[v2,2/8] sched/topology: introduce sched_numa_find_next_cpu()
Commit Message
The function searches for the next CPU in a given cpumask according to
NUMA topology, so that it traverses cpus per-hop.
If the CPU is the last cpu in a given hop, sched_numa_find_next_cpu()
switches to the next hop, and picks the first CPU from there, excluding
those already traversed.
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
include/linux/topology.h | 7 +++++++
kernel/sched/topology.c | 39 +++++++++++++++++++++++++++++++++++++++
2 files changed, 46 insertions(+)
Comments
On 19/04/23 22:19, Yury Norov wrote:
> +/*
> + * sched_numa_find_next_cpu() - given the NUMA topology, find the next cpu
> + * cpumask: cpumask to find a cpu from
> + * cpu: current cpu
> + * node: local node
> + * hop: (in/out) indicates distance order of current CPU to a local node
> + *
> + * The function searches for next cpu at a given NUMA distance, indicated
> + * by hop, and if nothing found, tries to find CPUs at a greater distance,
> + * starting from the beginning.
> + *
> + * Return: cpu, or >= nr_cpu_ids when nothing found.
> + */
> +int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
> +{
> + unsigned long *cur, *prev;
> + struct cpumask ***masks;
> + unsigned int ret;
> +
> + if (*hop >= sched_domains_numa_levels)
> + return nr_cpu_ids;
> +
> + masks = rcu_dereference(sched_domains_numa_masks);
> + cur = cpumask_bits(masks[*hop][node]);
> + if (*hop == 0)
> + ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
> + else {
> + prev = cpumask_bits(masks[*hop - 1][node]);
> + ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
> + }
> +
> + if (ret < nr_cpu_ids)
> + return ret;
> +
> + *hop += 1;
> + return sched_numa_find_next_cpu(cpus, 0, node, hop);
sched_domains_numa_levels is a fairly small number, so the recursion depth
isn't something we really need to worry about - still, the iterative
variant of this is fairly straightforward to get to:
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index e850f16c003ae..4c9a9e48fef6d 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2151,23 +2151,27 @@ int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsi
struct cpumask ***masks;
unsigned int ret;
- if (*hop >= sched_domains_numa_levels)
- return nr_cpu_ids;
+ /*
+ * Reset @cpu to 0 when increasing @hop, since CPU numbering has no
+ * relationship with NUMA distance: a search at @hop+1 may yield CPUs
+ * of lower ID than previously seen!
+ */
+ for (; *hop >= sched_domains_numa_levels; *hop += 1, cpu = 0) {
+ masks = rcu_dereference(sched_domains_numa_masks);
+ cur = cpumask_bits(masks[*hop][node]);
+
+ if (*hop == 0) {
+ ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
+ } else {
+ prev = cpumask_bits(masks[*hop - 1][node]);
+ ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
+ }
- masks = rcu_dereference(sched_domains_numa_masks);
- cur = cpumask_bits(masks[*hop][node]);
- if (*hop == 0)
- ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
- else {
- prev = cpumask_bits(masks[*hop - 1][node]);
- ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
+ if (ret < nr_cpu_ids)
+ return ret;
}
- if (ret < nr_cpu_ids)
- return ret;
-
- *hop += 1;
- return sched_numa_find_next_cpu(cpus, 0, node, hop);
+ return nr_cpu_ids;
}
EXPORT_SYMBOL_GPL(sched_numa_find_next_cpu);
On Tue, Apr 25, 2023 at 10:54:56AM +0100, Valentin Schneider wrote:
> On 19/04/23 22:19, Yury Norov wrote:
> > +/*
> > + * sched_numa_find_next_cpu() - given the NUMA topology, find the next cpu
> > + * cpumask: cpumask to find a cpu from
> > + * cpu: current cpu
> > + * node: local node
> > + * hop: (in/out) indicates distance order of current CPU to a local node
> > + *
> > + * The function searches for next cpu at a given NUMA distance, indicated
> > + * by hop, and if nothing found, tries to find CPUs at a greater distance,
> > + * starting from the beginning.
> > + *
> > + * Return: cpu, or >= nr_cpu_ids when nothing found.
> > + */
> > +int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
> > +{
> > + unsigned long *cur, *prev;
> > + struct cpumask ***masks;
> > + unsigned int ret;
> > +
> > + if (*hop >= sched_domains_numa_levels)
> > + return nr_cpu_ids;
> > +
> > + masks = rcu_dereference(sched_domains_numa_masks);
> > + cur = cpumask_bits(masks[*hop][node]);
> > + if (*hop == 0)
> > + ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
> > + else {
> > + prev = cpumask_bits(masks[*hop - 1][node]);
> > + ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
> > + }
> > +
> > + if (ret < nr_cpu_ids)
> > + return ret;
> > +
> > + *hop += 1;
> > + return sched_numa_find_next_cpu(cpus, 0, node, hop);
>
> sched_domains_numa_levels is a fairly small number, so the recursion depth
> isn't something we really need to worry about - still, the iterative
> variant of this is fairly straightforward to get to:
This is a tail recursion. Compiler normally converts it into the loop just
as well. At least, my GCC does.
On 25/04/23 22:26, Yury Norov wrote:
> On Tue, Apr 25, 2023 at 10:54:56AM +0100, Valentin Schneider wrote:
>> On 19/04/23 22:19, Yury Norov wrote:
>> > +/*
>> > + * sched_numa_find_next_cpu() - given the NUMA topology, find the next cpu
>> > + * cpumask: cpumask to find a cpu from
>> > + * cpu: current cpu
>> > + * node: local node
>> > + * hop: (in/out) indicates distance order of current CPU to a local node
>> > + *
>> > + * The function searches for next cpu at a given NUMA distance, indicated
>> > + * by hop, and if nothing found, tries to find CPUs at a greater distance,
>> > + * starting from the beginning.
>> > + *
>> > + * Return: cpu, or >= nr_cpu_ids when nothing found.
>> > + */
>> > +int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
>> > +{
>> > + unsigned long *cur, *prev;
>> > + struct cpumask ***masks;
>> > + unsigned int ret;
>> > +
>> > + if (*hop >= sched_domains_numa_levels)
>> > + return nr_cpu_ids;
>> > +
>> > + masks = rcu_dereference(sched_domains_numa_masks);
>> > + cur = cpumask_bits(masks[*hop][node]);
>> > + if (*hop == 0)
>> > + ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
>> > + else {
>> > + prev = cpumask_bits(masks[*hop - 1][node]);
>> > + ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
>> > + }
>> > +
>> > + if (ret < nr_cpu_ids)
>> > + return ret;
>> > +
>> > + *hop += 1;
>> > + return sched_numa_find_next_cpu(cpus, 0, node, hop);
>>
>> sched_domains_numa_levels is a fairly small number, so the recursion depth
>> isn't something we really need to worry about - still, the iterative
>> variant of this is fairly straightforward to get to:
>
> This is a tail recursion. Compiler normally converts it into the loop just
> as well. At least, my GCC does.
I'd hope so in 2023! I still prefer the iterative approach as I find it
more readable, but I'm not /too/ strongly attached to it.
@@ -247,6 +247,7 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
#ifdef CONFIG_NUMA
int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node);
+int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop);
extern const struct cpumask *sched_numa_hop_mask(unsigned int node, unsigned int hops);
#else
static __always_inline int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
@@ -254,6 +255,12 @@ static __always_inline int sched_numa_find_nth_cpu(const struct cpumask *cpus, i
return cpumask_nth(cpu, cpus);
}
+static __always_inline
+int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
+{
+ return find_next_bit(cpumask_bits(cpus), small_cpumask_bits, cpu);
+}
+
static inline const struct cpumask *
sched_numa_hop_mask(unsigned int node, unsigned int hops)
{
@@ -2130,6 +2130,45 @@ int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
}
EXPORT_SYMBOL_GPL(sched_numa_find_nth_cpu);
+/*
+ * sched_numa_find_next_cpu() - given the NUMA topology, find the next cpu
+ * cpumask: cpumask to find a cpu from
+ * cpu: current cpu
+ * node: local node
+ * hop: (in/out) indicates distance order of current CPU to a local node
+ *
+ * The function searches for next cpu at a given NUMA distance, indicated
+ * by hop, and if nothing found, tries to find CPUs at a greater distance,
+ * starting from the beginning.
+ *
+ * Return: cpu, or >= nr_cpu_ids when nothing found.
+ */
+int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
+{
+ unsigned long *cur, *prev;
+ struct cpumask ***masks;
+ unsigned int ret;
+
+ if (*hop >= sched_domains_numa_levels)
+ return nr_cpu_ids;
+
+ masks = rcu_dereference(sched_domains_numa_masks);
+ cur = cpumask_bits(masks[*hop][node]);
+ if (*hop == 0)
+ ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
+ else {
+ prev = cpumask_bits(masks[*hop - 1][node]);
+ ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
+ }
+
+ if (ret < nr_cpu_ids)
+ return ret;
+
+ *hop += 1;
+ return sched_numa_find_next_cpu(cpus, 0, node, hop);
+}
+EXPORT_SYMBOL_GPL(sched_numa_find_next_cpu);
+
/**
* sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops away from
* @node