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

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Message-ID: <20240126211133.GHZbQgBfqX4Qkdbmu_@fat_crate.local>
Date: Fri, 26 Jan 2024 22:11:33 +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 08:49:03PM +0000, Luck, Tony wrote:
> Every patch that adds new code or data structures adds to the kernel
> memory footprint. Each should be considered on its merits. The basic
> question being:
> 
>    "Is the new functionality worth the cost?"
> 
> Where does it end? It would end if Linus declared:
> 
>   "Linux is now complete. Stop sending patches".
> 
> I.e. it is never going to end.

No, it's not that - it is the merit thing which determines.

> 1) PPIN
> Cost = 8 bytes.
> Benefit: Emdeds a system identifier into the trace record so there
> can be no ambiguity about which machine generated this error.
> Also definitively indicates which socket on a multi-socket system.
> 
> 2) MICROCODE
> Cost = 4 bytes
> Benefit: Certainty about the microcode version active on the core
> at the time the error was detected.
> 
> RAS = Reliability, Availability, Serviceability
> 
> These changes fall into the serviceability bucket. They make it
> easier to diagnose what went wrong.

So does dmesg. Let's add it to the tracepoint...

But no, that's not the right question to ask.

It is rather: which bits of information are very relevant to an error
record and which are transient enough so that they cannot be gathered
from a system by other means or only gathered in a difficult way, and
should be part of that record.

The PPIN is not transient but you have to go map ->extcpu to the PPIN so
adding it to the tracepoint is purely a convenience thing. More or less.

The microcode revision thing I still don't buy but it is already there
so whateva...

So we'd need a rule hammered out and put there in a prominent place so
that it is clear what goes into struct mce and what not.

Thx.

-- 
Regards/Gruss,
    Boris.

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