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

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Date:	Tue, 28 Apr 2015 18:22:29 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	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 Tue, Apr 28, 2015 at 11:35:21AM -0400, Don Zickus wrote:
> Your solution seems much simpler. :-)

... and I love simpler :-)

> I followed up in another email stating I mis-spoke.  I forgot this still
> uses the NMI_LOCAL shared NMI.  So every perf NMI, will also call the GHES
> handler to make sure NMIs did not piggy back each other.  So I don't believe

And this is something we should really fix - perf and RAS should
not have anything to do with each other. But I don't know the NMI
code to even have an idea how. I don't even know whether we can
differentiate NMIs, hell, I can't imagine the hardware giving us a
different NMI reason through get_nmi_reason(). Maybe that byte returned
from NMI_REASON_PORT is too small and hangs on too much legacy crap to
even be usable. Questions over questions...

> the NMI reason lock is called a majority of the time (except when the NMI is
> swallowed, but that is under heavy perf load...).

..

> We both agree the mechanics of the spinlock are overkill here and cause much
> cache contention.  Simplifying it to just 'reads' and return removes most of
> the problem.

Right.

-- 
Regards/Gruss,
    Boris.

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