linux-kernel - Re: Warning in irq_work_queue

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Date:	Fri, 4 Sep 2015 17:11:54 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Warning in irq_work_queue_on()

On Thu, Sep 03, 2015 at 09:58:40AM +0200, Peter Zijlstra wrote:
> 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.

So, after our discussion in IRC, I checked how domains are rebuild on hotplug
ops and it appears that partition_sched_domain() is called on CPU_DOWN_PREPARE
only. The CPU shouldn't be on the domain tree after that.

(Correct me if I'm wrong, I really am not an expert in the domain handling code.
As you said that we can't guarantee that a CPU in the domain tree is in the cpu_online_mask,
I'm likely wrong somewhere).

This is then followed by synchronize_sched(). Which means that after that, the
new version of the CPU domains (with the offlining CPU excluded) is visible
everywhere while the CPU is still in cpu_online_mask.

And finally stop machine runs and the CPU is cleared out of cpu_online_mask.
So I'm probably missing something, otherwise we could find a CPU in the domain
tree that is not in cpu_online_mask.
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