov 2011 12:44:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>
Subject: Re: Perhaps a side effect regarding NMI returns

On Tue, Nov 29, 2011 at 12:35 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> There may be a problem where you are profiling at a high frequency, and
> then hit this and suddenly, your high frequency NMIs are stopped till
> the next interrupt. If you are running at 100HZ, the next interrupt may
> not happen for 10ms, where we don't have an NMI triggered until then.

It could be much worse than that with no-HZ and some CPU-intensive
load with no timers. I don't think "sysret" enables NMI's either, so
return-to-user-mode may well leave NMI blocked.

So I really think this might be a real issue. And the simplest
approach seems to be to just remove the code. Something like the
attached (TOTALLY UNTESTED!) patch.

Hmm?

                  Linus

View attachment "patch.diff" of type "text/x-patch" (1400 bytes)