sched_ext: idle: Accept an arbitrary cpumask in scx_select_cpu_dfl()

Many scx schedulers implement their own hard or soft-affinity rules
to support topology characteristics, such as heterogeneous architectures
(e.g., big.LITTLE, P-cores/E-cores), or to categorize tasks based on
specific properties (e.g., running certain tasks only in a subset of
CPUs).

Currently, there is no mechanism that allows to use the built-in idle
CPU selection policy to an arbitrary subset of CPUs. As a result,
schedulers often implement their own idle CPU selection policies, which
are typically similar to one another, leading to a lot of code
duplication.

To address this, modify scx_select_cpu_dfl() to accept an arbitrary
cpumask, that can be used by the BPF schedulers to apply the existent
built-in idle CPU selection policy to a subset of allowed CPUs.

With this concept the idle CPU selection policy becomes the following:
 - always prioritize CPUs from fully idle SMT cores (if SMT is enabled),
 - select the same CPU if it's idle and in the allowed CPUs,
 - select an idle CPU within the same LLC, if the LLC cpumask is a
   subset of the allowed CPUs,
 - select an idle CPU within the same node, if the node cpumask is a
   subset of the allowed CPUs,
 - select an idle CPU within the allowed CPUs.

This functionality will be exposed through a dedicated kfunc in a
separate patch.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/ext_idle.c

@@ -49,6 +49,7 @@ static struct scx_idle_cpus **scx_idle_node_masks;
 /*
  * Local per-CPU cpumasks (used to generate temporary idle cpumasks).
  */
+static DEFINE_PER_CPU(cpumask_var_t, local_idle_cpumask);
 static DEFINE_PER_CPU(cpumask_var_t, local_llc_idle_cpumask);
 static DEFINE_PER_CPU(cpumask_var_t, local_numa_idle_cpumask);
 
@@ -417,13 +418,15 @@ static inline bool task_affinity_all(const struct task_struct *p)
  *     branch prediction optimizations.
  *
  * 3. Pick a CPU within the same LLC (Last-Level Cache):
- *   - if the above conditions aren't met, pick a CPU that shares the same LLC
- *     to maintain cache locality.
+ *   - if the above conditions aren't met, pick a CPU that shares the same
+ *     LLC, if the LLC domain is a subset of @cpus_allowed, to maintain
+ *     cache locality.
  *
  * 4. Pick a CPU within the same NUMA node, if enabled:
- *   - choose a CPU from the same NUMA node to reduce memory access latency.
+ *   - choose a CPU from the same NUMA node, if the node cpumask is a
+ *     subset of @cpus_allowed, to reduce memory access latency.
  *
- * 5. Pick any idle CPU usable by the task.
+ * 5. Pick any idle CPU within the @cpus_allowed domain.
  *
  * Step 3 and 4 are performed only if the system has, respectively,
  * multiple LLCs / multiple NUMA nodes (see scx_selcpu_topo_llc and
@@ -446,6 +449,39 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 	int node = scx_cpu_node_if_enabled(prev_cpu);
 	s32 cpu;
 
+	preempt_disable();
+
+	/*
+	 * Determine the subset of CPUs usable by @p within @cpus_allowed.
+	 */
+	if (allowed != p->cpus_ptr) {
+		struct cpumask *local_cpus = this_cpu_cpumask_var_ptr(local_idle_cpumask);
+
+		if (task_affinity_all(p)) {
+			allowed = cpus_allowed;
+		} else if (cpumask_and(local_cpus, cpus_allowed, p->cpus_ptr)) {
+			allowed = local_cpus;
+		} else {
+			cpu = -EBUSY;
+			goto out_enable;
+		}
+
+		/*
+		 * If @prev_cpu is not in the allowed CPUs, skip topology
+		 * optimizations and try to pick any idle CPU usable by the
+		 * task.
+		 *
+		 * If %SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled, prioritize
+		 * the current node, as it may optimize some waker->wakee
+		 * workloads.
+		 */
+		if (!cpumask_test_cpu(prev_cpu, allowed)) {
+			node = scx_cpu_node_if_enabled(smp_processor_id());
+			cpu = scx_pick_idle_cpu(allowed, node, flags);
+			goto out_enable;
+		}
+	}
+
 	/*
 	 * This is necessary to protect llc_cpus.
 	 */
@@ -610,6 +646,8 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 
 out_unlock:
 	rcu_read_unlock();
+out_enable:
+	preempt_enable();
 
 	return cpu;
 }
@@ -641,6 +679,8 @@ void scx_idle_init_masks(void)
 
 	/* Allocate local per-cpu idle cpumasks */
 	for_each_possible_cpu(i) {
+		BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_idle_cpumask, i),
+					       GFP_KERNEL, cpu_to_node(i)));
 		BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_llc_idle_cpumask, i),
 					       GFP_KERNEL, cpu_to_node(i)));
 		BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_numa_idle_cpumask, i),