Re: [PATCH] sched/debug: avoid executing show_state and causing rcu stall warning

[Liu Song]

> * Liu Song <liusong@xxxxxxxxxxxxxxxxx> wrote:
>
> > From: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
> >
> > If the number of CPUs is large, "sysrq_sched_debug_show" will execute for
> > a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my
> > 128-core machine, the rcu stall warning will be triggered. Moreover,
> > sysrq_sched_debug_show does not need to be protected by rcu_read_lock,
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > and no rcu stall warning will appear after adjustment.
> That doesn't mean it doesn't have to be protected by *any* lock - which
> your patch implements AFAICS.
>
> There's a couple of lines such as:
>
>          for_each_online_cpu(cpu) {

Hi,

Here I refer to the implementation of "sysrq_timer_list_show", and I 
don't see any lock.

Maybe there is a problem with the implementation of "sysrq_timer_list_show".


Thanks

> ... which need to be protected against CPU hotplug events.
>
> I haven't checked any of the deeper code to see what RCU or other
> protection it may need, but clearly you didn't either ...
>
> Thanks,
>
> 	Ingo