linux-kernel - [PATCH] sched: don't wait in kthread

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Date:	Mon, 29 Jul 2013 22:19:08 +0500
From:	Abbas Raza <abbas_raza@...tor.com>
To:	linux-kernel@...r.kernel.org
Cc:	Abbas Raza <Abbas_Raza@...tor.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: [PATCH] sched: don't wait in kthread_bind for presmp initcalls

From: Abbas Raza <Abbas_Raza@...tor.com>

wait_task_inactive shouldn't be called in kthread_bind for presmp
initcalls the same way it is done in !SMP case.

more info here:
http://permalink.gmane.org/gmane.linux.kernel.embedded/4046

This patch improves boot time for pre-smp initcalls as given below

initcall spawn_ksoftirqd+0x0/0x54 returned 0 after 0 usecs
initcall init_workqueues+0x0/0x358 returned 0 after 0 usecs
initcall cpu_stop_init+0x0/0xcc returned 0 after 0 usecs

boot time without this patch:

initcall spawn_ksoftirqd+0x0/0x54 returned 0 after 9765 usecs
initcall init_workqueues+0x0/0x358 returned 0 after 9765 usecs
initcall cpu_stop_init+0x0/0xcc returned 0 after 19531 usecs

Signed-off-by: Abbas Raza <Abbas_Raza@...tor.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>

---
 kernel/kthread.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b579af5..80f19b5 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -221,10 +221,15 @@ EXPORT_SYMBOL(kthread_create_on_node);
  */
 void kthread_bind(struct task_struct *p, unsigned int cpu)
 {
-	/* Must have done schedule() in kthread() before we set_task_cpu */
-	if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
-		WARN_ON(1);
-		return;
+	if (!((num_online_cpus() == 1) && !cpu)) {
+		/*
+		 * Must have done schedule() in kthread() before
+		 * we set_task_cpu
+		 */
+		if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
+			WARN_ON(1);
+			return;
+		}
 	}
 
 	/* It's safe because the task is inactive. */
-- 
1.8.3.2

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