[PATCH v7 7/9] fs/resctrl: Do not invoke smp_processor_id() in preemptible context

[Chen Yu]

From: Tony Luck <tony.luck@xxxxxxxxx>

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, and report a wrong CPU via rmid_read::err.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Tested-by: Hongyu Ning <hongyu.ning@xxxxxxxxxxxxxxx>
---
v6->v7:
    Describe the current code instead of framing the changelog as a bug
    fix. (Reinette Chatre)
    Drop the now-merged lore.kernel.org link. (Reinette Chatre)
---
 fs/resctrl/monitor.c | 44 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index a932a1fea818..c08d2f90d721 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -422,9 +422,37 @@ 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_on_correct_domain() - Check if current CPU is in the correct
+ *			      domain for 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_on_correct_domain(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;
@@ -457,9 +485,6 @@ 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))
-		return -EINVAL;
 	if (rr->is_mbm_cntr)
 		rr->err = resctrl_arch_cntr_read(rr->r, d, closid, rmid, cntr_id,
 						 rr->evt->evtid, &tval);
@@ -477,7 +502,6 @@ 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;
@@ -495,10 +519,6 @@ 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))
-		return -EINVAL;
-
 	/*
 	 * Legacy files must report the sum of an event across all
 	 * domains that share the same L3 cache instance.
@@ -529,7 +549,11 @@ 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_on_correct_domain(rr)) {
+			rr->err = -EIO;
+			return -EINVAL;
+		}
+
 		if (rr->hdr)
 			return __l3_mon_event_count(rdtgrp, rr);
 		else
-- 
2.25.1