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

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Date: Sat, 16 Mar 2024 00:32:31 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Reinette Chatre <reinette.chatre@...el.com>, Rex Nie
 <rex.nie@...uarmicro.com>, james.morse@....com, "x86@...nel.org"
 <x86@...nel.org>, "Luck, Tony" <tony.luck@...el.com>, Babu Moger
 <babu.moger@....com>, Peter Newman <peternewman@...gle.com>, Borislav
 Petkov <bp@...en8.de>
Cc: 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 Thu, Mar 14 2024 at 08:25, Reinette Chatre wrote:
>> 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
>> cat: read error: No such file or directory
>> 
>> This is the call trace when cat llc_occupancy:
>> rdtgroup_mondata_show()
>> 	domid = md.u.domid
>> 	d = resctrl_arch_find_domain(r, domid)
>> 
>> d is null here because of lossing precision
>> 
>> Signed-off-by: Rex Nie <rex.nie@...uarmicro.com>
>> ---
>>  fs/resctrl/internal.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
>> index 7a6f46b4edd0..096317610949 100644
>> --- a/fs/resctrl/internal.h
>> +++ b/fs/resctrl/internal.h
>> @@ -94,7 +94,7 @@ union mon_data_bits {
>>  	struct {
>>  		unsigned int rid		: 10;
>>  		enum resctrl_event_id evtid	: 8;
>> -		unsigned int domid		: 14;
>> +		u32				domid;
>>  	} u;
>>  };
>>  
>
> resctrl currently supports 32bit builds. Fixing this issue* in this way
> would first require that resctrl (the architecture independent fs part)
> depend on X86_64. Is this a change that everybody will be comfortable
> with?

Why? Making mon_data_bits::u larger in the way it has been done does not
have any dependency on 32 or 64 builds unless I'm missing something.

> (Of course, there are other solutions available to address the issue mentioned
> in this patch that do not require depending on X86_64, but I would like
> to take this moment to understand the sentiment surrounding continuing support
> for 32bit resctrl.)

Caring about 32biit resctrl on x86 is a pointless exercise.

Thanks,

        tglx