linux-kernel - Re: [PATCHv2] x86/mce: Avoid reading every machine check bank register twice.

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Date:	Thu, 19 Apr 2012 18:03:47 +0200
From:	Borislav Petkov <bp@...64.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Borislav Petkov <bp@...64.org>,
	Chen Gong <gong.chen@...ux.intel.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Subject: Re: [PATCHv2] x86/mce: Avoid reading every machine check bank
 register twice.

On Wed, Apr 18, 2012 at 03:19:40PM -0700, Luck, Tony wrote:
> Reading machine check bank registers is slow. There is a trend of
> increasing the number of banks, and the number of cores. The main section
> of do_machine_check() is a serialized section where each cpu in turn
> checks every bank. Even on a little two socket SandyBridge-EP system
> that multiplies out as:
> 
> 	2 sockets * 8 cores * 2 hyperthreads * 20 banks = 640 MSRs
> 
> We already scan the banks in parallel in mce_no_way_out() to see if there
> is a fatal error anywhere in the system. If we build a cache of VALID
> bits during this scan, we can avoid uselessly re-reading banks that have
> no data. Note that this cache is only a hint. If the valid bit is set in a
> shared bank, all cpus that share that bank will see it during the parallel
> scan, but the first to find it in the sequential scan will (usually) clear
> the bank.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
> 
> Version 2:
> 	+ mce_no_way_out() now scans all banks to build the full bitmap (instead of
> 	  breaking out early if it saw a fatal issue).
> 	+ changed name of bitmap from "hint" to "valid_banks"

Looks good.

Acked-by: Borislav Petkov <borislav.petkov@....com>

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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