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

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Message-ID: <CA+55aFxyBYFbx=AGgJ35Dx+U8KVPktesHetYnQrYpt1QGKTrbQ@mail.gmail.com>
Date:	Tue, 29 Nov 2011 12:12:44 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Steven Rostedt <rostedt@...dmis.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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Turner <pjt@...gle.com>
Subject: Re: Perhaps a side effect regarding NMI returns

On Tue, Nov 29, 2011 at 12:09 PM, Andi Kleen <andi@...stfloor.org> wrote:
>
> Not sure what problem Steven thinks is there. Once you
> switched the stack any nesting is fine.

The problem is that NMI's are blocked!

We're potentially switching to another process, there is no guarantee
there will be an "iret" *anywhere* in any patch for a long time. So
any subsequent NMI's will not be able to come in.

> The reason I added them originally is to prevent the race of remote
> kernel events being delayed for a long time. With Frederic's nohz work
> this will be more important in the future. Today it would be eventually
> picked up by the regular timer interrupts.

You're just delaying another kind of event: the next NMI.

And NMI's really shouldn't be triggering any scheduling-related events afaik.

                Linus
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