Re: [PATCH v4 1/3] fs/resctrl: Report a temporary RMID shortage per CLOSID

[Ben Horgan <ben.horgan@arm.com>]

Hi Jinjie,

On 05/11/2026 07:09, Jinjie Ruan wrote:
> An RMID that is freed is held in limbo until its cache occupancy drops
> below the threshold, after which it returns to the free list. A request
> that fails because of limbo is thus temporary and worth retrying, which
> resctrl reports to userspace as -EBUSY. A request that fails because
> every RMID is in use is permanent and reported as -ENOSPC.
> 
> Where an RMID is only meaningful together with a CLOSID, a request can
> only be satisfied by an RMID belonging to the requested CLOSID, so the
> distinction has to be drawn per CLOSID. Instead it is drawn system wide.
> -ENOSPC is reported when the free list holds no RMID for the requested
> CLOSID, even when that CLOSID has RMIDs in limbo. -EBUSY is reported
> when the free list is empty and another CLOSID has an RMID in limbo,
> even when the requested CLOSID has none. Userspace is thus told to give
> up on a shortage that may clear on its own, and to retry one that cannot
> clear until it removes a group itself.
> 
> Architectures where the RMID is independent of the CLOSID always match
> the first entry on the free list, so neither case arises there and only
> MPAM systems are affected.
> 
> Draw the distinction from the number of RMIDs the requested CLOSID has
> in limbo where the RMID depends on the CLOSID, and from the system wide
> count otherwise. Drop the separate test for an empty free list, which
> the search handles on its own.
> 
> Fixes: c4c0376eefe1 ("x86/resctrl: Allow RMID allocation to be scoped by CLOSID")
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Thanks for fixing this MPAM oversight.

Reviewed-by: Ben Horgan <ben.horgan@arm.com>

> Cc: James Morse <james.morse@arm.com>
> Cc: Reinette Chatre <reinette.chatre@intel.com>
> Cc: Babu Moger <babu.moger@amd.com>
> Cc: <stable@vger.kernel.org> # 7.1.x
> ---
>  fs/resctrl/monitor.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
> index 04e4ea612b7c..b8f4e1ff4083 100644
> --- a/fs/resctrl/monitor.c
> +++ b/fs/resctrl/monitor.c
> @@ -207,9 +207,6 @@ static struct rmid_entry *resctrl_find_free_rmid(u32 closid)
>  	struct rmid_entry *itr;
>  	u32 itr_idx, cmp_idx;
>  
> -	if (list_empty(&rmid_free_lru))
> -		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
> @@ -225,7 +222,17 @@ static struct rmid_entry *resctrl_find_free_rmid(u32 closid)
>  			return itr;
>  	}
>  
> -	return ERR_PTR(-ENOSPC);
> +	/*
> +	 * No free RMID matched. If this CLOSID has RMIDs in limbo, return
> +	 * -EBUSY so userspace can retry. Otherwise, the CLOSID is genuinely
> +	 * out of RMIDs. When RMIDs are not per-CLOSID, any RMID in limbo
> +	 * delays the allocation.
> +	 */
> +	if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
> +		return closid_num_dirty_rmid[closid] ? ERR_PTR(-EBUSY) :
> +						       ERR_PTR(-ENOSPC);
> +
> +	return rmid_limbo_count ? ERR_PTR(-EBUSY) : ERR_PTR(-ENOSPC);
>  }
>  
>  /**