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

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Message-ID: <3ce123c2-3e04-4ff0-99f3-bd402a47ecb8@intel.com>
Date: Thu, 14 Mar 2024 17:51:52 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Peter Newman <peternewman@...gle.com>, 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>
Subject: Re: 32bit resctrl? (was Re: [PATCH v2] fs/resctrl: fix domid loss
 precision issue)



On 3/14/2024 5:04 PM, Peter Newman wrote:
> 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?

I propose that Rex includes the information about Arm's cache IDs [1]
in future changelogs.

Reinette


[1] https://lore.kernel.org/lkml/KL1PR0601MB577303C9D0B1247436BB06F8E6242@KL1PR0601MB5773.apcprd06.prod.outlook.com/