linux-kernel - Re: 32bit resctrl? (was Re: [PATCH v2] fs/resctrl: fix domid loss precision issue)

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Message-ID: <CALPaoChnAt9+2=iVzpownX-bN+mb3Ujn_C=_QS2qsAph=LPSiA@mail.gmail.com>
Date: Thu, 14 Mar 2024 17:04:44 -0700
From: Peter Newman <peternewman@...gle.com>
To: Rex Nie <rex.nie@...uarmicro.com>
Cc: james.morse@....com, "x86@...nel.org" <x86@...nel.org>, "Luck, Tony" <tony.luck@...el.com>, 
	Babu Moger <babu.moger@....com>, Borislav Petkov <bp@...en8.de>, fenghua.yu@...el.com, 
	ilpo.jarvinen@...ux.intel.com, linux-kernel@...r.kernel.org, 
	Reinette Chatre <reinette.chatre@...el.com>
Subject: Re: 32bit resctrl? (was Re: [PATCH v2] fs/resctrl: fix domid loss
 precision issue)

Hi Rex,

On Thu, Mar 14, 2024 at 8:25 AM Reinette Chatre
<reinette.chatre@...el.com> wrote:
>
> +x86 maintainers, Tony, Babu, Peter
>
> Hi Everybody,
>
> On 3/12/2024 12:53 AM, Rex Nie wrote:
> > Below statement from mkdir_mondata_subdir function will loss precision,
> > because it assigns int to 14 bits bitfield.
> >       priv.u.domid = d->id;
> >
> > On some platforms(e.g.,x86), the max cache_id is the amount of L3 caches,
> > so it is not in the range of 0x3fff. But some platforms use higher
> > cache_id, e.g., arm uses cache_id as locator for cache MSC. This will
> > cause below issue if cache_id > 0x3fff likes:
> > /sys/fs/resctrl/mon_groups/p1/mon_data/mon_L3_1048564 # cat llc_occupancy

This domain ID seems unreadable in decimal and I'm wondering whether
it's the best value to use to identify the domain. Does this system
have over 1 million L3 domains?

Thanks!
-Peter