linux-kernel - Re: [PATCH v2] x86/mce: Distirbute the clear operation of mces_seen to Per-CPU rather than only monarch CPU

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Date:	Wed, 21 May 2014 11:19:58 +0800
From:	Chen Yucong <slaoub@...il.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	tony.luck@...el.com, bp@...en8.de, ak@...ux.intel.com,
	ying.huang@...el.com, linux-kernel@...r.kernel.org,
	linux-edac@...r.kernel.org
Subject: Re: [PATCH v2] x86/mce: Distirbute the clear operation of mces_seen
 to Per-CPU rather than only monarch CPU

On Wed, 2014-05-21 at 11:43 +0900, Hidetoshi Seto wrote:
> (2014/05/21 11:03), Chen Yucong wrote:
> > On Wed, 2014-05-21 at 10:40 +0900, Hidetoshi Seto wrote:
> >> (2014/05/20 11:11), Chen Yucong wrote:
> >>> mces_seen is a Per-CPU variable which should only be accessed by Per-CPU as possible. So the
> >>> clear operation of mces_seen should also be lcoal to Per-CPU rather than monarch CPU.
> >>
> >> I don't think it should be local.
> >> Originally what we want to have here is memory to save mces_seen for each online cpus,
> >> such as a global array like mces_seen[cpus]. But at same time we don't want to preallocate
> >> big array enough for max possible cpus. So we use per-cpu store instead. 
> >>
> > But mces_seen will just be updated by Per-CPU rather than monarch CPU.
> > It is only read by monarch CPU.
> 
> Because mce status registers are per-cpu and monarch cannot access subjects' registers
> directly,
Right. This is one reason why we need to distribute the clear operation
to Per-CPU. And in fact it exactly assigns per-cpu property to
mces_seen.

>  all subjects read it's status for monarch, store the status to memory for monarch,
> and then monarch gather all status to make decision for all.

mce_regin, which is only called by monarch CPU, can be used for system
panics as quickly as possible if there is a truly data corrupting error.
But Monarch CPU don't have to help all other CPU to clean mces_clean.
One advantage of Per-CPU is the isolation of errors propagation, being
so, why do not we clean mces_seen by Per-CPU?

thx!
cyc
> 
> At last monarch kindly clear gathered status for all.
> It will be one of important steps to ready for next mce events.
> 
> I think you should clarify why "distributing the clear operation" is required here.
> What is the benefit?
> 



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