[PATCH v6 23/23] virt/steal_monitor: Optimise decrease_preferred_cpus when all CPUs are housekeeping

[Shrikanth Hegde <sshegde@linux.ibm.com>]

In most of the configurations one wouldn't specify nohz_full= or
isolated= option. That makes all CPU are part of housekeeping domain.

Since decrease_preferred_cpus aims to find the last CPU which
doesn't belong to the first housekeeping core, that could be optimized
when all CPUs are housekeeping cores. Simply look at last CPU in
the preferred mask and check if it is housekeeping core or not.

Time taken for decrease_preferred_cpus on 480 CPU (60 Cores, SMT8):
Without optimization:  around 6us.
With optimization:  around 3us.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
v5->v6:
- Optimize for the common case where nohz_full=<empty>

 drivers/virt/steal_monitor/defaults.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/drivers/virt/steal_monitor/defaults.c b/drivers/virt/steal_monitor/defaults.c
index 70dcfb1ce4cb..5206a5f7af48 100644
--- a/drivers/virt/steal_monitor/defaults.c
+++ b/drivers/virt/steal_monitor/defaults.c
@@ -53,7 +53,7 @@ unsigned int __weak get_num_cpus_steal_ratio(void)
 
 void __weak decrease_preferred_cpus(struct steal_monitor *ctx)
 {
-	int tmp_cpu, first_hk_cpu;
+	int tmp_cpu, first_hk_cpu, last_cpu;
 	const struct cpumask *first_hk_core;
 	int target_cpu = nr_cpu_ids;
 
@@ -62,14 +62,30 @@ void __weak decrease_preferred_cpus(struct steal_monitor *ctx)
 	first_hk_cpu = cpumask_first_and(housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
 					 cpu_active_mask);
 
-	if (first_hk_cpu >= nr_cpu_ids)
+	last_cpu = cpumask_last(cpu_preferred_mask);
+
+	if (first_hk_cpu >= nr_cpu_ids || last_cpu >= nr_cpu_ids)
 		return;
 
 	first_hk_core = get_core_mask(first_hk_cpu);
 
-	/* Always leave first housekeeping core as preferred. */
-	for_each_cpu_andnot(tmp_cpu, cpu_preferred_mask, first_hk_core)
-		target_cpu = tmp_cpu;
+	/*
+	 * Always leave first housekeeping core as preferred.
+	 * In most configurations housekeeping core will be at beginning,
+	 * i.e there is no nohz_full= or isolcpus=.
+	 * Skip the loop in that case.
+	 */
+	if (!cpumask_test_cpu(last_cpu, first_hk_core)) {
+		target_cpu = last_cpu;
+	} else {
+		/*
+		 * Since one may specify nohz_full= for any set of CPUs,
+		 * Find the last CPU which doesn't belong to the
+		 * protected first housekeeping core.
+		 */
+		for_each_cpu_andnot(tmp_cpu, cpu_preferred_mask, first_hk_core)
+			target_cpu = tmp_cpu;
+	}
 
 	/* Only the first housekeeping core remains */
 	if (target_cpu >= nr_cpu_ids)
-- 
2.47.3