[PATCH 1/2] swqueue: Control if a task should be queued on swq in select_idle_sibling()

[Gautham R. Shenoy]

If select_idle_sibling() fails to find an idle CPU to wakeup the task
on, then update the newly defined sched_add_to_swq field in its task
struct.

Use the value in this field to later on to determine if the task
should also be queued on the shared-wakequeue of the LLC of the target
CPU.

This extends the use of shared-wakequeue to cases when the target of a
wakeup is the current CPU instead of the task's previous CPU.

Signed-off-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
---
 include/linux/sched.h |  6 ++++++
 kernel/sched/fair.c   | 15 ++++++++-------
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b64fec55a381..38005262a7fe 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -910,6 +910,12 @@ struct task_struct {
 	 */
 	unsigned			sched_remote_wakeup:1;
 
+	/*
+	 * Bit used by select_idle_sibling() to signal enqueuing the
+	 * task on a shared wakequeue when it failed find an idle CPU.
+	 */
+	unsigned			sched_add_to_swq:1;
+
 	/* Bit to tell LSMs we're in execve(): */
 	unsigned			in_execve:1;
 	unsigned			in_iowait:1;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e311d1c3b992..fe33f6b13299 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -215,21 +215,17 @@ static void swqueue_enqueue(struct rq *rq, struct task_struct *p, int enq_flags)
 {
 	unsigned long flags;
 	struct swqueue *swqueue;
-	bool task_migrated = enq_flags & ENQUEUE_MIGRATED;
-	bool task_wakeup = enq_flags & ENQUEUE_WAKEUP;
 
 	/*
 	 * Only enqueue the task in the shared wakequeue if:
 	 *
 	 * - SWQUEUE is enabled
-	 * - The task is on the wakeup path
-	 * - The task wasn't purposefully migrated to the current rq by
-	 *   select_task_rq()
-	 * - The task isn't pinned to a specific CPU
+	 * - select_idle_sibling() didn't find an idle CPU to enqueue this wakee task.
 	 */
-	if (!task_wakeup || task_migrated || p->nr_cpus_allowed == 1)
+	if (!p->sched_add_to_swq)
 		return;
 
+	p->sched_add_to_swq = 0;
 	swqueue = rq_swqueue(rq);
 	spin_lock_irqsave(&swqueue->lock, flags);
 	list_add_tail(&p->swqueue_node, &swqueue->list);
@@ -7361,6 +7357,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if ((unsigned)i < nr_cpumask_bits)
 		return i;
 
+	/*
+	 * No idle sibling was found. Ok to queue this task on the
+	 * shared wakequeue of the target.
+	 */
+	p->sched_add_to_swq = 1;
 	return target;
 }
 
-- 
2.25.1


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