[PATCH 08/24] sched: Split DEQUEUE_SLEEP from deactivate_task()

[Peter Zijlstra]

As a preparation for dequeue_task() failing, and a second code-path
needing to take care of the 'success' path, split out the DEQEUE_SLEEP
path from deactivate_task().

Much thanks to Libo for spotting and fixing a TASK_ON_RQ_MIGRATING
ordering fail.

Fixed-by: Libo Chen <libo.chen@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c  |   23 +++++++++++++----------
 kernel/sched/sched.h |   14 ++++++++++++++
 2 files changed, 27 insertions(+), 10 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2036,12 +2036,23 @@ void activate_task(struct rq *rq, struct
 
 void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
 {
-	WRITE_ONCE(p->on_rq, (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING);
+	WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);
 	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 
+	/*
+	 * Code explicitly relies on TASK_ON_RQ_MIGRATING begin set *before*
+	 * dequeue_task() and cleared *after* enqueue_task().
+	 */
+
 	dequeue_task(rq, p, flags);
 }
 
+static void block_task(struct rq *rq, struct task_struct *p, int flags)
+{
+	if (dequeue_task(rq, p, DEQUEUE_SLEEP | flags))
+		__block_task(rq, p);
+}
+
 /**
  * task_curr - is this task currently executing on a CPU?
  * @p: the task in question.
@@ -6486,9 +6497,6 @@ static void __sched notrace __schedule(u
 				!(prev_state & TASK_NOLOAD) &&
 				!(prev_state & TASK_FROZEN);
 
-			if (prev->sched_contributes_to_load)
-				rq->nr_uninterruptible++;
-
 			/*
 			 * __schedule()			ttwu()
 			 *   prev_state = prev->state;    if (p->on_rq && ...)
@@ -6500,12 +6508,7 @@ static void __sched notrace __schedule(u
 			 *
 			 * After this, schedule() must not care about p->state any more.
 			 */
-			deactivate_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);
-
-			if (prev->in_iowait) {
-				atomic_inc(&rq->nr_iowait);
-				delayacct_blkio_start();
-			}
+			block_task(rq, prev, DEQUEUE_NOCLOCK);
 		}
 		switch_count = &prev->nvcsw;
 	}
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -68,6 +68,7 @@
 #include <linux/wait_api.h>
 #include <linux/wait_bit.h>
 #include <linux/workqueue_api.h>
+#include <linux/delayacct.h>
 
 #include <trace/events/power.h>
 #include <trace/events/sched.h>
@@ -2591,6 +2592,19 @@ static inline void sub_nr_running(struct
 	sched_update_tick_dependency(rq);
 }
 
+static inline void __block_task(struct rq *rq, struct task_struct *p)
+{
+	WRITE_ONCE(p->on_rq, 0);
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
+	if (p->sched_contributes_to_load)
+		rq->nr_uninterruptible++;
+
+	if (p->in_iowait) {
+		atomic_inc(&rq->nr_iowait);
+		delayacct_blkio_start();
+	}
+}
+
 extern void activate_task(struct rq *rq, struct task_struct *p, int flags);
 extern void deactivate_task(struct rq *rq, struct task_struct *p, int flags);