[PATCH] arm_mpam: Round the CDP RMID count down to whole CLOSIDs

[Jinjie Ruan]

在 2026/11/4 22:21, Ben Horgan 写道:
> Hi Jinjie,

Hi Ben,

> 
> For patches touching the glue between MPAM and resctrl, essentially mpam_resctrl.c, the prefix used
> is 'arm_mpam: resctrl' rather than just 'arm_mpam'.

The following will be done in this format.

> 
> On 04/11/2026 10:16, Jinjie Ruan wrote:
>> resctrl_arch_set_cdp_enabled() halves the whole RMID index space, while
>> the resctrl core halves the CLOSID count: schemata_list_add() divides a
>> CDP resource's schema by two, and closid_init() takes the smallest
>> across the schemata.
>>
>> When the PMG count and the CLOSID count are both odd, halving the
>> product counts RMIDs that belong to a CLOSID resctrl never allocates.
>> With 3 PMGs and 5 CLOSIDs MPAM reports 7 RMIDs, but the 2 CLOSIDs that
>> resctrl has times 3 PMGs only allow 6. The count is visible in
>> /sys/fs/resctrl/info/L3_MON/num_rmids, so userspace reads a value that
>> includes RMIDs no CLOSID can use; 
> 
> Good spot.
> 
> It occurs to me that 'num_rmids' is also too high on a monitor only system too as there is only one
> PARTID that can be used.  Can you see a clean way to fix this in one go?

I will have a try to fix both problems in a clean way, thank you very much.

> 
> Thanks,
> 
> Ben
> 
> RMID allocation itself is bounded by
>> the free list.
>>
>> Derive the number of RMID groups from the index space and halve that, so
>> the count follows resctrl_arch_system_num_rmid_idx().
>>
>> Fixes: 264c285999fc ("arm_mpam: resctrl: Add monitor initialisation and domain boilerplate")
>> Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
>> Cc: <stable at vger.kernel.org> # 7.1.x
>> Cc: James Morse <james.morse at arm.com>
>> Cc: Ben Horgan <ben.horgan at arm.com>
>> Cc: Reinette Chatre <reinette.chatre at intel.com>
>> Cc: Fenghua Yu <fenghuay at nvidia.com>
>> ---
>>  drivers/resctrl/mpam_resctrl.c | 12 +++++++++---
>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
>> index 5468367776e2..d3f957162b68 100644
>> --- a/drivers/resctrl/mpam_resctrl.c
>> +++ b/drivers/resctrl/mpam_resctrl.c
>> @@ -217,10 +217,16 @@ 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
>> +	if (enable) {
>> +		u32 num_group;
>> +
>> +		/* Halve the number of RMID groups, keeping whole PMG rows. */
>> +		num_group = resctrl_arch_system_num_rmid_idx() /
>> +			    (mpam_pmg_max + 1);
>> +		l3->mon.num_rmid = (mpam_pmg_max + 1) * (num_group / 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)
> 

-- 
Best regards,
Jinjie