[PATCH v2 05/17] sched/core: allow only preferred CPUs in is_cpu_allowed

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

When possible, choose a preferred CPUs to pick.

Push task mechanism uses stopper thread which going to call
select_fallback_rq and use this mechanism to pick only a preferred CPU.

When task is affined only to non-preferred CPUs it should continue to
run there. Detect that by checking if cpus_ptr and cpu_preferred_mask
interesect or not.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/core.c  | 17 ++++++++++++++---
 kernel/sched/sched.h | 12 ++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7ea05a7a717b..336e7c694eb7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2463,9 +2463,16 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (is_migration_disabled(p))
 		return cpu_online(cpu);
 
-	/* Non kernel threads are not allowed during either online or offline. */
-	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu);
+	/*
+	 * Non kernel threads are not allowed during either online or offline.
+	 * Ensure it is a preferred CPU to avoid further contention
+	 */
+	if (!(p->flags & PF_KTHREAD)) {
+		if (!cpu_active(cpu))
+			return false;
+		if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
+			return false;
+	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2475,6 +2482,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (cpu_dying(cpu))
 		return false;
 
+	/* Try on preferred CPU first */
+	if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
+		return false;
+
 	/* But are allowed during online. */
 	return cpu_online(cpu);
 }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 88e0c93b9e21..7271af2ca64f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -4130,4 +4130,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
 
 #include "ext.h"
 
+#ifdef CONFIG_PARAVIRT
+static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
+{
+	return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+}
+#else
+static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
+{
+	return true;
+}
+#endif
+
 #endif /* _KERNEL_SCHED_SCHED_H */
-- 
2.47.3