linux-kernel - Re: [BUGFIX -v7] x86, MCE: Fix bugs and issues of MCE log ring buffer

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Date:	Mon, 5 Oct 2009 10:51:46 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	Huang Ying <ying.huang@...el.com>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUGFIX -v7] x86, MCE: Fix bugs and issues of MCE log ring buffer

2009/10/5 Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>:
> Hi Huang,
>
> Huang Ying wrote:
>> Current MCE log ring buffer has following bugs and issues:
>>
>> - On larger systems the 32 size buffer easily overflow, losing events.
>>
>> - We had some reports of events getting corrupted which were also
>>   blamed on the ring buffer.
>>
>> - There's a known livelock, now hit by more people, under high error
>>   rate.
>>
>> We fix these bugs and issues via making MCE log ring buffer as
>> lock-less per-CPU ring buffer.
>
> Now I have a real problem on the small MCE log buffer on my new large
> system with Nehalem which has many cpus/banks in one socket...
> So I'd like to solve the problem asap.  I think this problem might block
> some distros to support new processor.
>
> Last week I reviewed your patch again and noticed that it is doing a lot
> of changes at once.  I suppose that this method must be one of reasons
> why your patch seems to be so hard to review, and why it is taking long
> time to be accepted by x86 maintainers.
>
> Fortunately I had some spare time so I carefully broke your patch into
> some purpose-designed pieces.  It would be the most significant change
> that now there are 2 steps to convert the buffer structure - 1) to make
> it per-CPU and 2) to make it ring buffer.
>
> Also I fixed some problem in your patch, found on the way to make this
> patch set.  I'll explain about my changes later using diff from your
> change.  Comments are welcomed.
>
> Thanks,
> H.Seto
>


Looks like the conversion of MCE log into a TRACE_EVENT is still in
discussion whereas the
current issues are urgent.

So the need is to have a more stable ring buffer. But this one is an ad-hoc one.
We already have a general purpose per-cpu/lockless ring buffer implementation in
kernel/trace/ring_buffer.c
And it's not only used by tracing, it's generally available.

I think it would be nicer to use it to avoid a proliferation of
unstable ring buffers
inside the kernel.
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