[PATCH v4 06/20] sched/core: allow only preferred CPUs in is_cpu_allowed

[Shrikanth Hegde]

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
intersect or not.

Since is_cpu_allowed can be called directly or repeatedly in
select_fallback_rq, encode the info in task_struct->has_preferred_cpu_state
if the path is via select_fallback_rq or not.
This helps to avoid N**2 complexity for the rare cases.

Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
---
v3->v4:
- Missing case of PF_KTHREAD is avoided.
- Add a new field in task_struct which encodes intersection of
  tasks affinity and preferred CPUs and path its coming from.

 include/linux/sched.h |  1 +
 kernel/sched/core.c   | 34 ++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h  | 18 ++++++++++++++++++
 3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index fc6ecb3869dd..2d0b1a6d50ac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1657,6 +1657,7 @@ struct task_struct {
 #ifdef CONFIG_UNWIND_USER
 	struct unwind_task_info		unwind_info;
 #endif
+	int				has_preferred_cpu_state;
 
 	/* CPU-specific state of this task: */
 	struct thread_struct		thread;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9e16946c9d62..714816cfa975 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2500,6 +2500,8 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
  */
 static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 {
+	bool task_check_preferred_cpu = false;
+
 	/* When not in the task's cpumask, no point in looking further. */
 	if (!task_allowed_on_cpu(p, cpu))
 		return false;
@@ -2508,9 +2510,22 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (is_migration_disabled(p))
 		return cpu_online(cpu);
 
+	/*
+	 * This is essential to maintain user affinities when preferred
+	 * CPUs change. A task pinned on non-preferred CPU should continue
+	 * to run there, since this is non-user triggered.
+	 *
+	 * If CPU is non-preferred and task can run on other CPUs which are
+	 * currently preferred, then choose those other CPUs instead
+	 */
+	task_check_preferred_cpu = !cpu_preferred(cpu) && task_has_preferred_cpus(p);
+
 	/* Non kernel threads are not allowed during either online or offline. */
-	if (!(p->flags & PF_KTHREAD))
+	if (!(p->flags & PF_KTHREAD)) {
+		if (task_check_preferred_cpu)
+			return false;
 		return cpu_active(cpu);
+	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2520,6 +2535,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (cpu_dying(cpu))
 		return false;
 
+	/* Try on preferred CPU first if possible*/
+	if (task_check_preferred_cpu)
+		return false;
+
 	/* But are allowed during online. */
 	return cpu_online(cpu);
 }
@@ -3549,6 +3568,14 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	enum { cpuset, possible, fail } state = cpuset;
 	int dest_cpu;
 
+	/*
+	 * Cache value whether task's affinity spans preferred CPUs.
+	 * This helps to avoid repeating the same for each CPU
+	 * later in the loop. Encode call to is_cpu_allowed coming
+	 * via select_fallback_rq.
+	 */
+	p->has_preferred_cpu_state = task_has_preferred_cpus(p) << 8 | 0x1;
+
 	/*
 	 * If the node that the CPU is on has been offlined, cpu_to_node()
 	 * will return -1. There is no CPU on the node, and we should
@@ -3560,7 +3587,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 		/* Look for allowed, online CPU in same node. */
 		for_each_cpu(dest_cpu, nodemask) {
 			if (is_cpu_allowed(p, dest_cpu))
-				return dest_cpu;
+				goto clear_and_return;
 		}
 	}
 
@@ -3604,6 +3631,8 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 		}
 	}
 
+clear_and_return:
+	p->has_preferred_cpu_state = 0;
 	return dest_cpu;
 }
 
@@ -4612,6 +4641,7 @@ static void __sched_fork(u64 clone_flags, struct task_struct *p)
 	init_numa_balancing(clone_flags, p);
 	p->wake_entry.u_flags = CSD_TYPE_TTWU;
 	p->migration_pending = NULL;
+	p->has_preferred_cpu_state = 0;
 	init_sched_mm(p);
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c7c2dea65edd..38fd84b0b8f8 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -4213,4 +4213,22 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
 
 #include "ext.h"
 
+/*
+ * has_preferred_cpu_state is encoding two bits of information.
+ * First Byte is to encode where the call to is_cpu_allowed coming from.
+ * Second Byte is to encode the intersection of task affinity
+ * and cpu_preferred_mask.
+ *
+ * If 1st Byte is set, call to is_cpu_allowed coming from select_fallback_rq.
+ * That helps to avoid repeated calculation keeping time complexity same.
+ */
+static inline bool task_has_preferred_cpus(struct task_struct *p)
+{
+	int cached_value = p->has_preferred_cpu_state;
+
+	if (cached_value & 0x1)
+		return p->has_preferred_cpu_state >> 8;
+	else
+		return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
+}
 #endif /* _KERNEL_SCHED_SCHED_H */
-- 
2.47.3