Re: [PATCH v5 05/24] x86/resctrl: Allow RMID allocation to be scoped by CLOSID

[Reinette Chatre]

Hi James,

On 7/28/2023 9:42 AM, James Morse wrote:
> -int alloc_rmid(void)
> +static struct rmid_entry *resctrl_find_free_rmid(u32 closid)
>  {
> -	struct rmid_entry *entry;
> -
> -	lockdep_assert_held(&rdtgroup_mutex);
> +	struct rmid_entry *itr;
> +	u32 itr_idx, cmp_idx;
>  
>  	if (list_empty(&rmid_free_lru))
> -		return rmid_limbo_count ? -EBUSY : -ENOSPC;
> +		return rmid_limbo_count ? ERR_PTR(-EBUSY) : ERR_PTR(-ENOSPC);
> +
> +	list_for_each_entry(itr, &rmid_free_lru, list) {
> +		/*
> +		 * Get the index of this free RMID, and the index it would need
> +		 * to be if it were used with this CLOSID.
> +		 * If the CLOSID is irrelevant on this architecture, these will
> +		 * always be the same meaning the compiler can reduce this loop
> +		 * to a single list_entry_first() call.
> +		 */
> +		itr_idx = resctrl_arch_rmid_idx_encode(itr->closid, itr->rmid);
> +		cmp_idx = resctrl_arch_rmid_idx_encode(closid, itr->rmid);
> +
> +		if (itr_idx == cmp_idx)
> +			return itr;
> +	}
> +
> +	return ERR_PTR(-ENOSPC);
> +}
> +
> +/*
> + * For MPAM the RMID value is not unique, and has to be considered with
> + * the CLOSID. The (CLOSID, RMID) pair is allocated on all domains, which
> + * allows all domains to be managed by a single limbo list.
> + * Each domain also has a rmid_busy_llc to reduce the work of the limbo handler.
> + */

I find the above comment to be contradicting - it talks about a single limbo list
yet there is "also" a limbo list/bitmask per domain. Should "single limbo list"
perhaps be "single free list"?

Reinette