linux-kernel - [patch] don't preempt not TASK

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Date:	Fri, 20 Mar 2009 10:43:56 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	mingo@...e.hu
Cc:	peterz@...radead.org, roland@...hat.com, efault@....de,
	rjw@...k.pl, jdike@...toit.com,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: [patch] don't preempt not TASK_RUNNING tasks

Ingo,

I tested this one, and I think it makes sense in any case as an
optimization.  It should also be good for -stable kernels.

Does it look OK?

Thanks,
Miklos
----

From: Miklos Szeredi <mszeredi@...e.cz>

This patch fixes bug #12208:

  http://bugzilla.kernel.org/show_bug.cgi?id=12208

Don't preempt tasks in preempt_schedule() if they are already in the
process of going to sleep.  Otherwise the task would wake up only to
go to sleep again.

Due to the way wait_task_inactive() works this can also drastically
slow down ptrace:

 - task A is ptracing task B
 - task B stops on a trace event
 - task A is woken up and preempts task B
 - task A calls ptrace on task B, which does ptrace_check_attach()
 - this calls wait_task_inactive(), which sees that task B is still on the runq
 - task A goes to sleep for a jiffy
 - ...

Since UML does lots of the above sequences, those jiffies quickly add
up to make it slow as hell.

Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
CC: stable@...nel.org
---
 kernel/sched.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux.git/kernel/sched.c
===================================================================
--- linux.git.orig/kernel/sched.c	2009-03-20 09:40:47.000000000 +0100
+++ linux.git/kernel/sched.c	2009-03-20 10:28:56.000000000 +0100
@@ -4632,6 +4632,10 @@ asmlinkage void __sched preempt_schedule
 	if (likely(ti->preempt_count || irqs_disabled()))
 		return;
 
+	/* No point in preempting we are just about to go to sleep. */
+	if (current->state != TASK_RUNNING)
+		return;
+
 	do {
 		add_preempt_count(PREEMPT_ACTIVE);
 		schedule();
--
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