kthread: Do not preempt current task if it is going to call schedule()

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lkml.kernel.org/r/20200306070133.18335-2-cl@rock-chips.com

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b262f47..bfbfa48 100644 (file)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -199,8 +199,15 @@ static void __kthread_parkme(struct kthread *self)
                if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
                        break;
 
+               /*
+                * Thread is going to call schedule(), do not preempt it,
+                * or the caller of kthread_park() may spend more time in
+                * wait_task_inactive().
+                */
+               preempt_disable();
                complete(&self->parked);
-               schedule();
+               schedule_preempt_disabled();
+               preempt_enable();
        }
        __set_current_state(TASK_RUNNING);
 }
@@ -245,8 +252,14 @@ static int kthread(void *_create)
        /* OK, tell user we're spawned, wait for stop or wakeup */
        __set_current_state(TASK_UNINTERRUPTIBLE);
        create->result = current;
+       /*
+        * Thread is going to call schedule(), do not preempt it,
+        * or the creator may spend more time in wait_task_inactive().
+        */
+       preempt_disable();
        complete(done);
-       schedule();
+       schedule_preempt_disabled();
+       preempt_enable();
 
        ret = -EINTR;
        if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {