Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

[Tejun Heo <tj@kernel.org>]

(cc'ing Waiman)

On Mon, Jul 11, 2022 at 06:46:29PM +0100, Qais Yousef wrote:
> Have you tried running with PROVE_LOCKDEP enabled? It'll help print a useful
> output about the DEADLOCK. But your explanation was good and clear to me.

I don't think lockdep would be able to track CPU1 -> CPU2 dependency here
unfortunately.

> AFAIU:
> 
> 
> CPU0                                     CPU1                                   CPU2
> 
> // attach task to a different
> // cpuset cgroup via sysfs
> __acquire(cgroup_threadgroup_rwsem)
> 
>                                          // pring up CPU2 online
>                                          __acquire(cpu_hotplug_lock)
>                                          // wait for CPU2 to come online
>                                                                                 // bringup cpu online
>                                                                                 // call cpufreq_online() which tries to create sugov kthread
> __acquire(cpu_hotplug_lock)                                                     copy_process()
>                                                                                    cgroup_can_fork()
>                                                                                       cgroup_css_set_fork()
>                                                                                       __acquire(cgroup_threadgroup_rwsem)
> // blocks forever                        // blocks forever                            // blocks forever
> 
> 
> Is this a correct summary of the problem?
> 
> The locks are held in reverse order and we end up with a DEADLOCK.
> 
> I believe the same happens on offline it's just the path to hold the
> cgroup_threadgroup_rwsem on CPU2 is different.
> 
> This will be a tricky one. Your proposed patch might fix it for this case, but
> if there's anything else that creates a kthread when a cpu goes online/offline
> then we'll hit the same problem again.
> 
> I haven't reviewed your patch to be honest, but I think worth seeing first if
> there's something that can be done at the 'right level' first.
> 
> Needs head scratching from my side at least. This is the not the first type of
> locking issue between hotplug and cpuset :-/

Well, the only thing I can think of is always grabbing cpus_read_lock()
before grabbing threadgroup_rwsem. Waiman, what do you think?

Thanks.

-- 
tejun