linux-kernel - Re: [PATCH v2 0/2] Update mce

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Date: Fri, 26 Jan 2024 19:56:49 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Luck, Tony" <tony.luck@...el.com>
Cc: Avadhut Naik <avadhut.naik@....com>,
	"linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yazen.ghannam@....com" <yazen.ghannam@....com>,
	"avadnaik@....com" <avadnaik@....com>
Subject: Re: [PATCH v2 0/2] Update mce_record tracepoint

On Fri, Jan 26, 2024 at 05:10:20PM +0000, Luck, Tony wrote:
> 12 extra bytes divided by (say) 64GB (a very small server these days, may laptop has that much)
>    = 0.00000001746%
> 
> We will need 57000 changes like this one before we get to 0.001% :-)

You're forgetting that those 12 bytes repeat per MCE tracepoint logged.
And there's other code which adds more 0.01% here and there, well,
because we can.

> But the key there is keeping the details of the source machine attached to
> the error record. My first contact with machine check debugging is always
> just the raw error record (from mcelog, rasdaemon, or console log).

Yes, that is somewhat sensible reason to have the PPIN together with the
MCE record.

> Knowing which microcode version was loaded on a core *at the time of
> the error* is critical. 

So is the rest of the debug info you're going to need from that machine.
And yet we're not adding that to the tracepoint.

> You've spent enough time with Ashok and Thomas tweaking the Linux
> microcode driver to know that going back to the machine the next day
> to ask about microcode version has a bunch of ways to get a wrong
> answer.

Huh, what does that have to do with this?

IIUC, if someone changes something on the system, whether that is
updating microcode or swapping a harddrive or swapping memory or
whatever, that invalidates the errors reported, pretty much.

You can't put it all in the trace record, you just can't. 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette