Commit 26c7295b authored Mar 06, 2020 by Liang Chen Committed by Peter Zijlstra Mar 20, 2020

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

parent c32b4308

kernel/kthread.c

+15 −2

Original line number	Diff line number	Diff line
		@@ -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)) {

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