Commit c2d8b2a5 authored by Andrea Righi's avatar Andrea Righi Committed by Tejun Heo
Browse files

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: default avatarAndrea Righi <arighi@nvidia.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 23c63a96

kernel/sched/ext_idle.c

+44 −4
Original line number Diff line number Diff line
@@ -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),