[RFC PATCH 24/38] arm_mpam: resctrl: Allow resctrl to allocate monitors

James Morse
2025-12-05T22:01:06Z

When resctrl wants to read a domain's 'QOS_L3_OCCUP', it needs
to allocate a monitor on the corresponding resource. Monitors are
allocated by class instead of component.

MBM monitors are much more complicated, if there are enough monitors,
they will be pre-allocated and free-running. If ABMC is in use instead
then 'some' are pre-allocated in a different way, and need assigning.

Add helpers to allocate a CSU monitor. These helper return an out
of range value for MBM counters.

Allocating a montitor context is expected to block until hardware
resources become available. This only makes sense for QOS_L3_OCCUP
as unallocated MBM counters are losing data.

Signed-off-by: James Morse <james.morse@arm.com>
---
Since ABMC got merged it may be possible to remove the monitor alloc
call for MBM counters from resctrl as this work is now done by ABMC's
assign call.
---
drivers/resctrl/mpam_internal.h | 14 ++++++-
drivers/resctrl/mpam_resctrl.c | 68 +++++++++++++++++++++++++++++++++
include/linux/arm_mpam.h | 4 ++
3 files changed, 85 insertions(+), 1 deletion(-)

diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 05101186af17..3a68ebd498fa 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -32,6 +32,14 @@ DECLARE_STATIC_KEY_FALSE(mpam_enabled);
#define PACKED_FOR_KUNIT
#endif

+/*
+ * This 'mon' values must not alias an actual monitor, so must be larger than
+ * U16_MAX, but not be confused with an errno value, so smaller than
+ * (u32)-SZ_4K.
+ * USE_PRE_ALLOCATED is used to avoid confusion with an actual monitor.
+ */
+#define USE_PRE_ALLOCATED (U16_MAX + 1)
+
static inline bool mpam_is_enabled(void)
{
return static_branch_likely(&mpam_enabled);
@@ -215,7 +223,11 @@ enum mon_filter_options {
};

struct mon_cfg {
- u16 mon;
+ /*
+ * mon must be large enough to hold out of range values like
+ * USE_RMID_IDX
+ */
+ u32 mon;
u8 pmg;
bool match_pmg;
bool csu_exclude_clean;
diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 22ad5dd3c383..a2b238d47117 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -21,6 +21,8 @@

#include "mpam_internal.h"

+DECLARE_WAIT_QUEUE_HEAD(resctrl_mon_ctx_waiters);
+
/*
* The classes we've picked to map to resctrl resources, wrapped
* in with their resctrl structure.
@@ -287,6 +289,72 @@ struct rdt_resource *resctrl_arch_get_resource(enum resctrl_res_level l)
return &mpam_resctrl_controls[l].resctrl_res;
}

+static int resctrl_arch_mon_ctx_alloc_no_wait(enum resctrl_event_id evtid)
+{
+ struct mpam_resctrl_mon *mon = &mpam_resctrl_counters[evtid];
+
+ if (!mon->class)
+ return -EINVAL;
+
+ switch (evtid) {
+ case QOS_L3_OCCUP_EVENT_ID:
+ /* With CDP, one monitor gets used for both code/data reads */
+ return mpam_alloc_csu_mon(mon->class);
+ case QOS_L3_MBM_LOCAL_EVENT_ID:
+ case QOS_L3_MBM_TOTAL_EVENT_ID:
+ return USE_PRE_ALLOCATED;
+ default:
+ return -EOPNOTSUPP;
+ }
+}
+
+void *resctrl_arch_mon_ctx_alloc(struct rdt_resource *r,
+ enum resctrl_event_id evtid)
+{
+ DEFINE_WAIT(wait);
+ int *ret;
+
+ ret = kmalloc(sizeof(*ret), GFP_KERNEL);
+ if (!ret)
+ return ERR_PTR(-ENOMEM);
+
+ do {
+ prepare_to_wait(&resctrl_mon_ctx_waiters, &wait,
+ TASK_INTERRUPTIBLE);
+ *ret = resctrl_arch_mon_ctx_alloc_no_wait(evtid);
+ if (*ret == -ENOSPC)
+ schedule();
+ } while (*ret == -ENOSPC && !signal_pending(current));
+ finish_wait(&resctrl_mon_ctx_waiters, &wait);
+
+ return ret;
+}
+
+static void resctrl_arch_mon_ctx_free_no_wait(enum resctrl_event_id evtid,
+ u32 mon_idx)
+{
+ struct mpam_resctrl_mon *mon = &mpam_resctrl_counters[evtid];
+
+ if (!mon->class)
+ return;
+
+ if (evtid == QOS_L3_OCCUP_EVENT_ID)
+ mpam_free_csu_mon(mon->class, mon_idx);
+
+ wake_up(&resctrl_mon_ctx_waiters);
+}
+
+void resctrl_arch_mon_ctx_free(struct rdt_resource *r,
+ enum resctrl_event_id evtid, void *arch_mon_ctx)
+{
+ u32 mon_idx = *(u32 *)arch_mon_ctx;
+
+ kfree(arch_mon_ctx);
+ arch_mon_ctx = NULL;
+
+ resctrl_arch_mon_ctx_free_no_wait(evtid, mon_idx);
+}
+
static bool cache_has_usable_cpor(struct mpam_class *class)
{
struct mpam_props *cprops = &class->props;
diff --git a/include/linux/arm_mpam.h b/include/linux/arm_mpam.h
index 385554ceb452..e1461e32af75 100644
--- a/include/linux/arm_mpam.h
+++ b/include/linux/arm_mpam.h
@@ -63,6 +63,10 @@ u32 resctrl_arch_rmid_idx_encode(u32 closid, u32 rmid);
void resctrl_arch_rmid_idx_decode(u32 idx, u32 *closid, u32 *rmid);
u32 resctrl_arch_system_num_rmid_idx(void);

+struct rdt_resource;
+void *resctrl_arch_mon_ctx_alloc(struct rdt_resource *r, enum resctrl_event_id evtid);
+void resctrl_arch_mon_ctx_free(struct rdt_resource *r, enum resctrl_event_id evtid, void *ctx);
+
/**
* mpam_register_requestor() - Register a requestor with the MPAM driver
* @partid_max: The maximum PARTID value the requestor can generate.
--
2.39.5