[PATCH 02/10] x86/intel_cqm: Modify hot cpu notification handling

[Vikas Shivappa]

This patch modifies hot cpu notification handling in Intel cache
monitoring:

 - to add a new cpu to the cqm_cpumask(which has one cpu per package)
  during cpu start, it uses the existing package<->core map instead of
  looping through all cpus in cqm_cpumask.
 - to search for the next online sibling during cpu exit, it uses the
 cpumask_any_online_but instead of looping through all online cpus.  In
 large systems with large number of cpus the time taken to loop may be
 expensive and also the time increase linearly.

Signed-off-by: Vikas Shivappa <vikas.shivappa@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event_intel_cqm.c | 47 +++++++++++++++---------------
 1 file changed, 24 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
index 1880761..3c38031 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -1236,15 +1236,21 @@ static struct pmu intel_cqm_pmu = {
 
 static inline void cqm_pick_event_reader(int cpu)
 {
-	int phys_id = topology_physical_package_id(cpu);
-	int i;
+	static cpumask_t tmp;
 
-	for_each_cpu(i, &cqm_cpumask) {
-		if (phys_id == topology_physical_package_id(i))
-			return;	/* already got reader for this socket */
-	}
+	/*
+	 * Grab the mutex so that the cqm_cpumask doesn't change
+	 * while intel_cqm_rmid_stabilize() is running.
+	 */
+	mutex_lock(&cache_mutex);
+	cpumask_and(&tmp, &cqm_cpumask, topology_core_cpumask(cpu));
 
-	cpumask_set_cpu(cpu, &cqm_cpumask);
+	/*
+	 * Pick a reader if there isn't one already.
+	 */
+	if (cpumask_empty(&tmp))
+		cpumask_set_cpu(cpu, &cqm_cpumask);
+	mutex_unlock(&cache_mutex);
 }
 
 static void intel_cqm_cpu_prepare(unsigned int cpu)
@@ -1262,24 +1268,21 @@ static void intel_cqm_cpu_prepare(unsigned int cpu)
 
 static void intel_cqm_cpu_exit(unsigned int cpu)
 {
-	int phys_id = topology_physical_package_id(cpu);
 	int i;
 
 	/*
 	 * Is @cpu a designated cqm reader?
 	 */
-	if (!cpumask_test_and_clear_cpu(cpu, &cqm_cpumask))
+	mutex_lock(&cache_mutex);
+	if (!cpumask_test_and_clear_cpu(cpu, &cqm_cpumask)) {
+		mutex_unlock(&cache_mutex);
 		return;
-
-	for_each_online_cpu(i) {
-		if (i == cpu)
-			continue;
-
-		if (phys_id == topology_physical_package_id(i)) {
-			cpumask_set_cpu(i, &cqm_cpumask);
-			break;
-		}
 	}
+
+	i = cpumask_any_online_but(topology_core_cpumask(cpu), cpu);
+	if (i < nr_cpu_ids)
+		cpumask_set_cpu(i, &cqm_cpumask);
+	mutex_unlock(&cache_mutex);
 }
 
 static int intel_cqm_cpu_notifier(struct notifier_block *nb,
@@ -1288,15 +1291,13 @@ static int intel_cqm_cpu_notifier(struct notifier_block *nb,
 	unsigned int cpu  = (unsigned long)hcpu;
 
 	switch (action & ~CPU_TASKS_FROZEN) {
-	case CPU_UP_PREPARE:
+	case CPU_ONLINE:
 		intel_cqm_cpu_prepare(cpu);
+		cqm_pick_event_reader(cpu);
 		break;
-	case CPU_DOWN_PREPARE:
+	case CPU_DEAD:
 		intel_cqm_cpu_exit(cpu);
 		break;
-	case CPU_STARTING:
-		cqm_pick_event_reader(cpu);
-		break;
 	}
 
 	return NOTIFY_OK;
-- 
1.9.1

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