Re: [RFC PATCH 16/38] arm_mpam: resctrl: Add support for 'MB' resource

Gavin Shan
2025-12-12T04:27:24Z

Hi James and Ben,

On 12/6/25 7:58 AM, James Morse wrote:
> resctrl supports 'MB', as a percentage throttling of traffic somewhere
> after the L3. This is the control that mba_sc uses, so ideally the
> class chosen should be as close as possible to the counters used for
> mba_local.
>
> MB's percentage control should be backed either with the fixed point
> fraction MBW_MAX. The bandwidth portion bitmaps is not used as its
> tricky to pick which bits to use to avoid contention, and may be
> possible to expose this as something other than a percentage in the
> future.
>
> CC: Zeng Heng <zengheng4@huawei.com>
> Co-developed-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>>
> ---
> drivers/resctrl/mpam_resctrl.c | 212 ++++++++++++++++++++++++++++++++-
> 1 file changed, 211 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 55576d0caf12..b9f3f00d8cad 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -247,6 +247,33 @@ static bool cache_has_usable_cpor(struct mpam_class *class)
> return (class->props.cpbm_wd <= 32);
> }
>
> +static bool mba_class_use_mbw_max(struct mpam_props *cprops)
> +{
> + return (mpam_has_feature(mpam_feat_mbw_max, cprops) &&
> + cprops->bwa_wd);
> +}
> +
> +static bool class_has_usable_mba(struct mpam_props *cprops)
> +{
> + return mba_class_use_mbw_max(cprops);
> +}
> +
> +/*
> + * Calculate the worst-case percentage change from each implemented step
> + * in the control.
> + */
> +static u32 get_mba_granularity(struct mpam_props *cprops)
> +{
> + if (!mba_class_use_mbw_max(cprops))
> + return 0;
> +
> + /*
> + * bwa_wd is the number of bits implemented in the 0.xxx
> + * fixed point fraction. 1 bit is 50%, 2 is 25% etc.
> + */
> + return DIV_ROUND_UP(MAX_MBA_BW, 1 << cprops->bwa_wd);
> +}
> +
> /*
> * Each fixed-point hardware value architecturally represents a range
> * of values: the full range 0% - 100% is split contiguously into
> @@ -287,6 +314,96 @@ static u16 percent_to_mbw_max(u8 pc, struct mpam_props *cprops)
> return val;
> }
>
> +static u32 get_mba_min(struct mpam_props *cprops)
> +{
> + u32 val = 0;
> +
> + if (mba_class_use_mbw_max(cprops))
> + val = mbw_max_to_percent(val, cprops);
> + else
> + WARN_ON_ONCE(1);
> +
> + return val;
> +}
> +
> +/* Find the L3 cache that has affinity with this CPU */
> +static int find_l3_equivalent_bitmask(int cpu, cpumask_var_t tmp_cpumask)
> +{
> + u32 cache_id = get_cpu_cacheinfo_id(cpu, 3);
> +
> + lockdep_assert_cpus_held();
> +
> + return mpam_get_cpumask_from_cache_id(cache_id, 3, tmp_cpumask);
> +}
> +
> +/*
> + * topology_matches_l3() - Is the provided class the same shape as L3
> + * @victim: The class we'd like to pretend is L3.
> + *
> + * resctrl expects all the world's a Xeon, and all counters are on the
> + * L3. We play fast and loose with this, mapping counters on other
> + * classes - provided the CPU->domain mapping is the same kind of shape.
> + *
> + * Using cacheinfo directly would make this work even if resctrl can't
> + * use the L3 - but cacheinfo can't tell us anything about offline CPUs.
> + * Using the L3 resctrl domain list also depends on CPUs being online.
> + * Using the mpam_class we picked for L3 so we can use its domain list
> + * assumes that there are MPAM controls on the L3.
> + * Instead, this path eventually uses the mpam_get_cpumask_from_cache_id()
> + * helper which can tell us about offline CPUs ... but getting the cache_id
> + * to start with relies on at least one CPU per L3 cache being online at
> + * boot.
> + *
> + * Walk the victim component list and compare the affinity mask with the
> + * corresponding L3. The topology matches if each victim:component's affinity
> + * mask is the same as the CPU's corresponding L3's. These lists/masks are
> + * computed from firmware tables so don't change at runtime.
> + */
> +static bool topology_matches_l3(struct mpam_class *victim)
> +{
> + int cpu, err;
> + struct mpam_component *victim_iter;
> + cpumask_var_t __free(free_cpumask_var) tmp_cpumask;
> +
> + if (!alloc_cpumask_var(&tmp_cpumask, GFP_KERNEL))
> + return false;
> +
> + guard(srcu)(&mpam_srcu);
> + list_for_each_entry_srcu(victim_iter, &victim->components, class_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + if (cpumask_empty(&victim_iter->affinity)) {
> + pr_debug("class %u has CPU-less component %u - can't match L3!\n",
> + victim->level, victim_iter->comp_id);
> + return false;
> + }
> +
> + cpu = cpumask_any(&victim_iter->affinity);
> + if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
> + return false;
> +
> + cpumask_clear(tmp_cpumask);
> + err = find_l3_equivalent_bitmask(cpu, tmp_cpumask);
> + if (err) {
> + pr_debug("Failed to find L3's equivalent component to class %u component %u\n",
> + victim->level, victim_iter->comp_id);
> + return false;
> + }
> +
> + /* Any differing bits in the affinity mask? */
> + if (!cpumask_equal(tmp_cpumask, &victim_iter->affinity)) {
> + pr_debug("class %u component %u has Mismatched CPU mask with L3 equivalent\n"
> + "L3:%*pbl != victim:%*pbl\n",
> + victim->level, victim_iter->comp_id,
> + cpumask_pr_args(tmp_cpumask),
> + cpumask_pr_args(&victim_iter->affinity));
> +
> + return false;
> + }
> + }
> +
> + return true;
> +}
> +
> /* Test whether we can export MPAM_CLASS_CACHE:{2,3}? */
> static void mpam_resctrl_pick_caches(void)
> {
> @@ -330,10 +447,63 @@ static void mpam_resctrl_pick_caches(void)
> }
> }
>
> +static void mpam_resctrl_pick_mba(void)
> +{
> + struct mpam_class *class, *candidate_class = NULL;
> + struct mpam_resctrl_res *res;
> +
> + lockdep_assert_cpus_held();
> +
> + guard(srcu)(&mpam_srcu);
> + list_for_each_entry_srcu(class, &mpam_classes, classes_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + struct mpam_props *cprops = &class->props;
> +
> + if (class->level < 3) {
> + pr_debug("class %u is before L3\n", class->level);
> + continue;
> + }
> +
> + if (!class_has_usable_mba(cprops)) {
> + pr_debug("class %u has no bandwidth control\n",
> + class->level);
> + continue;
> + }
> +
> + if (!cpumask_equal(&class->affinity, cpu_possible_mask)) {
> + pr_debug("class %u has missing CPUs\n", class->level);
> + continue;
> + }
> +
> + if (!topology_matches_l3(class)) {
> + pr_debug("class %u topology doesn't match L3\n",
> + class->level);
> + continue;
> + }
> +
> + /*
> + * mba_sc reads the mbm_local counter, and waggles the MBA
> + * controls. mbm_local is implicitly part of the L3, pick a
> + * resource to be MBA that as close as possible to the L3.
> + */
> + if (!candidate_class || class->level < candidate_class->level)
> + candidate_class = class;
> + }
> +
> + if (candidate_class) {
> + pr_debug("selected class %u to back MBA\n",
> + candidate_class->level);
> + res = &mpam_resctrl_controls[RDT_RESOURCE_MBA];
> + res->class = candidate_class;
> + exposed_alloc_capable = true;
> + }
> +}
> +
> static int mpam_resctrl_control_init(struct mpam_resctrl_res *res,
> enum resctrl_res_level type)
> {
> struct mpam_class *class = res->class;
> + struct mpam_props *cprops = &class->props;
> struct rdt_resource *r = &res->resctrl_res;
>
> switch (res->resctrl_res.rid) {
> @@ -362,6 +532,20 @@ static int mpam_resctrl_control_init(struct mpam_resctrl_res *res,
> * 'all the bits' is the correct answer here.
> */
> r->cache.shareable_bits = resctrl_get_default_ctrl(r);
> + break;
> + case RDT_RESOURCE_MBA:
> + r->alloc_capable = true;
> + r->schema_fmt = RESCTRL_SCHEMA_RANGE;
> + r->ctrl_scope = RESCTRL_L3_CACHE;
> +
> + r->membw.delay_linear = true;
> + r->membw.throttle_mode = THREAD_THROTTLE_UNDEFINED;
> + r->membw.min_bw = get_mba_min(cprops);
> + r->membw.max_bw = MAX_MBA_BW;
> + r->membw.bw_gran = get_mba_granularity(cprops);
> +
> + r->name = "MB";
> +
> break;
> default:
> break;
> @@ -377,7 +561,17 @@ static int mpam_resctrl_pick_domain_id(int cpu, struct mpam_component *comp)
> if (class->type == MPAM_CLASS_CACHE)
> return comp->comp_id;
>
> - /* TODO: repaint domain ids to match the L3 domain ids */
> + if (topology_matches_l3(class)) {
> + /* Use the corresponding L3 component ID as the domain ID */
> + int id = get_cpu_cacheinfo_id(cpu, 3);
> +
> + /* Implies topology_matches_l3() made a mistake */
> + if (WARN_ON_ONCE(id == -1))
> + return comp->comp_id;
> +
> + return id;
> + }
> +
> /*
> * Otherwise, expose the ID used by the firmware table code.
> */
> @@ -419,6 +613,12 @@ u32 resctrl_arch_get_config(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> case RDT_RESOURCE_L3:
> configured_by = mpam_feat_cpor_part;
> break;
> + case RDT_RESOURCE_MBA:
> + if (mpam_has_feature(mpam_feat_mbw_max, cprops)) {
> + configured_by = mpam_feat_mbw_max;
> + break;
> + }
> + fallthrough;
> default:
> return resctrl_get_default_ctrl(r);
> }
> @@ -430,6 +630,8 @@ u32 resctrl_arch_get_config(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> switch (configured_by) {
> case mpam_feat_cpor_part:
> return cfg->cpbm;
> + case mpam_feat_mbw_max:
> + return mbw_max_to_percent(cfg->mbw_max, cprops);
> default:
> return resctrl_get_default_ctrl(r);
> }
> @@ -474,6 +676,13 @@ int resctrl_arch_update_one(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> cfg.cpbm = cfg_val;
> mpam_set_feature(mpam_feat_cpor_part, &cfg);
> break;
> + case RDT_RESOURCE_MBA:
> + if (mpam_has_feature(mpam_feat_mbw_max, cprops)) {
> + cfg.mbw_max = percent_to_mbw_max(cfg_val, cprops);
> + mpam_set_feature(mpam_feat_mbw_max, &cfg);
> + break;
> + }
> + fallthrough;

I think mpam_feat_mbw_min peroperly need to be cleared in '&cfg', whose content
is copied from that of the component. mpam_feat_mbw_min may have been existing
in '&cfg' and struct mpam_config::mbw_min won't be updated correctly in the
subsequent call mpam_extend_config(). It means register MPAMCFG_MBW_MIN isn't
updated correctly.

On NVidia's grace-hopper machine, I got:

host$ mount none -tresctrl /sys/fs/resctrl/
host$ mkdir -p /sys/fs/resctrl/all
host$ mkdir -p /sys/fs/resctrl/test
host$ cat /proc/dump_feat_regs
MPAMF_IDR 0000008057010027
MAPMF_MBW_IDR 00000c07

host$ echo "MB:1=98" > /sys/fs/resctrl/test/schemata
host$ cat /proc/dump_cfg_regs
MPAMCFG_PART_SEL 00000002
MPAMCFG_MBW_MAX 0000f9ff
MPAMCFG_MBW_MIN 0000f000

host$ echo "MB:1=2" > /sys/fs/resctrl/test/schemata
host$ cat /proc/dump_cfg_regs
MPAMCFG_PART_SEL 00000002
MPAMCFG_MBW_MAX 000005ff
MPAMCFG_MBW_MIN 0000f000

With 'mpam_clear_feature(mpam_feat_mbw_min, &cfg);' applied here, the register
can be updated correctly. It also makes my (soft) MBW limiting tests happy.

host$ echo "MB:1=98" > /sys/fs/resctrl/test/schemata
host$ cat /proc/dump_cfg_regs
MPAMCFG_PART_SEL 00000002
MPAMCFG_MBW_MAX 0000f9ff
MPAMCFG_MBW_MIN 0000ea00

host$ echo "MB:1=2" > /sys/fs/resctrl/test/schemata
host$ cat /proc/dump_cfg_regs
MPAMCFG_PART_SEL 00000002
MPAMCFG_MBW_MAX 000005ff
MPAMCFG_MBW_MIN 00000200

Thanks,
Gavin

> default:
> return -EINVAL;
> }
> @@ -743,6 +952,7 @@ int mpam_resctrl_setup(void)
>
> /* Find some classes to use for controls */
> mpam_resctrl_pick_caches();
> + mpam_resctrl_pick_mba();
>
> /* Initialise the resctrl structures from the classes */
> for (i = 0; i < RDT_NUM_RESOURCES; i++) {