linux-kernel - Re: [PATCH] sched/debug: initialize sd_sysctl_cpus if !CONFIG_CPUMASK

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Date:   Fri, 1 Feb 2019 17:33:19 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Masayoshi Mizuma <msys.mizuma@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Subject: Re: [PATCH] sched/debug: initialize sd_sysctl_cpus if
 !CONFIG_CPUMASK_OFFSTACK

On 1/31/19 8:44 AM, Masayoshi Mizuma wrote:
> On Wed, Jan 30, 2019 at 09:14:00PM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 29, 2019 at 10:12:45AM -0500, Masayoshi Mizuma wrote:
>>> From: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
>>>
>>> register_sched_domain_sysctl() copies the cpu_possible_mask into
>>> sd_sysctl_cpus, but only if sd_sysctl_cpus hasn't already been
>>> allocated (ie, CONFIG_CPUMASK_OFFSTACK is set).  However, when
>>> CONFIG_CPUMASK_OFFSTACK is not set, sd_sysctl_cpus is left uninitialized
>>> (all zeroes) and the kernel may fail to initialize sched_domain sysctl
>>> entries for all possible cpus.
>>>
>>> This is visible to the user if the kernel is booted with maxcpus=n, or
>>> if ACPI tables have been modified to leave cpus offline, and then
>>> checking for missing /proc/sys/kernel/sched_domain/cpu* entries.
>>>
>>> Fix this by separating the allocataion and initialization, and adding
>>> a flag to initialize the possible cpu entries while system booting only.
>>>
>>> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
>>> Reviewed-by: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
>>> Tested-by: Syuuichirou Ishii <ishii.shuuichir@...fujitsu.com>
>>> Tested-by: Tarumizu, Kohei <tarumizu.kohei@...fujitsu.com>
>>> ---
>>>   kernel/sched/debug.c | 4 ++++
>>>   1 file changed, 4 insertions(+)
>>>
>>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>>> index de3de997e245..9c6637f3e21d 100644
>>> --- a/kernel/sched/debug.c
>>> +++ b/kernel/sched/debug.c
>>> @@ -310,6 +310,7 @@ static struct ctl_table *sd_alloc_ctl_cpu_table(int cpu)
>>>   
>>>   static cpumask_var_t		sd_sysctl_cpus;
>>>   static struct ctl_table_header	*sd_sysctl_header;
>>> +static int register_sched_domain_sysctl_on_boot = 1;
>>>   
>>>   void register_sched_domain_sysctl(void)
>>>   {
>>> @@ -344,9 +345,12 @@ void register_sched_domain_sysctl(void)
>>>   	if (!cpumask_available(sd_sysctl_cpus)) {
>>>   		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
>>>   			return;
>>> +	}
>>>   
>>> +	if (register_sched_domain_sysctl_on_boot) {
>>>   		/* init to possible to not have holes in @cpu_entries */
>>>   		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
>>> +		register_sched_domain_sysctl_on_boot = 0;
>>>   	}
>>>   
>>>   	for_each_cpu(i, sd_sysctl_cpus) {
>>
>> I change it like the below. By keeping the initial value 0 it can go
>> into .bss instead of .data.
> 
> Great, thanks!
> Should I re-post the patch as v2?
> 
> - Masa
> 

Peter's revision tests fine on my aarch64 box and would get my ACK, 
however it ends up getting re-posted :)

Thanks,

-- Joe

>>
>> --- a/kernel/sched/debug.c
>> +++ b/kernel/sched/debug.c
>> @@ -315,6 +315,7 @@ void register_sched_domain_sysctl(void)
>>   {
>>   	static struct ctl_table *cpu_entries;
>>   	static struct ctl_table **cpu_idx;
>> +	static bool init_done = false;
>>   	char buf[32];
>>   	int i;
>>   
>> @@ -344,7 +345,10 @@ void register_sched_domain_sysctl(void)
>>   	if (!cpumask_available(sd_sysctl_cpus)) {
>>   		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
>>   			return;
>> +	}
>>   
>> +	if (!init_done) {
>> +		init_done = true;
>>   		/* init to possible to not have holes in @cpu_entries */
>>   		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
>>   	}


-- Joe