linux-kernel - Re: [patch] fix uml slowness caused by ptrace preemption bug on host

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Message-Id: <E1LkZjf-0002vu-84@pomaz-ex.szeredi.hu>
Date:	Fri, 20 Mar 2009 09:05:47 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	roland@...hat.com
CC:	miklos@...redi.hu, peterz@...radead.org, efault@....de,
	rjw@...k.pl, jdike@...toit.com, mingo@...e.hu, oleg@...hat.com,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fix uml slowness caused by ptrace preemption bug on host

On Thu, 19 Mar 2009, Roland McGrath wrote:
> I'm no scheduler expert and I don't know whether the exact placement in
> your change is the optimal one.  But it's certainly fine from a ptrace
> perspective.

I'm no scheduler expert either.

Maybe a more generic solution in the scheduler (something like this
totally untested patch) would be better?  What say you, scheduler
experts?

Thanks,
Miklos


Index: linux.git/kernel/sched.c
===================================================================
--- linux.git.orig/kernel/sched.c	2009-03-18 12:53:47.000000000 +0100
+++ linux.git/kernel/sched.c	2009-03-20 08:58:13.000000000 +0100
@@ -4629,7 +4629,8 @@ asmlinkage void __sched preempt_schedule
 	 * If there is a non-zero preempt_count or interrupts are disabled,
 	 * we do not want to preempt the current task. Just return..
 	 */
-	if (likely(ti->preempt_count || irqs_disabled()))
+	if (likely(ti->preempt_count || irqs_disabled() ||
+		   current->state != TASK_RUNNING))
 		return;
 
 	do {
--
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