linux-kernel - Re: Scheduler regression from caffcdd8d27ba78730d5540396ce72ad022aff2c

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Message-ID: <53C79013.1020808@arm.com>
Date:	Thu, 17 Jul 2014 10:57:55 +0200
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Bruno Wolff III <bruno@...ff.to>
CC:	Josh Boyer <jwboyer@...hat.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Scheduler regression from caffcdd8d27ba78730d5540396ce72ad022aff2c

On 17/07/14 05:09, Bruno Wolff III wrote:
> On Thu, Jul 17, 2014 at 01:18:36 +0200,
>    Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> So the output of
>>
>> $ cat /proc/sys/kernel/sched_domain/cpu*/domain*/*
>>
>> would be handy too.

Thanks, this was helpful.
I see from the sched domain layout that you have SMT (domain0) and DIE 
(domain1) level. So on this system, the MC level gets degenerated 
(sd_degenerate() in kernel/sched/core.c).
I fail so far to see how this can have an effect on the memory of the 
sched groups. But I can try to fake this situation on one of my platforms.

There is also the possibility that the memory for sched_group sg is not 
(completely) zeroed out:

   sg = kzalloc_node(sizeof(struct sched_group) + cpumask_size(),
			GFP_KERNEL, cpu_to_node(j));


   struct sched_group {
	...
	 * NOTE: this field is variable length. (Allocated dynamically
	 * by attaching extra space to the end of the structure,
	 * depending on how many CPUs the kernel has booted up with)
	 */
	unsigned long cpumask[0];
};

so that the cpumask of a sched group is not 0 and can only be cured by 
an explicit cpumask_clear(sched_group_cpus(sg)) in build_sched_groups() 
on this kind of machine.

>
> Attached and added to the bug.
>
>> Just to make sure, you do have 'CONFIG_X86_32=y' and '# CONFIG_NUMA is
>> not set' in your build?
>
> Yes.
>
> I probably won't be able to get /proc/schedstat on my next test since the
> system will probably crash right away. However, I probably will have a
> much faster rebuild and might still be able to get the info for you
> before I leave tomorrow.
>


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