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

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Message-ID: <alpine.LNX.2.00.1504010941040.843@pobox.suse.cz>
Date:	Wed, 1 Apr 2015 09:45:53 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Borislav Petkov <bp@...en8.de>
cc:	linux-edac <linux-edac@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	"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 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.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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