Re: [PATCH v3] arm_mpam: resctrl: Limit num_rmids to the CLOSIDs in use

[Ben Horgan]

Hi Jinjie,

On 06/11/2026 08:19, Jinjie Ruan wrote:
> On MPAM an RMID is a PMG within a PARTID, so resctrl can only allocate
> the PMGs of the CLOSIDs it uses. info/L3_MON/num_rmids is instead set
> to the whole PARTID and PMG space when the monitors are initialised,
> and halved when CDP is enabled. It reports more RMIDs than resctrl can
> allocate in two cases:
> 
> - With CDP each CLOSID takes two PARTIDs, and resctrl halves the
>   CLOSID count rounding down, while MPAM halves the whole RMID space.
>   With 5 PARTIDs and 2 PMGs resctrl has 2 CLOSIDs and 4 RMIDs, but
>   num_rmids reads 5. The count is too high whenever the number of
>   PARTIDs is odd and there is more than one PMG.
> 
> - On a monitor-only system there are no control groups and resctrl
>   only uses the default CLOSID, but num_rmids still covers every
>   PARTID. With 5 PARTIDs and 2 PMGs it reads 10 while only 2 RMIDs
>   can be allocated.
> 
> RMID allocation is limited to the CLOSIDs in use, so only the value
> reported to userspace is too high. Creating as many monitor groups as
> num_rmids allows fails with -ENOSPC before that number is reached.
> 
> Work out the number of CLOSIDs with the rules resctrl applies when it
> is mounted: schemata_list_add() halves a resource's CLOSID count
> while CDP is enabled on it, rounding down, closid_init() takes the
> smallest across the resources, and a monitor-only system has only the
> default CLOSID. Report (pmg_max + 1) RMIDs for each of those CLOSIDs,
> counting at monitor initialisation and from
> resctrl_arch_set_cdp_enabled(), which resctrl calls whenever a mount
> or unmount enables or disables CDP.
> 
> Fixes: 264c285999fc ("arm_mpam: resctrl: Add monitor initialisation and domain boilerplate")
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 7.1.x
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Ben Horgan <ben.horgan@xxxxxxx>
> Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Cc: Fenghua Yu <fenghuay@xxxxxxxxxx>
> ---
> v3:
> - Drop resctrl_arch_set_mon_rmid_count() and count the CLOSIDs in the
>   MPAM driver instead, as Ben suggested.
> - v2: https://lore.kernel.org/all/20261105123304.2281987-1-ruanjinjie@xxxxxxxxxx/
> v2:
> - Also fix num_rmids on monitor-only systems, as Ben suggested.
> - Retitle from "arm_mpam: Round the CDP RMID count down to whole CLOSIDs".
> - v1: https://lore.kernel.org/all/20261104101622.563280-1-ruanjinjie@xxxxxxxxxx/
> ---
>  drivers/resctrl/mpam_resctrl.c | 45 +++++++++++++++++++++++-----------
>  1 file changed, 31 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 5468367776e2..6859849f299b 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -191,6 +191,34 @@ static void mpam_resctrl_monitor_sync_abmc_vals(struct rdt_resource *l3)
>  	 */
>  }
>  
> +/* The RMID count follows the CLOSIDs resctrl uses. */
> +static void mpam_resctrl_set_num_rmid(void)
> +{
> +	struct mpam_resctrl_res *res = &mpam_resctrl_controls[RDT_RESOURCE_L3];
> +	struct rdt_resource *l3 = &res->resctrl_res;
> +	enum resctrl_res_level rid;
> +	u32 num_closid = UINT_MAX;
> +
> +	for_each_mpam_resctrl_control(res, rid) {
> +		u32 res_num_closid;
> +
> +		if (!res->resctrl_res.alloc_capable)
> +			continue;
> +
> +		res_num_closid = mpam_partid_max + 1;
> +		if (res->cdp_enabled)
> +			res_num_closid /= 2;
> +
> +		num_closid = min(num_closid, res_num_closid);
> +	}

There are global variables mpam_partid_max, cdp_enabled and a helper resctrl_arch_alloc_capable().
Using these should make this simpler.

> +
> +	/* Monitor-only systems have only the default CLOSID. */
> +	if (num_closid == UINT_MAX)
> +		num_closid = 1;
> +
> +	l3->mon.num_rmid = (mpam_pmg_max + 1) * num_closid;
> +}
> +
>  int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>  {
>  	u32 partid_i = RESCTRL_RESERVED_CLOSID, partid_d = RESCTRL_RESERVED_CLOSID;
> @@ -217,11 +245,6 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>  	cdp_enabled = enable;
>  	mpam_resctrl_controls[rid].cdp_enabled = enable;
>  
> -	if (enable)
> -		l3->mon.num_rmid = resctrl_arch_system_num_rmid_idx() / 2;
> -	else
> -		l3->mon.num_rmid = resctrl_arch_system_num_rmid_idx();
> -
>  	/* The mbw_max feature can't hide cdp as it's a per-partid maximum. */
>  	if (cdp_enabled && !mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled)
>  		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = false;
> @@ -238,6 +261,8 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>  	if (!cdp_enabled && mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>  		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
>  
> +	mpam_resctrl_set_num_rmid();
> +

Why move the location of where num_rmid is set?

Thanks,

Ben

>  	if (enable) {
>  		if (mpam_partid_max < 1)
>  			return -EINVAL;
> @@ -1344,15 +1369,7 @@ static int mpam_resctrl_monitor_init(struct mpam_resctrl_mon *mon,
>  	l3->name = "L3";
>  	l3->mon_scope = RESCTRL_L3_CACHE;
>  
> -	/*
> -	 * num-rmid is the upper bound for the number of monitoring groups that
> -	 * can exist simultaneously, including the default monitoring group for
> -	 * each control group. Hence, advertise the whole rmid_idx space even
> -	 * though each control group has its own pmg/rmid space. Unfortunately,
> -	 * this does mean userspace needs to know the architecture to correctly
> -	 * interpret this value.
> -	 */
> -	l3->mon.num_rmid = resctrl_arch_system_num_rmid_idx();
> +	mpam_resctrl_set_num_rmid();
>  
>  	if (type == QOS_L3_MBM_TOTAL_EVENT_ID) {
>  		int err;