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

[Reinette Chatre]

+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
> 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@xxxxxxxxxxxxxxx>
> ---
>  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?

(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.)

Thank you.

Reinette

* Please note that this is not an urgent fix but instead a preparatory change
  for future Arm support.