Messages in this thread | | | Date | Mon, 5 Sep 2022 12:46:28 +0300 | Subject | Re: [PATCH v3 8/9] sched/topology: Introduce for_each_numa_hop_cpu() | From | Tariq Toukan <> |
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On 8/25/2022 9:12 PM, Valentin Schneider wrote: > The recently introduced sched_numa_hop_mask() exposes cpumasks of CPUs > reachable within a given distance budget, but this means each successive > cpumask is a superset of the previous one. > > Code wanting to allocate one item per CPU (e.g. IRQs) at increasing > distances would thus need to allocate a temporary cpumask to note which > CPUs have already been visited. This can be prevented by leveraging > for_each_cpu_andnot() - package all that logic into one ugl^D fancy macro. > > Signed-off-by: Valentin Schneider <vschneid@redhat.com> > --- > include/linux/topology.h | 37 +++++++++++++++++++++++++++++++++++++ > 1 file changed, 37 insertions(+) > > diff --git a/include/linux/topology.h b/include/linux/topology.h > index 13b82b83e547..6c671dc3252c 100644 > --- a/include/linux/topology.h > +++ b/include/linux/topology.h > @@ -254,5 +254,42 @@ static inline const struct cpumask *sched_numa_hop_mask(int node, int hops) > } > #endif /* CONFIG_NUMA */ > > +/** > + * for_each_numa_hop_cpu - iterate over CPUs by increasing NUMA distance, > + * starting from a given node. > + * @cpu: the iteration variable. > + * @node: the NUMA node to start the search from. > + * > + * Requires rcu_lock to be held. > + * Careful: this is a double loop, 'break' won't work as expected. > + * > + * > + * Implementation notes: > + * > + * Providing it is valid, the mask returned by > + * sched_numa_hop_mask(node, hops+1) > + * is a superset of the one returned by > + * sched_numa_hop_mask(node, hops) > + * which may not be that useful for drivers that try to spread things out and > + * want to visit a CPU not more than once. > + * > + * To accommodate for that, we use for_each_cpu_andnot() to iterate over the cpus > + * of sched_numa_hop_mask(node, hops+1) with the CPUs of > + * sched_numa_hop_mask(node, hops) removed, IOW we only iterate over CPUs > + * a given distance away (rather than *up to* a given distance). > + * > + * hops=0 forces us to play silly games: we pass cpu_none_mask to > + * for_each_cpu_andnot(), which turns it into for_each_cpu(). > + */ > +#define for_each_numa_hop_cpu(cpu, node) \ > + for (struct { const struct cpumask *curr, *prev; int hops; } __v = \ > + { sched_numa_hop_mask(node, 0), NULL, 0 }; \ > + !IS_ERR_OR_NULL(__v.curr); \ > + __v.hops++, \ > + __v.prev = __v.curr, \ > + __v.curr = sched_numa_hop_mask(node, __v.hops)) \ > + for_each_cpu_andnot(cpu, \ > + __v.curr, \ > + __v.hops ? __v.prev : cpu_none_mask) >
Hiding two nested loops together in one for_each_* macro leads to unexpected behavior for the standard usage of 'break/continue'.
for_each_numa_hop_cpu(cpu, node) { if (condition) break; <== will terminate the inner loop only, but it's invisible to the human developer/reviewer. }
These bugs will not be easy to spot in code review.
> #endif /* _LINUX_TOPOLOGY_H */
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