Re: [OFFLIST PATCH 2/2] cgroup: use subsystem-specific rstat locks to avoid contention

[JP Kobryn <inwardvessel@gmail.com>]

On 5/21/25 4:33 PM, Shakeel Butt wrote:
> On Wed, May 21, 2025 at 04:23:44PM -0700, Shakeel Butt wrote:
>> On Thu, May 22, 2025 at 12:23:44AM +0200, Klara Modin wrote:
>>> Hi,
>>>
>>> On 2025-04-28 23:15:58 -0700, Shakeel Butt wrote:
>>>> Please ignore this patch as it was sent by mistake.
>>>
>>> This seems to have made it into next:
>>>
>>> 748922dcfabd ("cgroup: use subsystem-specific rstat locks to avoid contention")
>>>
>>> It causes a BUG and eventually a panic on my Raspberry Pi 1:
>>>
>>> WARNING: CPU: 0 PID: 0 at mm/percpu.c:1766 pcpu_alloc_noprof (mm/percpu.c:1766 (discriminator 2))
>>> illegal size (0) or align (4) for percpu allocation
>>
>> Ok this config is without CONFIG_SMP and on such configs we have:
>>
>> typedef struct { } arch_spinlock_t;
>>
>> So, we are doing ss->rstat_ss_cpu_lock = alloc_percpu(0).
>>
>> Hmm, let me think more on how to fix this.
>>
> 
> I think following is the simplest fix:
> 
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index 7dd396ae3c68..aab09495192e 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -511,7 +511,10 @@ int __init ss_rstat_init(struct cgroup_subsys *ss)
>   	int cpu;
>   
>   	if (ss) {
> -		ss->rstat_ss_cpu_lock = alloc_percpu(raw_spinlock_t);
> +		size_t size = sizeof(raw_spinlock_t) ?: 1;
> +
> +		ss->rstat_ss_cpu_lock = __alloc_percpu(size,
> +						__alignof__(raw_spinlock_t));

Thanks for narrowing this one down so fast. Would this approach be more
straightforward?

if (ss) {
#ifdef CONFIG_SMP
	ss->rstat_ss_cpu_lock = alloc_percpu(raw_spinlock_t);
#endif

Since on non-smp the lock functions are no-ops, leaving the ss cpu lock
can perhaps be left NULL. I could include a comment as well explaining
why.

>   		if (!ss->rstat_ss_cpu_lock)
>   			return -ENOMEM;
>   	}