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

From: Jiri Kosina <jkosina@suse.cz>
To: Borislav Petkov <bp@suse.de>
Cc: linux-edac <linux-edac@vger.kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>, Tony Luck <tony.luck@intel.com>,
	Tomasz Nowicki <tomasz.nowicki@linaro.org>,
	"Chen, Gong" <gong.chen@linux.intel.com>,
	Wolfram Sang <wsa@the-dreams.de>, Lv Zheng <lv.zheng@intel.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
	Huang Ying <ying.huang@intel.com>
Subject: Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader
Date: Thu, 23 Apr 2015 10:39:58 +0200 (CEST)	[thread overview]
Message-ID: <alpine.LNX.2.00.1504231038470.3695@pobox.suse.cz> (raw)
In-Reply-To: <20150401134913.GB18523@pd.tnic>

On Wed, 1 Apr 2015, Borislav Petkov wrote:

> > > From: Jiri Kosina <jkosina@suse.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.

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