[PATCH v9 10/12] fs/resctrl: Do not invoke smp_processor_id() in preemptible context

[Chen Yu <yu.c.chen@intel.com>]

From: Tony Luck <tony.luck@intel.com>

An event flagged with mon_evt::any_cpu is read directly in preemptible task
context instead of being dispatched to a CPU of the monitoring domain. Only
Intel AET events set this flag, so __mon_event_count() rejects an any_cpu L3
event with WARN_ON_ONCE(). The L3 read paths can therefore call
smp_processor_id() unconditionally to check that the current CPU belongs to the
domain being read.

MMIO-based LLC occupancy counters can be read from any CPU, so the L3 read
paths have to tolerate an any_cpu event. Move that check into a helper which
returns early for any_cpu events, avoiding smp_processor_id() in preemptible
context.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
---
Changes since v8:
- Rename cpu_on_correct_domain() to cpu_can_read_event() because the
  domain is not always relevant to the answer. (Reinette Chatre)
- Drop "report a wrong CPU via rmid_read::err" from the changelog, it is
  obvious from the diff. (Reinette Chatre)

 fs/resctrl/monitor.c | 47 +++++++++++++++++++++++++++++++-------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 73413cb128ea..5060640b4f50 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -422,9 +422,36 @@ static void mbm_cntr_free(struct rdt_l3_mon_domain *d, int cntr_id)
 	memset(&d->cntr_cfg[cntr_id], 0, sizeof(*d->cntr_cfg));
 }
 
+/**
+ * cpu_can_read_event() - Check if the current CPU can read the event.
+ * @rr: The rmid_read structure containing event and domain information.
+ *
+ * Context: Preemptible process context when @rr->evt->any_cpu is set.
+ *          Non-migratable process context (via smp_call_on_cpu()) or
+ *          non-preemptible context (via smp_call_function_any()) when
+ *          the event must be read on a specific CPU.
+ * Return: true if the current CPU can read this event, false otherwise.
+ */
+static bool cpu_can_read_event(struct rmid_read *rr)
+{
+	int cpu;
+
+	/* Any CPU is OK for this event */
+	if (rr->evt->any_cpu)
+		return true;
+
+	cpu = smp_processor_id();
+
+	/* Single domain. Must be on a CPU in that domain. */
+	if (rr->hdr)
+		return cpumask_test_cpu(cpu, &rr->hdr->cpu_mask);
+
+	/* Summing domains that share a cache, must be on a CPU for that cache. */
+	return cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map);
+}
+
 static int __l3_mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 {
-	int cpu = smp_processor_id();
 	u32 closid = rdtgrp->closid;
 	u32 rmid = rdtgrp->mon.rmid;
 	struct rdt_l3_mon_domain *d;
@@ -457,11 +484,6 @@ static int __l3_mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 		return 0;
 	}
 
-	/* Reading a single domain, must be on a CPU in that domain. */
-	if (!cpumask_test_cpu(cpu, &d->hdr.cpu_mask)) {
-		rr->err = -EIO;
-		return -EINVAL;
-	}
 	if (rr->is_mbm_cntr)
 		rr->err = resctrl_arch_cntr_read(rr->r, d, closid, rmid, cntr_id,
 						 rr->evt->evtid, &tval);
@@ -479,7 +501,6 @@ static int __l3_mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 
 static int __l3_mon_event_count_sum(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 {
-	int cpu = smp_processor_id();
 	u32 closid = rdtgrp->closid;
 	u32 rmid = rdtgrp->mon.rmid;
 	struct rdt_l3_mon_domain *d;
@@ -497,12 +518,6 @@ static int __l3_mon_event_count_sum(struct rdtgroup *rdtgrp, struct rmid_read *r
 		return -EINVAL;
 	}
 
-	/* Summing domains that share a cache, must be on a CPU for that cache. */
-	if (!cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map)) {
-		rr->err = -EIO;
-		return -EINVAL;
-	}
-
 	/*
 	 * Legacy files must report the sum of an event across all
 	 * domains that share the same L3 cache instance.
@@ -538,7 +553,11 @@ static int __mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 {
 	switch (rr->r->rid) {
 	case RDT_RESOURCE_L3:
-		WARN_ON_ONCE(rr->evt->any_cpu);
+		if (!cpu_can_read_event(rr)) {
+			rr->err = -EIO;
+			return -EINVAL;
+		}
+
 		if (rr->hdr)
 			return __l3_mon_event_count(rdtgrp, rr);
 		else
-- 
2.43.0