Re: [PATCH 5/6] sched_ext: idle: Per-node idle cpumasks

Andrea Righi
2025-02-08T08:47:24Z

On Fri, Feb 07, 2025 at 12:30:37PM -1000, Tejun Heo wrote:
> Hello,
>
> On Fri, Feb 07, 2025 at 09:40:52PM +0100, Andrea Righi wrote:
> > +/*
> > + * cpumasks to track idle CPUs within each NUMA node.
> > + *
> > + * If SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled, a single global cpumask
> > + * from is used to track all the idle CPUs in the system.
> > + */
> > +struct idle_cpus {
> > cpumask_var_t cpu;
> > cpumask_var_t smt;
> > -} idle_masks CL_ALIGNED_IF_ONSTACK;
> > +};
>
> Can you prefix the type name with scx_?
>
> Unrelated to this series but I wonder whether we can replace "smt" with
> "core" in the future to become more consistent with how the terms are used
> in the kernel:
>
> struct scx_idle_masks {
> cpumask_var_t cpus;
> cpumask_var_t cores;
> };
>
> We expose "smt" name through kfuncs but we can rename that to "core" through
> compat macros later too.
>
> > +/*
> > + * Find the best idle CPU in the system, relative to @node.
> > + */
> > +s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> > +{
> > + nodemask_t unvisited = NODE_MASK_ALL;
> > + s32 cpu = -EBUSY;
> > +
> > + if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
> > + return pick_idle_cpu_from_node(cpus_allowed, NUMA_NO_NODE, flags);
> > +
> > + /*
> > + * If an initial node is not specified, start with the current
> > + * node.
> > + */
> > + if (node == NUMA_NO_NODE)
> > + node = numa_node_id();
> > +
> > + /*
> > + * Traverse all nodes in order of increasing distance, starting
> > + * from @node.
> > + *
> > + * This loop is O(N^2), with N being the amount of NUMA nodes,
> > + * which might be quite expensive in large NUMA systems. However,
> > + * this complexity comes into play only when a scheduler enables
> > + * SCX_OPS_BUILTIN_IDLE_PER_NODE and it's requesting an idle CPU
> > + * without specifying a target NUMA node, so it shouldn't be a
> > + * bottleneck is most cases.
> > + *
> > + * As a future optimization we may want to cache the list of hop
> > + * nodes in a per-node array, instead of actually traversing them
> > + * every time.
> > + */
> > + for_each_numa_node(node, unvisited, N_POSSIBLE) {
> > + cpu = pick_idle_cpu_from_node(cpus_allowed, node, flags);
>
> Maybe rename pick_idle_cpu_in_node() to stay in sync with
> SCX_PICK_IDLE_IN_NODE? It's not like pick_idle_cpu_from_node() walks from
> the node, right? It just picks within the node.
>
> > @@ -460,38 +582,50 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool
> >
> > void scx_idle_reset_masks(void)
> > {
> > + int node;
> > +
> > + if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
> > + cpumask_copy(idle_cpumask(NUMA_NO_NODE)->cpu, cpu_online_mask);
> > + cpumask_copy(idle_cpumask(NUMA_NO_NODE)->smt, cpu_online_mask);
> > + return;
> > + }
> > +
> > /*
> > * Consider all online cpus idle. Should converge to the actual state
> > * quickly.
> > */
> > - cpumask_copy(idle_masks.cpu, cpu_online_mask);
> > - cpumask_copy(idle_masks.smt, cpu_online_mask);
> > -}
> > + for_each_node(node) {
> > + const struct cpumask *node_mask = cpumask_of_node(node);
> > + struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
> > + struct cpumask *idle_smts = idle_cpumask(node)->smt;
> > -void scx_idle_init_masks(void)
> > -{
> > - BUG_ON(!alloc_cpumask_var(&idle_masks.cpu, GFP_KERNEL));
> > - BUG_ON(!alloc_cpumask_var(&idle_masks.smt, GFP_KERNEL));
> > + cpumask_and(idle_cpus, cpu_online_mask, node_mask);
> > + cpumask_copy(idle_smts, idle_cpus);
> > + }
>
> nitpick: Maybe something like the following is more symmetric with the
> global case and easier to read?
>
> for_each_node(node) {
> const struct cpumask *node_mask = cpumask_of_node(node);
> cpumask_and(idle_cpumask(node)->cpu, cpu_online_mask, node_mask);
> cpumask_and(idle_cpumask(node)->smt, cpu_online_mask, node_mask);
> }
>
> > }
> >
> > static void update_builtin_idle(int cpu, bool idle)
> > {
> > - assign_cpu(cpu, idle_masks.cpu, idle);
> > + int node = idle_cpu_to_node(cpu);

Ok to all of the above.

>
> minor: I wonder whether idle_cpu_to_node() name is a bit confusing - why
> does a CPU being idle have anything to do with its node mapping? If there is
> a better naming convention, great. If not, it is what it is.

Maybe scx_cpu_to_node()? At the end it's just a wrapper to cpu_to_node(),
but from the scx perspective, so if NUMA-awareness is not enabled in scx,
it'd return NUMA_NO_NODE.

Thanks,
-Andrea