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

[Yury Norov <yury.norov@gmail.com>]

On Fri, Feb 07, 2025 at 09:40:52PM +0100, Andrea Righi wrote:
> Using a single global idle mask can lead to inefficiencies and a lot of
> stress on the cache coherency protocol on large systems with multiple
> NUMA nodes, since all the CPUs can create a really intense read/write
> activity on the single global cpumask.

Can you put your perf numbers here too?
 
> Therefore, split the global cpumask into multiple per-NUMA node cpumasks
> to improve scalability and performance on large systems.
> 
> The concept is that each cpumask will track only the idle CPUs within
> its corresponding NUMA node, treating CPUs in other NUMA nodes as busy.
> In this way concurrent access to the idle cpumask will be restricted
> within each NUMA node.
> 
> The split of multiple per-node idle cpumasks can be controlled using the
> SCX_OPS_BUILTIN_IDLE_PER_NODE flag.
> 
> By default SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled and a global
> host-wide idle cpumask is used, maintaining the previous behavior.
> 
> NOTE: if a scheduler explicitly enables the per-node idle cpumasks (via
> SCX_OPS_BUILTIN_IDLE_PER_NODE), scx_bpf_get_idle_cpu/smtmask() will
> trigger an scx error, since there are no system-wide cpumasks.
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/ext_idle.c | 242 ++++++++++++++++++++++++++++++++--------
>  kernel/sched/ext_idle.h |  11 +-
>  2 files changed, 203 insertions(+), 50 deletions(-)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index a3f2b00903ac2..4b90ec9018c1a 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -18,25 +18,88 @@ DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
>  DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_per_node);
>  
>  #ifdef CONFIG_SMP
> -#ifdef CONFIG_CPUMASK_OFFSTACK
> -#define CL_ALIGNED_IF_ONSTACK
> -#else
> -#define CL_ALIGNED_IF_ONSTACK __cacheline_aligned_in_smp
> -#endif
> -
>  /* Enable/disable LLC aware optimizations */
>  DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
>  
>  /* Enable/disable NUMA aware optimizations */
>  DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
>  
> -static struct {
> +/*
> + * 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;
> +};
> +
> +/*
> + * Global host-wide idle cpumasks (used when SCX_OPS_BUILTIN_IDLE_PER_NODE
> + * is not enabled).
> + */
> +static struct idle_cpus scx_idle_global_masks;
> +
> +/*
> + * Per-node idle cpumasks.
> + */
> +static struct idle_cpus **scx_idle_node_masks;
> +
> +/*
> + * Initialize per-node idle cpumasks.
> + *
> + * In case of a single NUMA node or if NUMA support is disabled, only a
> + * single global host-wide cpumask will be initialized.
> + */
> +void scx_idle_init_masks(void)
> +{
> +	int node;
> +
> +	/* Allocate global idle cpumasks */
> +	BUG_ON(!alloc_cpumask_var(&scx_idle_global_masks.cpu, GFP_KERNEL));
> +	BUG_ON(!alloc_cpumask_var(&scx_idle_global_masks.smt, GFP_KERNEL));
> +
> +	/* Allocate per-node idle cpumasks */
> +	scx_idle_node_masks = kcalloc(num_possible_nodes(),
> +				      sizeof(*scx_idle_node_masks), GFP_KERNEL);
> +	BUG_ON(!scx_idle_node_masks);
> +
> +	for_each_node(node) {
> +		scx_idle_node_masks[node] = kzalloc_node(sizeof(**scx_idle_node_masks),
> +							 GFP_KERNEL, node);
> +		BUG_ON(!scx_idle_node_masks[node]);
> +
> +		BUG_ON(!alloc_cpumask_var_node(&scx_idle_node_masks[node]->cpu, GFP_KERNEL, node));
> +		BUG_ON(!alloc_cpumask_var_node(&scx_idle_node_masks[node]->smt, GFP_KERNEL, node));
> +	}
> +}
> +
> +/*
> + * Return the idle masks associated to a target @node.
> + */
> +static struct idle_cpus *idle_cpumask(int node)
> +{
> +	return node == NUMA_NO_NODE ? &scx_idle_global_masks : scx_idle_node_masks[node];
> +}
> +
> +/*
> + * Return the node id associated to a target idle CPU (used to determine
> + * the proper idle cpumask).
> + */
> +static int idle_cpu_to_node(int cpu)
> +{
> +	if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
> +		return NUMA_NO_NODE;
> +
> +	return cpu_to_node(cpu);
> +}
>  
>  bool scx_idle_test_and_clear_cpu(int cpu)
>  {
> +	int node = idle_cpu_to_node(cpu);
> +	struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
> +
>  #ifdef CONFIG_SCHED_SMT
>  	/*
>  	 * SMT mask should be cleared whether we can claim @cpu or not. The SMT
> @@ -45,33 +108,38 @@ bool scx_idle_test_and_clear_cpu(int cpu)
>  	 */
>  	if (sched_smt_active()) {
>  		const struct cpumask *smt = cpu_smt_mask(cpu);
> +		struct cpumask *idle_smts = idle_cpumask(node)->smt;
>  
>  		/*
>  		 * If offline, @cpu is not its own sibling and
>  		 * scx_pick_idle_cpu() can get caught in an infinite loop as
> -		 * @cpu is never cleared from idle_masks.smt. Ensure that @cpu
> -		 * is eventually cleared.
> +		 * @cpu is never cleared from the idle SMT mask. Ensure that
> +		 * @cpu is eventually cleared.
>  		 *
>  		 * NOTE: Use cpumask_intersects() and cpumask_test_cpu() to
>  		 * reduce memory writes, which may help alleviate cache
>  		 * coherence pressure.
>  		 */
> -		if (cpumask_intersects(smt, idle_masks.smt))
> -			cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
> -		else if (cpumask_test_cpu(cpu, idle_masks.smt))
> -			__cpumask_clear_cpu(cpu, idle_masks.smt);
> +		if (cpumask_intersects(smt, idle_smts))
> +			cpumask_andnot(idle_smts, idle_smts, smt);
> +		else if (cpumask_test_cpu(cpu, idle_smts))
> +			__cpumask_clear_cpu(cpu, idle_smts);
>  	}
>  #endif
> -	return cpumask_test_and_clear_cpu(cpu, idle_masks.cpu);
> +
> +	return cpumask_test_and_clear_cpu(cpu, idle_cpus);
>  }
>  
> -s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
> +/*
> + * Pick an idle CPU in a specific NUMA node.
> + */
> +s32 pick_idle_cpu_from_node(const struct cpumask *cpus_allowed, int node, u64 flags)
>  {
>  	int cpu;
>  
>  retry:
>  	if (sched_smt_active()) {
> -		cpu = cpumask_any_and_distribute(idle_masks.smt, cpus_allowed);
> +		cpu = cpumask_any_and_distribute(idle_cpumask(node)->smt, cpus_allowed);
>  		if (cpu < nr_cpu_ids)
>  			goto found;
>  
> @@ -79,7 +147,7 @@ s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
>  			return -EBUSY;
>  	}
>  
> -	cpu = cpumask_any_and_distribute(idle_masks.cpu, cpus_allowed);
> +	cpu = cpumask_any_and_distribute(idle_cpumask(node)->cpu, cpus_allowed);
>  	if (cpu >= nr_cpu_ids)
>  		return -EBUSY;
>  
> @@ -90,6 +158,55 @@ s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
>  		goto retry;
>  }
>  
> +/*
> + * 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);
> +		if (cpu >= 0)
> +			break;
> +
> +		/*
> +		 * Check if the search is restricted to the same core or
> +		 * the same node.
> +		 */
> +		if (flags & SCX_PICK_IDLE_IN_NODE)
> +			break;

If SCX_PICK_IDLE_IN_NODE is set, you can avoid the loop at all, right?
Just:
	if (flags & SCX_PICK_IDLE_IN_NODE)
	        return pick_idle_cpu_from_node(cpus_allowed, node, flags);

	for_each_numa_node(node, unvisited, N_POSSIBLE) {
		cpu = pick_idle_cpu_from_node(cpus_allowed, node, flags);
		if (cpu >= 0)
			return cpu;
        }

Thanks,
Yury