linux-kernel - Re: [RFC 1/3] Unified NMI delayed call mechanism

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Date:	Mon, 14 Jun 2010 09:54:03 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Huang Ying <ying.huang@...el.com>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC 1/3] Unified NMI delayed call mechanism

On Mon, Jun 14, 2010 at 12:45:21PM +0900, Hidetoshi Seto wrote:
> (2010/06/12 19:25), Ingo Molnar wrote:
> > 
> > * Huang Ying <ying.huang@...el.com> wrote:
> > 
> >> NMI can be triggered even when IRQ is masked. So it is not safe for NMI 
> >> handler to call some functions. One solution is to delay the call via self 
> >> interrupt, so that the delayed call can be done once the interrupt is 
> >> enabled again. This has been implemented in MCE and perf event. This patch 
> >> provides a unified version and make it easier for other NMI semantic handler 
> >> to take use of the delayed call.
> > 
> > Instead of introducing this extra intermediate facility please use the same 
> > approach the unified NMI watchdog is using (see latest -tip): a perf event 
> > callback gives all the extra functionality needed.
> > 
> > The MCE code needs to be updated to use that - and then it will be integrated 
> > into the events framework.
> 
> Hi Ingo,
> 
> I think this "NMI delayed call mechanism" could be a part of "the events
> framework" that we are planning to get in kernel soon.  At least APEI will
> use NMI to report some hardware events (likely error) to kernel.  So I
> suppose we will go to have a delayed call as an event handler for APEI.
> 
> Generally speaking "event" can occur independently of the situation.
> NMI can tell us some of external events, expecting urgent reaction for
> the event, but we cannot do everything in NMI context.  Or we might have
> a sudden urge to generate an internal event while interrupts are disabled.
> 
> I agree that generating a self interrupt is reasonable solution.
> Note that it could be said that both of "MCE handled (=event log should
> be delivered to userland asap)" and "perf events pending (=pending events
> should be handled asap)" are kind of internal event that requires urgent
> handling in non-NMI kernel context.  One question here is why we should
> have different vectors for these events that uses same mechanism.

I think the perf event subsytem can log events in NMI context already and
deliver them to userspace when the NMI is done.  This is why I think Ingo
wants MCE to be updated to sit on top of the perf event subsytem to avoid
re-invent everything again.

Then again I do not know enough about the MCE stuff to understand what you
mean when an event comes in but you can't handle it in an NMI-safe
context.  An example would be helpful.

Cheers,
Don
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