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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, May 21, 2025 at 04:33:35PM -0700, 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));
>  		if (!ss->rstat_ss_cpu_lock)
>  			return -ENOMEM;
>  	}

JP suggested to avoid the allocation altogether on such configs. So,
something like the following.

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 7dd396ae3c68..d0c88903d033 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -510,7 +510,7 @@ int __init ss_rstat_init(struct cgroup_subsys *ss)
 {
 	int cpu;
 
-	if (ss) {
+	if (ss && sizeof(raw_spinlock_t)) {
 		ss->rstat_ss_cpu_lock = alloc_percpu(raw_spinlock_t);
 		if (!ss->rstat_ss_cpu_lock)
 			return -ENOMEM;


Tejun which one do you prefer?