Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

[Jiri Kosina]

On Wed, 1 Apr 2015, Borislav Petkov wrote:

> > > From: Jiri Kosina <jkosina@xxxxxxx>
> > > 
> > > Since GHES sources are global, we theoretically need only a single CPU
> > > reading them per NMI instead of a thundering herd of CPUs waiting on a
> > > spinlock in NMI context for no reason at all.
> > 
> > I originally wasn't 100% sure whether GHES sources are global (i.e. if it 
> > really doesn't matter which CPU is reading the registers), but looking at 
> > the code more it actually seems that this is really the right thing to do.
> > 
> > Rationale: ghes_ioremap_pfn_nmi() always ioremaps() (exclusively) the page 
> > with the registers, performs apei_read() (which is ghes-source specific, 
> > but not CPU-specific) and unmaps the page again.
> > 
> > There is nothing that would make this CPU-specific. Adding Huang Ying (the 
> > original author of the code) to confirm this. Huang?
> > 
> > > Do that.
> > 
> > I think this should indeed be pushed forward. It fixes horrible spinlock 
> > contention on systems which are under NMI storm (such as when perf is 
> > active) unrelated to GHES.
> 
> Right, so I tested injecting an error without your patch and same
> behavior. So it all points at global sources AFAICT. It would be cool,
> though, if someone who knows the fw confirms unambiguously.

Three weeks have passed, therefore I find this an appropriate time for a 
friendly ping :)

Rafael? Naoya? Huang?

This fixes a contention spinlock problem in NMI observed on a real HW, so 
it would be really nice to have it fixed.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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