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    Subject[PATCH v2 4/5] sched/topology: Introduce for_each_numa_hop_cpu()
    Date
    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)

    #endif /* _LINUX_TOPOLOGY_H */
    --
    2.31.1
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