Re: [PATCH v4 3/3] arm_mpam: resctrl: Limit num_rmids to the CLOSIDs in use

[Gavin Shan <gshan@redhat.com>]

On 11/11/26 6:13 PM, 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.
> 
> Count the CLOSIDs resctrl uses instead: every resource has
> mpam_partid_max + 1 CLOSIDs, halved and rounded down while CDP is
> enabled, and a system with no resource that can allocate uses only the
> default CLOSID. Report (pmg_max + 1) RMIDs for each of those CLOSIDs.
> Count 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@huawei.com>
> Cc: <stable@vger.kernel.org> # 7.1+
> Cc: James Morse <james.morse@arm.com>
> Cc: Ben Horgan <ben.horgan@arm.com>
> Cc: Reinette Chatre <reinette.chatre@intel.com>
> Cc: Fenghua Yu <fenghuay@nvidia.com>
> ---
> v4:
> - Simplify the fix with mpam_partid_max, cdp_enabled
>    and resctrl_arch_alloc_capable() as Ben suggested.
> 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@huawei.com/
> 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@huawei.com/
> ---
>   drivers/resctrl/mpam_resctrl.c | 30 +++++++++++++++++-------------
>   1 file changed, 17 insertions(+), 13 deletions(-)

Reviewed-by: Gavin Shan <gshan@redhat.com>