Re: Warning in irq_work_queue_on()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 03, 2015 at 02:03:51AM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 03, 2015 at 12:24:27AM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 02, 2015 at 11:50:22PM +0200, Frederic Weisbecker wrote:
> > > > > [  875.703227]  [<ffffffff810c2d74>] tick_nohz_full_kick_cpu+0x44/0x50
> > > 
> > > It happens in nohz full, but I'm not sure the guilty is nohz full.
> > > 
> > > The problem here is that wake_up_nohz_cpu() selects a CPU that is offline.
> > 
> > wake_up_nohz_cpu() doesn't do any such thing. Where does the selection
> > logic live?
> 
> Err, got confused with get_nohz_timer_target(). But yeah wake_up_nohz_cpu() is
> called with a CPU that is chosen by mod_timer() -> get_nohz_timer_target().
> 
> > 
> > > But this shouldn't happen. Either it selects a CPU that is in the domain tree,
> > > and I suspect offline CPUs aren't supposed to be there, or it selects the current
> > > CPU. And if the CPU is offlined, it shouldn't be running some kthread...
> > 
> > Do no assume things like that.. always check with the active mask.
> 
> Hmm, so perhaps we need something like this (makes me realize that
> the is_housekeeping_cpu() passes the wrong argument, no issue in practice
> since nohz full aren't in the domain tree but I still need to fix that along).
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0902e4d..2c10a69 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -628,7 +628,7 @@ int get_nohz_timer_target(void)
>  
>  	rcu_read_lock();
>  	for_each_domain(cpu, sd) {
> -		for_each_cpu(i, sched_domain_span(sd)) {
> +		for_each_cpu_and(i, sched_domain_span(sd), cpu_online_mask) {

cpu_active_mask, we clear that when we start killing the cpu. online
only gets cleared once the cpu is actually dead.

>  			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
>  				cpu = i;
>  				goto unlock;
>