Re: [PATCH v4 5/7] sched/topology: Introduce sched_numa_hop_mask()

[Yury Norov <yury.norov@gmail.com>]

On Sun, Sep 25, 2022 at 11:05:18AM -0700, Yury Norov wrote:
> On Fri, Sep 23, 2022 at 04:55:40PM +0100, Valentin Schneider wrote:
> > Tariq has pointed out that drivers allocating IRQ vectors would benefit
> > from having smarter NUMA-awareness - cpumask_local_spread() only knows
> > about the local node and everything outside is in the same bucket.
> > 
> > sched_domains_numa_masks is pretty much what we want to hand out (a cpumask
> > of CPUs reachable within a given distance budget), introduce
> > sched_numa_hop_mask() to export those cpumasks.
> > 
> > Link: http://lore.kernel.org/r/20220728191203.4055-1-tariqt@nvidia.com
> > Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> > ---
> >  include/linux/topology.h | 12 ++++++++++++
> >  kernel/sched/topology.c  | 31 +++++++++++++++++++++++++++++++
> >  2 files changed, 43 insertions(+)
> > 
> > diff --git a/include/linux/topology.h b/include/linux/topology.h
> > index 4564faafd0e1..3e91ae6d0ad5 100644
> > --- a/include/linux/topology.h
> > +++ b/include/linux/topology.h
> > @@ -245,5 +245,17 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
> >  	return cpumask_of_node(cpu_to_node(cpu));
> >  }
> >  
> > +#ifdef CONFIG_NUMA
> > +extern const struct cpumask *sched_numa_hop_mask(int node, int hops);
> > +#else
> > +static inline const struct cpumask *sched_numa_hop_mask(int node, int hops)
> > +{
> > +	if (node == NUMA_NO_NODE && !hops)
> > +		return cpu_online_mask;
> > +
> > +	return ERR_PTR(-EOPNOTSUPP);
> > +}
> > +#endif	/* CONFIG_NUMA */
> > +
> >  
> >  #endif /* _LINUX_TOPOLOGY_H */
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 8739c2a5a54e..ee77706603c0 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -2067,6 +2067,37 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
> >  	return found;
> >  }
> >  
> > +/**
> > + * sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops away.
> > + * @node: The node to count hops from.
> > + * @hops: Include CPUs up to that many hops away. 0 means local node.
> > + *
> > + * Requires rcu_lock to be held. Returned cpumask is only valid within that
> > + * read-side section, copy it if required beyond that.
> > + *
> > + * Note that not all hops are equal in distance; see sched_init_numa() for how
> > + * distances and masks are handled.
> > + *
> > + * Also note that this is a reflection of sched_domains_numa_masks, which may change
> > + * during the lifetime of the system (offline nodes are taken out of the masks).
> > + */
> > +const struct cpumask *sched_numa_hop_mask(int node, int hops)
> > +{
> > +	struct cpumask ***masks = rcu_dereference(sched_domains_numa_masks);
> > +
> > +	if (node == NUMA_NO_NODE && !hops)
> > +		return cpu_online_mask;
> > +
> > +	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
> > +		return ERR_PTR(-EINVAL);
> 
> This looks like a sanity check. If so, it should go before the snippet
> above, so that client code would behave consistently.
> 
> > +
> > +	if (!masks)
> > +		return NULL;
> 
> In (node == NUMA_NO_NODE && !hops) case you return online cpus. Here
> you return NULL just to convert it to cpu_online_mask in the caller.
> This looks inconsistent. So, together with the above comment, this
> makes me feel that you'd do it like this:
> 
>  const struct cpumask *sched_numa_hop_mask(int node, int hops)
>  {
> 	struct cpumask ***masks;
> 
> 	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
>         {
>  #ifdef CONFIG_SCHED_DEBUG
>                 pr_err(...);
>  #endif
> 		return ERR_PTR(-EINVAL);
>         }

It's an exported function, and any lame driver may crash the system by
dereferencing a random pointer.

You need to check the node for -2, -3, etc, because only -1 is a valid
negative value. For hops, it should be an unsigned int. Right?

> 
> 	if (node == NUMA_NO_NODE && !hops)
> 		return cpu_online_mask; /* or NULL */
> 
>         masks = rcu_dereference(sched_domains_numa_masks);
> 	if (!masks)
> 		return cpu_online_mask; /* or NULL */
> 
> 	return masks[hops][node];
>  }