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

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Date:	Fri, 18 Jun 2010 14:45:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	huang ying <huang.ying.caritas@...il.com>
Cc:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Huang Ying <ying.huang@...el.com>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.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


* huang ying <huang.ying.caritas@...il.com> wrote:

> Hi, Ingo,
> 
> On Fri, Jun 18, 2010 at 5:48 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Hidetoshi Seto <seto.hidetoshi@...fujitsu.com> 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. [...]
> >
> > My request was to make it part of perf events - which is a generic event 
> > logging framework. We dont really need/want a second 'events framework' as 
> > we have one already ;-)
> 
> This patchset is simple and straightforward, [...]

We wouldnt want to add another workqueue or memory allocation mechanism 
either, even if it was 'simple and straightforward'. We try to make things 
more generally useful.

> [...] it is just a delayed execution mechanism, not another 'events 
> framework'. There are several other NMI users other than perf, should we 
> integrate all NMI users into perf framework?

We already did so with the NMI watchdog. What other significant NMI event 
users do you have in mind?

> >> [...] ??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.
> >
> > Yep, that makes sense. I wasnt arguing against the functionality itself, i 
> > was arguing against the illogical layering that limits its utility. By 
> > making it part of perf events it becomes a generic part of that framework 
> > and can be used by anything that deals with events and uses that 
> > framework.
> 
> I think the the 'layering' in the patchset helps instead of 'limits' its 
> utility. It is designed to be as general as possible, so that it can be used 
> by both perf and other NMI users. Do you think so?

What other NMI users do you mean? EDAC/MCE is going to go utilize events as 
well (away from the horrible /dev/mcelog interface), the NMI watchdog already 
did it and the perf tool obviously does as well. There's a few leftovers like 
kcrash which isnt really event centric and i dont think it needs to be 
converted.

Thanks,

	Ingo
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