linux-kernel - Re: [Patch V1 1/3] x86, mce: MCE log size not enough for high core parts

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Date:	Thu, 24 Sep 2015 21:22:24 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	"Raj, Ashok" <ashok.raj@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>
Subject: Re: [Patch V1 1/3] x86, mce: MCE log size not enough for high core
 parts

On Thu, Sep 24, 2015 at 07:00:46PM +0000, Luck, Tony wrote:
> > If we get new ones logged in the meantime and userspace hasn't managed
> > to consume and delete the present ones yet, we overwrite the oldest ones
> > and set MCE_OVERFLOW like mce_log does now for mcelog. And that's no
> > difference in functionality than what we have now.
> 
> Ummmm. No.
> 
>                 for (;;) {
> 
>                         /*
>                          * When the buffer fills up discard new entries.
>                          * Assume that the earlier errors are the more
>                          * interesting ones:
>                          */
>                         if (entry >= MCE_LOG_LEN) {
>                                 set_bit(MCE_OVERFLOW,
>                                         (unsigned long *)&mcelog.flags);
>                                 return;
>                         }

Ah, we return. But we shouldn't return - we should overwrite. I believe
we've talked about the policy of overwriting old errors with new ones.

TBH, I don't think there's a 100%-correct policy to act according to
when our error logging buffers are full:

- we can overwrite old errors with new but then this way we might lose
the one important error record with which it all started.

- if we don't overwrite, we might fill up with "unimportant" correctable
error records and miss other, more important ones which happen now

- ...

We could try to implement some cheap heuristics which decide what and
when to overwrite but I'm sceptical it'll be always correct...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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