linux-kernel - Re: [PATCH 1/6] x86-mce: Modify CMCI poll interval to adjust for small check

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Message-ID: <CA+8MBbJ+FeQKZC9oVZsvrBptaY+24rVKWUXT02ETHMMoA-omuA@mail.gmail.com>
Date:	Thu, 10 Jul 2014 11:55:06 -0700
From:	Tony Luck <tony.luck@...il.com>
To:	Havard Skinnemoen <hskinnemoen@...gle.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ewout van Bekkum <ewout@...gle.com>,
	linux-edac <linux-edac@...r.kernel.org>
Subject: Re: [PATCH 1/6] x86-mce: Modify CMCI poll interval to adjust for
 small check_interval values.

On Thu, Jul 10, 2014 at 10:51 AM, Havard Skinnemoen
<hskinnemoen@...gle.com> wrote:
> What's the typical interrupt rate during a storm? We should make it
> significantly less frequent than that, otherwise there's no point
> switching to polling.
>
> IIRC we've seen at least several hundred CMCIs per second, so perhaps
> 100 ms would be a reasonable minimum? Or perhaps 10 ms, which is the
> current minimum polling interval enforced by mce_timer_fn.

I don't think we have a solid point to really declare "storm!".  The
CMCI rates between normal and abnormal rates are vast:

Normal rates are a few CMCI per year (or maybe per month ... if
you have a multi-terabyte machine perhaps even "per day" is normal).

So if you see two CMCI inside the same minute, you could declare
a storm.  Realistically we want the threshold a bit higher.

It then becomes a balance between seeing all the errors (so our PFA
mechanisms get enough data to spot bad pages and take action) and
processing so many interrupts that we begin to take a performamce
hit.

Once we do decide there is a storm - we know we have given up on
seeing all the errors ... the polling rate will only decide how fast we
can determine that the storm has ended.  I don't see a lot of value
in detecting the end at milli-second granularity. But we probably don't
want to give up minutes worth of PFA data if the storm does end.

-Tony
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