Date:	Tue, 25 Sep 2012 10:39:02 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-numa@...r.kernel.org, wency@...fujitsu.com, mingo@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Update sched_domains_numa_masks when new cpus are onlined.

Hi Peter:

Thanks for commenting this patch. :)

On 09/24/2012 05:38 PM, Peter Zijlstra wrote:
> Why are you cc'ing x86 and numa folks but not a single scheduler person
> when you're patching scheduler stuff?

First of all, I'm sorry for this. I thought it was a NUMA or memory
related problem. And I get your address from the get_maintainer.pl
script.

>
> On Tue, 2012-09-18 at 18:12 +0800, Tang Chen wrote:
>> Once array sched_domains_numa_masks is defined, it is never updated.
>> When a new cpu on a new node is onlined,
>
> Hmm, so there's hardware where you can boot with smaller nr_node_ids
> than possible.. I guess that makes sense.

Yes. nr_node_ids represents the max number of nodes the sustem supports.
And usually, we don't have that many.

>
>>   the coincident member in
>> sched_domains_numa_masks is not initialized, and all the masks are 0.
>> As a result, the build_overlap_sched_groups() will initialize a NULL
>> sched_group for the new cpu on the new node, which will lead to kernel panic.
>
> <snip>
>
>> This patch registers a new notifier for cpu hotplug notify chain, and
>> updates sched_domains_numa_masks every time a new cpu is onlined or offlined.
>
> Urgh, more hotplug notifiers.. a well.
>
>> Signed-off-by: Tang Chen<tangchen@...fujitsu.com>
>> ---
>>   kernel/sched/core.c |   62 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 files changed, 62 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index fbf1fd0..66b36ab 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -6711,6 +6711,14 @@ static void sched_init_numa(void)
>>   	 * numbers.
>>   	 */
>>
>> +	/*
>> +	 * Since sched_domains_numa_levels is also used in other functions as
>> +	 * an index for sched_domains_numa_masks[][], we should reset it here in
>> +	 * case sched_domains_numa_masks[][] fails to be initialized. And set it
>> +	 * to 'level' when sched_domains_numa_masks[][] is fully initialized.
>> +	 */
>> +	sched_domains_numa_levels = 0;
>
> This isn't strictly needed for this patch right? I don't see anybody
> calling sched_init_numa() a second time (although they should)..

Indeed, sched_init_numa() won't be called by anyone else. But I clear it
here in case the following memory allocation fail.

Actually, I want to use sched_domains_numa_levels to iterate all the
numa levels, such as in sched_domains_numa_masks_set().

Suppose sched_domains_numa_levels is 10. If allocating memory for
sched_domains_numa_masks[5] fails in sched_init_numa(), it will just 
return, but sched_domains_numa_masks[] only has 4 members. It
could be dangerous.

So I set sched_domains_numa_levels to 0, and reset it to level in the
end of sched_init_numa(), see below.

>
>>   	sched_domains_numa_masks = kzalloc(sizeof(void *) * level, GFP_KERNEL);
>>   	if (!sched_domains_numa_masks)
>>   		return;
>> @@ -6765,11 +6773,64 @@ static void sched_init_numa(void)
>>   	}
>>
>>   	sched_domain_topology = tl;
>> +
>> +	sched_domains_numa_levels = level;

And I set it to level here again.

>> +}
>> +
>> +static void sched_domains_numa_masks_set(int cpu)
>> +{
>> +	int i, j;
>> +	int node = cpu_to_node(cpu);
>> +
>> +	for (i = 0; i<  sched_domains_numa_levels; i++)
>> +		for (j = 0; j<  nr_node_ids; j++)
>> +			if (node_distance(j, node)<= sched_domains_numa_distance[i])
>> +				cpumask_set_cpu(cpu, sched_domains_numa_masks[i][j]);
>> +}
>> +
>> +static void sched_domains_numa_masks_clear(int cpu)
>> +{
>> +	int i, j;
>> +	for (i = 0; i<  sched_domains_numa_levels; i++)
>> +		for (j = 0; j<  nr_node_ids; j++)
>> +			cpumask_clear_cpu(cpu, sched_domains_numa_masks[i][j]);
>> +}
>
> Aside from the coding style nit of wanting braces over multi-line
> statements even though not strictly required, I really don't see how
> this could possibly be right..

Thanks for telling me that. I'll fix the coding style. :)

>
> We do this because nr_node_ids changed, right? This means the entire
> distance table grew/shrunk, which means we should do the level scan
> again.

It seems that nr_node_ids will not change once the system is up.
I'm not quite sure. If I am wrong, please tell me. :)

I found that nr_node_ids is defined as MAX_NUMNODES in mm/page_alloc.c

int nr_node_ids __read_mostly = MAX_NUMNODES;

And all the functions change this value are __init functions. So I think
nr_node_ids is just the possible nodes in the system, and it won't be
changed after the system initialization finished.

>
>> @@ -7218,6 +7279,7 @@ void __init sched_init_smp(void)
>>          mutex_unlock(&sched_domains_mutex);
>>          put_online_cpus();
>>
>> +       hotcpu_notifier(sched_domains_numa_masks_update, CPU_PRI_SCHED_ACTIVE);
>>          hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
>>          hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);
>
> OK, so you really want your notifier to run before cpuset_cpu_active
> because otherwise you get that crash, yet you fail with the whole order
> thing.. You should not _ever_ rely on registration order.
>
>

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