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From: "Zheng, Lv" <lv.zheng@intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: linux-edac <linux-edac@vger.kernel.org>,
	Jiri Kosina <jkosina@suse.cz>, Borislav Petkov <bp@suse.de>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	"Len Brown" <lenb@kernel.org>, "Luck, Tony" <tony.luck@intel.com>,
	Tomasz Nowicki <tomasz.nowicki@linaro.org>,
	"Chen, Gong" <gong.chen@linux.intel.com>,
	Wolfram Sang <wsa@the-dreams.de>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader
Date: Wed, 29 Apr 2015 00:24:40 +0000	[thread overview]
Message-ID: <1AE640813FDE7649BE1B193DEA596E880270FAD9@SHSMSX101.ccr.corp.intel.com> (raw)
In-Reply-To: <20150428135913.GD19025@pd.tnic>

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Hi,

> From: Borislav Petkov [mailto:bp@alien8.de]
> Sent: Tuesday, April 28, 2015 9:59 PM
> Subject: Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader
> 
> On Tue, Apr 28, 2015 at 01:38:41PM +0000, Zheng, Lv wrote:
> > > -	raw_spin_lock(&ghes_nmi_lock);
> > > +	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> > > +		return ret;
> > > +
> >
> > if (atomic_cmpxchg(&ghes_in_nmi, 0, 1))
> > 	return ret;
> 
> Ok, now I understand what you mean.
> 
> We absolutely want to use atomic_add_unless() because we get to save us
> the expensive
> 
> 	LOCK; CMPXCHG
> 
> if the value was already 1. Which is exactly what this patch is trying
> to avoid - a thundering herd of cores CMPXCHGing a global variable.

IMO, on most architectures, the "cmp" part should work just like what you've done with "if".
And on some architectures, if the "xchg" doesn't happen, the "cmp" part even won't cause a pipe line hazard.

Thanks and best regards
-Lv


> 
> I.e.,
> 
> 	movl	ghes_in_nmi(%rip), %ecx	# MEM[(const int *)&ghes_in_nmi], c
> 	cmpl	$1, %ecx	#, c
> 	je	.L311	#,				<--- exit here if ghes_in_nmi == 1.
> 	leal	1(%rcx), %edx	#, D.37163
> 	movl	%ecx, %eax	# c, c
> #APP
> # 177 "./arch/x86/include/asm/atomic.h" 1
> 	.pushsection .smp_locks,"a"
> .balign 4
> .long 671f - .
> .popsection
> 671:
> 	lock; cmpxchgl %edx,ghes_in_nmi(%rip)	# D.37163, MEM[(volatile u32 *)&ghes_in_nmi]
> # 0 "" 2
> #NO_APP
> 
> --
> Regards/Gruss,
>     Boris.
> 
> ECO tip #101: Trim your mails when you reply.
> --
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  reply	other threads:[~2015-04-29  0:24 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-27  9:22 [RFC PATCH 0/5] GHES NMI handler cleanup Borislav Petkov
2015-03-27  9:22 ` [RFC PATCH 1/5] GHES: Carve out error queueing in a separate function Borislav Petkov
2015-03-27  9:22 ` [RFC PATCH 2/5] GHES: Carve out the panic functionality Borislav Petkov
2015-03-27  9:22 ` [RFC PATCH 3/5] GHES: Panic right after detection Borislav Petkov
2015-03-27  9:22 ` [RFC PATCH 4/5] GHES: Elliminate double-loop in the NMI handler Borislav Petkov
2015-03-27  9:22 ` [RFC PATCH 5/5] GHES: Make NMI handler have a single reader Borislav Petkov
2015-04-01  7:45   ` Jiri Kosina
2015-04-01 13:49     ` Borislav Petkov
2015-04-23  8:39       ` Jiri Kosina
2015-04-23  8:59         ` Borislav Petkov
2015-04-23 18:00           ` Luck, Tony
2015-04-27 20:23             ` Borislav Petkov
2015-04-28 14:30     ` Don Zickus
2015-04-28 14:42       ` Don Zickus
2015-04-28 14:55       ` Borislav Petkov
2015-04-28 15:35         ` Don Zickus
2015-04-28 16:22           ` Borislav Petkov
2015-04-28 18:44             ` Don Zickus
2015-05-04 15:40               ` Borislav Petkov
2015-04-27  3:16   ` Zheng, Lv
2015-04-27  8:46     ` Borislav Petkov
2015-04-28  0:44       ` Zheng, Lv
2015-04-28  2:24       ` Zheng, Lv
2015-04-28  7:38         ` Borislav Petkov
2015-04-28 13:38   ` Zheng, Lv
2015-04-28 13:59     ` Borislav Petkov
2015-04-29  0:24       ` Zheng, Lv [this message]
2015-04-29  0:49       ` Zheng, Lv
2015-04-29  8:13         ` Borislav Petkov
2015-04-30  8:05           ` Zheng, Lv
2015-04-30  8:48             ` Borislav Petkov
2015-05-02  0:34               ` Zheng, Lv

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