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

[Liu Song]

> * Liu Song <liusong@xxxxxxxxxxxxxxxxx> wrote:
>
> > > * 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".
> But we are talking about sysrq_sched_debug_show(), which your patch tries
> to relax the RCU locking of.

Hi,

I'm not sure for_each_online_cpu && print_cpu must need a lock to 
protect, so I refer to other codes

under kernel that reference the implementation. It looks like some 
places use "get_online_cpus" to prevent

cpu hotplug, but many places don't have obvious protection, so I'm also 
confused if protection is necessarily

required.


Thanks

> Thanks,
>
> 	Ingo