[RFC PATCH v3 05/10] sched/fair: Don't consider paravirt CPUs for wakeup and load balance

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

load balancer for fair class looks at sched domain and active cpus to consider
spreading the load. mask out the paravirt CPUs so that tasks doesn't spread to
those.

At wakeup, don't select a paravirt CPU.

Expect minimal impact when it is disabled.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df8dc389af8e..3dc76525b32c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8563,7 +8563,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		if (!is_rd_overutilized(this_rq()->rd)) {
 			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
 			if (new_cpu >= 0)
-				return new_cpu;
+				goto check_new_cpu;
 			new_cpu = prev_cpu;
 		}
 
@@ -8605,7 +8605,12 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	}
 	rcu_read_unlock();
 
-	return new_cpu;
+	/* If newly found or prev_cpu is a paravirt cpu, use current cpu */
+check_new_cpu:
+	if (is_cpu_paravirt(new_cpu))
+		return cpu;
+	else
+		return new_cpu;
 }
 
 /*
@@ -11734,6 +11739,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 
 	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
 
+#ifdef CONFIG_PARAVIRT
+	/* Don't spread load to paravirt CPUs */
+	if (static_branch_unlikely(&cpu_paravirt_push_tasks))
+		cpumask_andnot(cpus, cpus, cpu_paravirt_mask);
+#endif
+
 	schedstat_inc(sd->lb_count[idle]);
 
 redo:
-- 
2.47.3