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

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Message-ID: <20150401134913.GB18523@pd.tnic>
Date:	Wed, 1 Apr 2015 15:49:13 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	linux-edac <linux-edac@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>, Tony Luck <tony.luck@...el.com>,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>,
	"Chen, Gong" <gong.chen@...ux.intel.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Lv Zheng <lv.zheng@...el.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Huang Ying <ying.huang@...el.com>
Subject: Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

On Wed, Apr 01, 2015 at 09:45:53AM +0200, Jiri Kosina wrote:
> On Fri, 27 Mar 2015, Borislav Petkov wrote:
> 
> > From: Jiri Kosina <jkosina@...e.cz>
> > 
> > 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.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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