Re: [PATCH wq/for-4.5-fixes] workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup

[Mike Galbraith]

On Wed, 2016-02-03 at 14:28 -0500, Tejun Heo wrote:
> Hello,
> 
> On Wed, Feb 03, 2016 at 08:12:19PM +0100, Thomas Gleixner wrote:
> > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> > > Reported-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> > > Cc: Tang Chen <tangchen@xxxxxxxxxxxxxx>
> > > Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> > > Cc: Len Brown <len.brown@xxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx # v4.3+
> > 
> > 4.3+ ? Hasn't 874bbfe600a6 been backported to older stable kernels?
> > 
> > Adding a 'Fixes: 874bbfe600a6 ...' tag is what you really want here.
> 
> Oops, you're right.  Will add that once Mike confirms the fix.
> 
> > > @@ -570,6 +570,16 @@ static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
> > >  > > > 	> > > 	> > > 	> > > 	> > > 	> > > 	> > >   int node)
> > >  {
> > >  > > > 	> > > assert_rcu_or_wq_mutex_or_pool_mutex(wq);
> > > +
> > > +> > > 	> > > /*
> > > +> > > 	> > >  * XXX: @node can be NUMA_NO_NODE if CPU goes offline while a
> > > +> > > 	> > >  * delayed item is pending.  The plan is to keep CPU -> NODE
> > > +> > > 	> > >  * mapping valid and stable across CPU on/offlines.  Once that
> > > +> > > 	> > >  * happens, this workaround can be removed.
> > 
> > So what happens if the complete node is offline?
> 
> pool_workqueue lookup itself should be fine as dfl_pwq is assigned to
> all nodes by default.  When the node comes back online, things can
> break currently because cpu to node mapping may change.  That's what
> Tang has been working on.

That may make confirming the fix a bit problematic.  The crash I was
looking at happened on a Fujitsu box.

	-Mike