[PATCH v3 27/47] arm_mpam: resctrl: Add support for 'MB' resource

Ben Horgan
2026-01-12T17:02:20Z

From: James Morse <james.morse@arm.com>

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 or bandwidth portion bitmaps. 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>>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v2:
Code flow change
Commit message 'or'
---
drivers/resctrl/mpam_resctrl.c | 209 ++++++++++++++++++++++++++++++++-
1 file changed, 208 insertions(+), 1 deletion(-)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 3ca977527698..7402bf4293b6 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -248,6 +248,33 @@ 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
@@ -298,6 +325,94 @@ static u16 percent_to_mbw_max(u8 pc, struct mpam_props *cprops)
return val;
}

+static u32 get_mba_min(struct mpam_props *cprops)
+{
+ if (!mba_class_use_mbw_max(cprops)) {
+ WARN_ON_ONCE(1);
+ return 0;
+ }
+
+ return mbw_max_to_percent(0, cprops);
+}
+
+/* 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)
{
@@ -340,9 +455,62 @@ 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)
{
struct mpam_class *class = res->class;
+ struct mpam_props *cprops = &class->props;
struct rdt_resource *r = &res->resctrl_res;

switch (r->rid) {
@@ -372,6 +540,19 @@ static int mpam_resctrl_control_init(struct mpam_resctrl_res *res)
*/
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:
return -EINVAL;
}
@@ -386,7 +567,17 @@ 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. */
return comp->comp_id;
}
@@ -426,6 +617,12 @@ 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);
}
@@ -437,6 +634,8 @@ 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);
}
@@ -481,6 +680,13 @@ 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;
default:
return -EINVAL;
}
@@ -764,6 +970,7 @@ 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_each_mpam_resctrl_control(res, rid) {
--
2.43.0