linux-kernel - Re: [PATCH] x86/mce: Increase the size of the MCE pool from 2 to 8 pages

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Date:   Mon, 16 Oct 2023 10:47:11 -0400
From:   Yazen Ghannam <yazen.ghannam@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Sironi, Filippo" <sironi@...zon.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     yazen.ghannam@....com, "tony.luck@...el.com" <tony.luck@...el.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>
Subject: Re: [PATCH] x86/mce: Increase the size of the MCE pool from 2 to 8
 pages

On 10/16/23 10:24 AM, Dave Hansen wrote:
> On 10/16/23 07:14, Yazen Ghannam wrote:
>> 1) Keep the current config size for boot time. 
>> 2) Add a kernel parameter
>> and/or sysfs file to allow users to request additional genpool capacity.
>> 3) Use gen_pool_add(), or whichever, to add the capacity based on user
>> input. Maybe this can be expanded later to be automatic. But I think it
>> simpler to start with explicit user input.
> 
> I guarantee virtually nobody will ever use an explicit kernel interface
> to bump the size up.  It'll be the same exact folks that recompile their
> kernels.

Right, I agree. My intent was to help avoid a recompile if, for example,
the config size was not enough. Meaning even if a fleet has a custom
kernel with 8 pages of size, it may happen that more is needed. But "may
happen" isn't a strong reason to make a change here. :)

> 
> An automatic resizing one doesn't have to be fancy and only has to
> expand once:
>

Why once?

> static bool expanded = false;
> 
> ...
> 
> 	if (full && !expanded) {
> 		expand();
> 		expanded = true;
> 	}
> 
> It might be a _wee_ bit worse than that because you might have to queue
> some work outside of #MC context but seriously we're talking 10-ish
> lines of code.  It'd probably be even smaller than doing it when poked
> by userspace and wouldn't involve new ABI.

Okay, I'm with you here.

Thanks,
Yazen