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

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Date:	Sat, 5 Apr 2008 22:03:47 +0100
From:	Ken Moffat <zarniwhoop@...world.com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.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 Sat, Apr 05, 2008 at 08:10:43PM +0530, Srivatsa Vaddagiri wrote:
> 
> 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
> 
 Well, I'm no longer sure exactly what was in the config, but after
I had confirmed the reversion would fix 2.6.24.4 I _did_ try just
the second part of the patch applied to 2.6.25-rc8 and it gave a 60%
success rate across 10 tests.
> 
> 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.
> 

 I agree this is probably exposing a problem somewhere else.

> 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

 Will do, but it might be a day or so before I can get to this.

Thanks.

Ken
-- 
das eine Mal als Tragödie, das andere Mal als Farce
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