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Subject[PATCH v1 1/1] sched/fair: do not preempt current task if it is going to call schedule()
Date
From: Liang Chen <cl@rock-chips.com>

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@rock-chips.com>
---
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


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