linux-kernel - [PATCH v3 1/1] kthread: do not preempt current task if it is going to call schedule()

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Date:   Fri,  6 Mar 2020 15:01:33 +0800
From:   <cl@...k-chips.com>
To:     heiko@...ech.de
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        akpm@...ux-foundation.org, tglx@...utronix.de, mpe@...erman.id.au,
        surenb@...gle.com, ben.dooks@...ethink.co.uk,
        anshuman.khandual@....com, catalin.marinas@....com,
        will@...nel.org, keescook@...omium.org, luto@...capital.net,
        wad@...omium.org, mark.rutland@....com, geert+renesas@...der.be,
        george_davis@...tor.com, sudeep.holla@....com,
        linux@...linux.org.uk, gregkh@...uxfoundation.org, info@...ux.net,
        kstewart@...uxfoundation.org, allison@...utok.net,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        huangtao@...k-chips.com, Liang Chen <cl@...k-chips.com>
Subject: [PATCH v3 1/1] kthread: do not preempt current task if it is going to call schedule()

From: Liang Chen <cl@...k-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@...k-chips.com>
---
 kernel/kthread.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b262f47046ca..bfbfa481be3a 100644
--- 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)) {
-- 
2.17.1