linux-kernel - Re: Regression in gdm-2.18 since 2.6.24

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Message-ID: <20080405144042.GB24075@linux.vnet.ibm.com>
Date:	Sat, 5 Apr 2008 20:10:43 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Ken Moffat <zarniwhoop@...world.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "Rafael J. Wysocki" <rjw@...k.pl>,
	lkml <linux-kernel@...r.kernel.org>, a.p.zijlstra@...llo.nl,
	aneesh.kumar@...ux.vnet.ibm.com, dhaval@...ux.vnet.ibm.com,
	Balbir Singh <balbir@...ibm.com>, skumar@...ux.vnet.ibm.com
Subject: Re: Regression in gdm-2.18 since 2.6.24

On Fri, Apr 04, 2008 at 04:32:32PM +0100, Ken Moffat wrote:
> > Just to confirm, are you saying you applied patch below on top of
> > 2.6.25-rc8 and it solved your shutdown issues?
> > 
>  Yes.

Thanks for confirming that the patch I sent was what you had tried and
found it to fix your problem. That patch however is not something we
want to apply for 2.6.25-rc8 (since it will worsen interactivity for
other cases).

Given that you seem to be seeing the problem even without
CONFIG_GROUP_SCHED, only the second hunk of the patch seems to be making
a difference for your problem i.e just the hunk below applied on
2.6.25-rc8 (to kernel/sched_fair.c) should fix your problem too:

@@ -1145,7 +1145,7 @@ static void check_preempt_wakeup(struct 
 	 * More easily preempt - nice tasks, while not making
 	 * it harder for + nice tasks.
 	 */
-	if (unlikely(se->load.weight > NICE_0_LOAD))
+	if (unlikely(se->load.weight != NICE_0_LOAD))
 		gran = calc_delta_fair(gran, &se->load);

 	if (pse->vruntime + gran < se->vruntime)

[The first hunk is a no-op under !CONFIG_GROUP_SCHED, since
entity_is_task() is always 1 for !CONFIG_GROUP_SCHED]

This second hunk changes how fast + or - niced tasks get preempted. 

2.6.25-rc8 (Bad case):
	Sets preempt granularity for + niced tasks at 5ms (1 CPU)

2.6.25-rc8 + the hunk above (Good case):
	Sets preempt granularity for + niced tasks at >5ms

So bumping up preempt granularity for + niced tasks seems to make things
work for you. IMO the deeper problem lies somewhere else (perhaps is
some race issue in gdm itself), which is easily exposed with 2.6.25-rc8 
which lets + niced tasks be preempted quickly.

To help validate this, can you let us know the result of tuning preempt
granularity on native 2.6.25-rc8 (without any patches applied and
CONFIG_GROUP_SCHED disabled)?

	# echo 100000000 > /proc/sys/kernel/sched_wakeup_granularity_ns

To check if echo command worked, do:

	# cat /proc/sys/kernel/sched_wakeup_granularity_ns

It should return 100000000.

Now try shutting down thr' gdm and pls let me know if it makes a
difference.

-- 
Regards,
vatsa
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