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Date: Wed, 16 Jul 2014 21:17:32 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Josh Boyer <jwboyer@...hat.com>, Bruno Wolff III <bruno@...ff.to>
CC: "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
Hi Bruno and Josh,
On 16/07/14 17:17, Josh Boyer wrote:
> Adding Dietmar in since he is the original author.
>
> josh
>
> On Wed, Jul 16, 2014 at 09:55:46AM -0500, Bruno Wolff III wrote:
>> caffcdd8d27ba78730d5540396ce72ad022aff2c has been causing crashes
>> early in the boot process on one of three machines I have been
>> testing the kernel on. On that one machine it happens every boot. It
>> happens before netconsole is functional.
I tested this patch on two platforms (ARM TC2 and INTEL i5 M520) by
replacing the two lines (already with the new sg->sgc->capacity instead
of the old sg->sgp->power) by:
BUG_ON(!cpumask_empty(sched_group_cpus(sg)));
BUG_ON(sg->sgc->capacity);
The memory for sg is allocated and zeroed out in __sdt_alloc() with:
sgc = kzalloc_node(sizeof(struct sched_group_capacity) + cpumask_size(),
GFP_KERNEL, cpu_to_node(j));
The related call chain:
build_sched_domains()
__visit_domain_allocation_hell()
__sdt_alloc()
build_sched_groups()
>>
>> A partial revert of the commit fixes the problem. I do not know why
>> the commit is broken though.
>>
>> I have filed https://bugzilla.kernel.org/show_bug.cgi?id=80251 for
>> this issue.
From the issue, I see that the machine making trouble is an Xeon (2
processors w/ hyper-threading).
Could you please share:
cat /proc/cpuinfo and
cat /proc/schedstat (kernel config w/ CONFIG_SCHEDSTATS=y)
from this machine.
I don't think it is SMT (since it's also there on my INTEL i5 M520
(arch/x86/configs/x86_64_defconfig).
Could you also put the two BUG_ON lines into build_sched_groups()
[kernel/sched/core.c] wo/ the cpumask_clear() and setting
sg->sgc->capacity to 0 and share the possible crash output as well?
>>
>> The problem happens on both Fedora and Linus kernels.
>>
>> git diff caffcdd8d27ba78730d5540396ce72ad022aff2c^ caffcdd8d27ba78730d5540396ce72ad022aff2c
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 45d077ed24fb..6340c601475d 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5794,8 +5794,6 @@ build_sched_groups(struct sched_domain *sd, int cpu)
>> continue;
>>
>> group = get_group(i, sdd, &sg);
>> - cpumask_clear(sched_group_cpus(sg));
>> - sg->sgp->power = 0;
>> cpumask_setall(sched_group_mask(sg));
>>
>> for_each_cpu(j, span) {
>>
>> By rc5 the second line can't be added back because the structure has
>> changed. However adding back cpumask_clear(sched_group_cpus(sg)); to
>> rc5 got things working for me again.
That's because 'sched: Let 'struct sched_group_power' care about CPU
capacity' (commit id 63b2ca30bdb3) changes the struct sched_group member
from struct sched_group_power *sgp to struct sched_group_capacity *sgc .
I.e. the second line becomes
sg->sgc->capacity = 0;
Thanks,
-- Dietmar
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