Re: [PATCH v2] sched/topology: Fix for_each_node_numadist() lockup with !CONFIG_NUMA

[Andrea Righi <arighi@nvidia.com>]

Hi Yury,

On Mon, Jun 09, 2025 at 11:03:29AM -0400, Yury Norov wrote:
> On Mon, Jun 09, 2025 at 01:35:36PM +0200, Andrea Righi wrote:
> > for_each_node_numadist() can lead to hard lockups on kernels built
> > without CONFIG_NUMA. For instance, the following was triggered by
> > sched_ext:
> > 
> >   watchdog: CPU5: Watchdog detected hard LOCKUP on cpu 5
> >   ...
> >   RIP: 0010:_find_first_and_bit+0x8/0x60
> >   ...
> >   Call Trace:
> >   <TASK>
> >    cpumask_any_and_distribute+0x49/0x80
> >    pick_idle_cpu_in_node+0xcf/0x140
> >    scx_bpf_pick_idle_cpu_node+0xaa/0x110
> >    bpf_prog_16ee5b1f077af006_pick_idle_cpu+0x57f/0x5de
> >    bpf_prog_df2ce5cfac58ce09_bpfland_select_cpu+0x37/0xf4
> >    bpf__sched_ext_ops_select_cpu+0x4b/0xb3
> > 
> > This happens because nearest_node_nodemask() always returns NUMA_NO_NODE
> > (-1) when CONFIG_NUMA is disabled, causing the loop to never terminate,
> > as the condition node >= MAX_NUMNODES is never satisfied.
> > 
> > Prevent this by providing a stub implementation based on
> > for_each_node_mask() when CONFIG_NUMA is disabled, which can safely
> > processes the single available node while still honoring the unvisited
> > nodemask.
> > 
> > Fixes: f09177ca5f242 ("sched/topology: Introduce for_each_node_numadist() iterator")
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  include/linux/topology.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > Changes in v2:
> >  - Provide a stub implementation for the !CONFIG_NUMA case
> >  - Link to v1: https://lore.kernel.org/all/20250603080402.170601-1-arighi@nvidia.com/
> > 
> > diff --git a/include/linux/topology.h b/include/linux/topology.h
> > index 33b7fda97d390..97c4f5fc75038 100644
> > --- a/include/linux/topology.h
> > +++ b/include/linux/topology.h
> > @@ -304,12 +304,17 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
> >   *
> >   * Requires rcu_lock to be held.
> >   */
> > +#ifdef CONFIG_NUMA
> 
> While there, can you expand this optimization to MAX_NUMNODES == 1
> case?
>         #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1)

Makes sense, will send a v3, thanks!

-Andrea

> 
> With that:
> 
> Acked-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> 
> Thanks,
> Yury
> 
> >  #define for_each_node_numadist(node, unvisited)					\
> >  	for (int __start = (node),						\
> >  	     (node) = nearest_node_nodemask((__start), &(unvisited));		\
> >  	     (node) < MAX_NUMNODES;						\
> >  	     node_clear((node), (unvisited)),					\
> >  	     (node) = nearest_node_nodemask((__start), &(unvisited)))
> > +#else
> > +#define for_each_node_numadist(node, unvisited)					\
> > +	for_each_node_mask((node), (unvisited))
> > +#endif
> >  
> >  /**
> >   * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
> > -- 
> > 2.49.0