[PATCH v1 1/1] sched/fair: do not preempt current task if it is going to call schedule()

[cl]

From: Liang Chen <cl@xxxxxxxxxxxxxx>

when we create a kthread with ktrhead_create_on_cpu(),the child thread
entry is ktread.c:ktrhead() which will be preempted by the parent after
call complete(done) while schedule() is not called yet,then the parent
will call wait_task_inactive(child) but the child is still on the runqueue,
so the parent will schedule_hrtimeout() for 1 jiffy,it will waste a lot of
time,especially on startup.

  parent                             child
ktrhead_create_on_cpu()
  wait_fo_completion(&done) -----> ktread.c:ktrhead()
                             |----- complete(done);--wakeup and preempted by parent
 kthread_bind() <------------|  |-> schedule();--dequeue here
  wait_task_inactive(child)     |
   schedule_hrtimeout(1 jiffy) -|

So we hope the child just wakeup parent but not preempted by parent, and the
child is going to call schedule() soon,then the parent will not call
schedule_hrtimeout(1 jiffy) as the child is already dequeue.

The same issue for ktrhead_park()&&kthread_parkme().
This patch can save 120ms on rk312x startup with CONFIG_HZ=300.

Signed-off-by: Liang Chen <cl@xxxxxxxxxxxxxx>
---
 arch/arm/include/asm/thread_info.h   |  1 +
 arch/arm64/include/asm/thread_info.h |  1 +
 include/linux/sched.h                | 15 +++++++++++++++
 kernel/kthread.c                     |  4 ++++
 kernel/sched/fair.c                  |  4 ++++
 5 files changed, 25 insertions(+)

diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 0d0d5178e2c3..51802991ba1f 100644
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -145,6 +145,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
 #define TIF_USING_IWMMXT	17
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
 #define TIF_RESTORE_SIGMASK	20
+#define TIF_GOING_TO_SCHED	27	/* task is going to call schedule() */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index f0cec4160136..332786f11dc3 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -78,6 +78,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define TIF_SVE_VL_INHERIT	24	/* Inherit sve_vl_onexec across exec */
 #define TIF_SSBD		25	/* Wants SSB mitigation */
 #define TIF_TAGGED_ADDR		26	/* Allow tagged user addresses */
+#define TIF_GOING_TO_SCHED	27	/* task is going to call schedule() */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 04278493bf15..cb9058d2cf0b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1768,6 +1768,21 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
 	return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED));
 }
 
+static inline void set_tsk_going_to_sched(struct task_struct *tsk)
+{
+	set_tsk_thread_flag(tsk, TIF_GOING_TO_SCHED);
+}
+
+static inline void clear_tsk_going_to_sched(struct task_struct *tsk)
+{
+	clear_tsk_thread_flag(tsk, TIF_GOING_TO_SCHED);
+}
+
+static inline int test_tsk_going_to_sched(struct task_struct *tsk)
+{
+	return unlikely(test_tsk_thread_flag(tsk, TIF_GOING_TO_SCHED));
+}
+
 /*
  * cond_resched() and cond_resched_lock(): latency reduction via
  * explicit rescheduling in places that are safe. The return
diff --git a/kernel/kthread.c b/kernel/kthread.c
index b262f47046ca..8a4e4c9cdc22 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -199,8 +199,10 @@ static void __kthread_parkme(struct kthread *self)
 		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
 			break;
 
+		set_tsk_going_to_sched(current);
 		complete(&self->parked);
 		schedule();
+		clear_tsk_going_to_sched(current);
 	}
 	__set_current_state(TASK_RUNNING);
 }
@@ -245,8 +247,10 @@ static int kthread(void *_create)
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_UNINTERRUPTIBLE);
 	create->result = current;
+	set_tsk_going_to_sched(current);
 	complete(done);
 	schedule();
+	clear_tsk_going_to_sched(current);
 
 	ret = -EINTR;
 	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3c8a379c357e..28a308743bf8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4330,6 +4330,8 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
 			hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
 		return;
 #endif
+	if (test_tsk_going_to_sched(rq_of(cfs_rq)->curr))
+		return;
 
 	if (cfs_rq->nr_running > 1)
 		check_preempt_tick(cfs_rq, curr);
@@ -6633,6 +6635,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
 	 */
 	if (test_tsk_need_resched(curr))
 		return;
+	if (test_tsk_going_to_sched(curr))
+		return;
 
 	/* Idle tasks are by definition preempted by non-idle tasks. */
 	if (unlikely(task_has_idle_policy(curr)) &&
-- 
2.17.1