[PATCH 1/9] 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
 same map 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 | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 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..9f4ee29 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -1236,15 +1236,15 @@ 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 */
-	}
+	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);
 }
 
 static void intel_cqm_cpu_prepare(unsigned int cpu)
@@ -1262,7 +1262,7 @@ 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);
+	static cpumask_t tmp;
 	int i;
 
 	/*
@@ -1271,15 +1271,12 @@ static void intel_cqm_cpu_exit(unsigned int cpu)
 	if (!cpumask_test_and_clear_cpu(cpu, &cqm_cpumask))
 		return;
 
-	for_each_online_cpu(i) {
-		if (i == cpu)
-			continue;
+	cpumask_and(&tmp, topology_core_cpumask(cpu), cpu_online_mask);
+	cpumask_clear_cpu(cpu, &tmp);
+	i = cpumask_any(&tmp);
 
-		if (phys_id == topology_physical_package_id(i)) {
-			cpumask_set_cpu(i, &cqm_cpumask);
-			break;
-		}
-	}
+	if (i < nr_cpu_ids)
+		cpumask_set_cpu(i, &cqm_cpumask);
 }
 
 static int intel_cqm_cpu_notifier(struct notifier_block *nb,
-- 
1.9.1

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