[RFC PATCH 5/6] fs/resctrl: Do not invoke smp_processor_id() in preemptible context

[Chen Yu]

From: Tony Luck <tony.luck@xxxxxxxxx>

__l3_mon_event_count() and __l3_mon_event_count_sum() call
smp_processor_id() to obtain the current CPU. However, some
monitor events can be read from any CPU in task context via
mon_event_count(); in that case the calling context is
preemptible and smp_processor_id() triggers a debug warning.

Fix this by skipping the current-CPU lookup when the event's
any_cpu flag is set, since such events do not need to run on a
specific CPU.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 fs/resctrl/monitor.c | 38 ++++++++++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 10 deletions(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 9fd901c78dc6..371ccae04892 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -417,9 +417,33 @@ 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));
 }
 
+/*
+ * Called from preemptible context via a direct call of mon_event_count() for
+ * events that can be read on any CPU.
+ * Called from preemptible but non-migratable process context (mon_event_count()
+ * via smp_call_on_cpu()) OR non-preemptible context (mon_event_count() via
+ * smp_call_function_any()) for events that need to be read on a specific CPU.
+ */
+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;
@@ -452,9 +476,6 @@ 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);
@@ -472,7 +493,6 @@ 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;
@@ -490,10 +510,6 @@ 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.
@@ -524,7 +540,9 @@ 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))
+			return -EINVAL;
+
 		if (rr->hdr)
 			return __l3_mon_event_count(rdtgrp, rr);
 		else
-- 
2.25.1