Re: [PATCH 07/10] sched_ext: Introduce per-node idle cpumasks

Yury Norov
2024-12-24T04:05:56Z

On Fri, Dec 20, 2024 at 04:11:39PM +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.
>
> 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.
>
> NOTE: if a scheduler 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.
>
> By default (when SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled), only the
> cpumask of node 0 is used as a single global flat CPU mask, maintaining
> the previous behavior.
>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

This is a rather big patch... Can you split it somehow? Maybe
introduce new functions in a separate patch, and use them in the
following patch(es)?

> ---
> kernel/sched/ext.c | 7 +-
> kernel/sched/ext_idle.c | 258 +++++++++++++++++++++++++++++++---------
> 2 files changed, 208 insertions(+), 57 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 148ec04d4a0a..143938e935f1 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3228,7 +3228,7 @@ static void handle_hotplug(struct rq *rq, bool online)
> atomic_long_inc(&scx_hotplug_seq);
>
> if (scx_enabled())
> - update_selcpu_topology();
> + update_selcpu_topology(&scx_ops);
>
> if (online && SCX_HAS_OP(cpu_online))
> SCX_CALL_OP(SCX_KF_UNLOCKED, cpu_online, cpu);
> @@ -5107,7 +5107,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
>
> check_hotplug_seq(ops);
> #ifdef CONFIG_SMP
> - update_selcpu_topology();
> + update_selcpu_topology(ops);
> #endif
> cpus_read_unlock();
>
> @@ -5800,8 +5800,7 @@ void __init init_sched_ext_class(void)
>
> BUG_ON(rhashtable_init(&dsq_hash, &dsq_hash_params));
> #ifdef CONFIG_SMP
> - BUG_ON(!alloc_cpumask_var(&idle_masks.cpu, GFP_KERNEL));
> - BUG_ON(!alloc_cpumask_var(&idle_masks.smt, GFP_KERNEL));
> + idle_masks_init();
> #endif
> scx_kick_cpus_pnt_seqs =
> __alloc_percpu(sizeof(scx_kick_cpus_pnt_seqs[0]) * nr_cpu_ids,
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index 4952e2793304..444f2a15f1d4 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -10,7 +10,14 @@
> * Copyright (c) 2024 Andrea Righi <arighi@nvidia.com>
> */
>
> +/*
> + * If NUMA awareness is disabled consider only node 0 as a single global
> + * NUMA node.
> + */
> +#define NUMA_FLAT_NODE 0

If it's a global idle node maybe

#define GLOBAL_IDLE_NODE 0

This actually bypasses NUMA, so it's weird to mention NUMA here.

> +
> static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
> +static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_per_node);
>
> static bool check_builtin_idle_enabled(void)
> {
> @@ -22,22 +29,82 @@ static bool check_builtin_idle_enabled(void)
> }
>
> #ifdef CONFIG_SMP
> -#ifdef CONFIG_CPUMASK_OFFSTACK
> -#define CL_ALIGNED_IF_ONSTACK
> -#else
> -#define CL_ALIGNED_IF_ONSTACK __cacheline_aligned_in_smp
> -#endif
> -
> -static struct {
> +struct idle_cpumask {
> cpumask_var_t cpu;
> cpumask_var_t smt;
> -} idle_masks CL_ALIGNED_IF_ONSTACK;
> +};

We already have struct cpumask, and this struct idle_cpumask may
mislead. Maybe struct idle_cpus or something?

> +
> +/*
> + * cpumasks to track idle CPUs within each NUMA node.
> + *
> + * If SCX_OPS_BUILTIN_IDLE_PER_NODE is not specified, a single flat cpumask
> + * from node 0 is used to track all idle CPUs system-wide.
> + */
> +static struct idle_cpumask **scx_idle_masks;
> +
> +static struct idle_cpumask *get_idle_mask(int node)

Didn't we agree to drop this 'get' thing?

> +{
> + if (node == NUMA_NO_NODE)
> + node = numa_node_id();
> + else if (WARN_ON_ONCE(node < 0 || node >= nr_node_ids))
> + return NULL;

Kernel users always provide correct parameters. I don't even think you
need to check for NO_NODE, because if I as user of your API need to
provide current node, I can use numa_node_id() just as well.

If you drop all that sanity bloating, your function will be a
one-liner, and the question is: do you need it at all?

We usually need such wrappers to apply 'const' qualifier or do some
housekeeping before dereferencing. But in this case you just return
a pointer, and I don't understand why local users can't do it
themself.

The following idle_mask_init() happily ignores just added accessor...

> + return scx_idle_masks[node];
> +}

> +
> +static struct cpumask *get_idle_cpumask(int node)
> +{
> + struct idle_cpumask *mask = get_idle_mask(node);
> +
> + return mask ? mask->cpu : cpu_none_mask;
> +}
> +
> +static struct cpumask *get_idle_smtmask(int node)
> +{
> + struct idle_cpumask *mask = get_idle_mask(node);
> +
> + return mask ? mask->smt : cpu_none_mask;
> +}

For those two guys... I think you agreed with Tejun that you don't
need them. To me the following is more verbose:

idle_cpus(node)->smt;

> +
> +static void idle_masks_init(void)
> +{
> + int node;
> +
> + scx_idle_masks = kcalloc(num_possible_nodes(), sizeof(*scx_idle_masks), GFP_KERNEL);
> + BUG_ON(!scx_idle_masks);
> +
> + for_each_node_state(node, N_POSSIBLE) {
> + scx_idle_masks[node] = kzalloc_node(sizeof(**scx_idle_masks), GFP_KERNEL, node);
> + BUG_ON(!scx_idle_masks[node]);
> +
> + BUG_ON(!alloc_cpumask_var_node(&scx_idle_masks[node]->cpu, GFP_KERNEL, node));
> + BUG_ON(!alloc_cpumask_var_node(&scx_idle_masks[node]->smt, GFP_KERNEL, node));
> + }
> +}
>
> static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
> static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
>
> +/*
> + * 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)
> +{
> + int node;
> +
> + if (static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
> + node = cpu_to_node(cpu);

Nit: can you just return cpu_to_node(cpu). This will save 3 LOCs

> + else
> + node = NUMA_FLAT_NODE;
> +
> + return node;
> +}
> +
> static bool test_and_clear_cpu_idle(int cpu)
> {
> + int node = idle_cpu_to_node(cpu);
> + struct cpumask *idle_cpus = get_idle_cpumask(node);
> +
> #ifdef CONFIG_SCHED_SMT
> /*
> * SMT mask should be cleared whether we can claim @cpu or not. The SMT
> @@ -46,33 +113,37 @@ static bool test_and_clear_cpu_idle(int cpu)
> */
> if (sched_smt_active()) {
> const struct cpumask *smt = cpu_smt_mask(cpu);
> + struct cpumask *idle_smts = get_idle_smtmask(node);
>
> /*
> * 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);
> }
>
> -static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
> +/*
> + * Pick an idle CPU in a specific NUMA node.
> + */
> +static 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(get_idle_smtmask(node), cpus_allowed);
> if (cpu < nr_cpu_ids)
> goto found;
>
> @@ -80,15 +151,57 @@ static 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(get_idle_cpumask(node), cpus_allowed);
> if (cpu >= nr_cpu_ids)
> return -EBUSY;
>
> found:
> if (test_and_clear_cpu_idle(cpu))
> return cpu;
> - else
> - goto retry;
> + goto retry;
> +}

Yes, I see this too. But to me minimizing your patch and preserving as
much history as you can is more important.

After all, newcomers should have a room to practice :)

> +
> +/*
> + * Find the best idle CPU in the system, relative to @node.
> + *
> + * If @node is NUMA_NO_NODE, start from the current node.
> + */

And if you don't invent this rule for kernel users, you don't need to
explain it everywhere.

> +static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> +{
> + nodemask_t hop_nodes = NODE_MASK_NONE;
> + s32 cpu = -EBUSY;
> +
> + if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
> + return pick_idle_cpu_from_node(cpus_allowed, NUMA_FLAT_NODE, flags);
> +
> + /*
> + * If a NUMA node was not specified, start with the current one.
> + */
> + if (node == NUMA_NO_NODE)
> + node = numa_node_id();

And enforce too...

> +
> + /*
> + * Traverse all nodes in order of increasing distance, starting
> + * from prev_cpu's 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_hop_node(n, node, hop_nodes, N_POSSIBLE) {
> + cpu = pick_idle_cpu_from_node(cpus_allowed, n, flags);
> + if (cpu >= 0)
> + break;
> + }
> +
> + return cpu;
> }
>
> /*
> @@ -208,7 +321,7 @@ static bool llc_numa_mismatch(void)
> * CPU belongs to a single LLC domain, and that each LLC domain is entirely
> * contained within a single NUMA node.
> */
> -static void update_selcpu_topology(void)
> +static void update_selcpu_topology(struct sched_ext_ops *ops)
> {
> bool enable_llc = false, enable_numa = false;
> unsigned int nr_cpus;
> @@ -298,6 +411,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> {
> const struct cpumask *llc_cpus = NULL;
> const struct cpumask *numa_cpus = NULL;
> + int node = idle_cpu_to_node(prev_cpu);
> s32 cpu;
>
> *found = false;
> @@ -355,9 +469,9 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> * piled up on it even if there is an idle core elsewhere on
> * the system.
> */
> - if (!cpumask_empty(idle_masks.cpu) &&
> - !(current->flags & PF_EXITING) &&
> - cpu_rq(cpu)->scx.local_dsq.nr == 0) {
> + if (!(current->flags & PF_EXITING) &&
> + cpu_rq(cpu)->scx.local_dsq.nr == 0 &&
> + !cpumask_empty(get_idle_cpumask(idle_cpu_to_node(cpu)))) {
> if (cpumask_test_cpu(cpu, p->cpus_ptr))
> goto cpu_found;
> }
> @@ -371,7 +485,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> /*
> * Keep using @prev_cpu if it's part of a fully idle core.
> */
> - if (cpumask_test_cpu(prev_cpu, idle_masks.smt) &&
> + if (cpumask_test_cpu(prev_cpu, get_idle_smtmask(node)) &&
> test_and_clear_cpu_idle(prev_cpu)) {
> cpu = prev_cpu;
> goto cpu_found;
> @@ -381,7 +495,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> * Search for any fully idle core in the same LLC domain.
> */
> if (llc_cpus) {
> - cpu = scx_pick_idle_cpu(llc_cpus, SCX_PICK_IDLE_CORE);
> + cpu = pick_idle_cpu_from_node(llc_cpus, node, SCX_PICK_IDLE_CORE);
> if (cpu >= 0)
> goto cpu_found;
> }
> @@ -390,15 +504,19 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> * Search for any fully idle core in the same NUMA node.
> */
> if (numa_cpus) {
> - cpu = scx_pick_idle_cpu(numa_cpus, SCX_PICK_IDLE_CORE);
> + cpu = scx_pick_idle_cpu(numa_cpus, node, SCX_PICK_IDLE_CORE);
> if (cpu >= 0)
> goto cpu_found;
> }
>
> /*
> * Search for any full idle core usable by the task.
> + *
> + * If NUMA aware idle selection is enabled, the search will
> + * begin in prev_cpu's node and proceed to other nodes in
> + * order of increasing distance.
> */
> - cpu = scx_pick_idle_cpu(p->cpus_ptr, SCX_PICK_IDLE_CORE);
> + cpu = scx_pick_idle_cpu(p->cpus_ptr, node, SCX_PICK_IDLE_CORE);
> if (cpu >= 0)
> goto cpu_found;
> }
> @@ -415,7 +533,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> * Search for any idle CPU in the same LLC domain.
> */
> if (llc_cpus) {
> - cpu = scx_pick_idle_cpu(llc_cpus, 0);
> + cpu = pick_idle_cpu_from_node(llc_cpus, node, 0);
> if (cpu >= 0)
> goto cpu_found;
> }
> @@ -424,7 +542,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> * Search for any idle CPU in the same NUMA node.
> */
> if (numa_cpus) {
> - cpu = scx_pick_idle_cpu(numa_cpus, 0);
> + cpu = pick_idle_cpu_from_node(numa_cpus, node, 0);
> if (cpu >= 0)
> goto cpu_found;
> }
> @@ -432,7 +550,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> /*
> * Search for any idle CPU usable by the task.
> */
> - cpu = scx_pick_idle_cpu(p->cpus_ptr, 0);
> + cpu = scx_pick_idle_cpu(p->cpus_ptr, node, 0);
> if (cpu >= 0)
> goto cpu_found;
>
> @@ -448,17 +566,33 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
>
> static void reset_idle_masks(void)
> {
> + int node;
> +
> + if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
> + cpumask_copy(get_idle_cpumask(NUMA_FLAT_NODE), cpu_online_mask);
> + cpumask_copy(get_idle_smtmask(NUMA_FLAT_NODE), 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_state(node, N_POSSIBLE) {
> + const struct cpumask *node_mask = cpumask_of_node(node);
> + struct cpumask *idle_cpu = get_idle_cpumask(node);
> + struct cpumask *idle_smt = get_idle_smtmask(node);
> +
> + cpumask_and(idle_cpu, cpu_online_mask, node_mask);
> + cpumask_copy(idle_smt, idle_cpu);

Tejun asked you to use cpumask_and() in both cases, didn't he?

> + }
> }
>
> void __scx_update_idle(struct rq *rq, bool idle)
> {
> int cpu = cpu_of(rq);
> + int node = idle_cpu_to_node(cpu);
> + struct cpumask *idle_cpu = get_idle_cpumask(node);
>
> if (SCX_HAS_OP(update_idle) && !scx_rq_bypassing(rq)) {
> SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle);
> @@ -466,24 +600,25 @@ void __scx_update_idle(struct rq *rq, bool idle)
> return;
> }
>
> - assign_cpu(cpu, idle_masks.cpu, idle);
> + assign_cpu(cpu, idle_cpu, idle);
>
> #ifdef CONFIG_SCHED_SMT
> if (sched_smt_active()) {
> const struct cpumask *smt = cpu_smt_mask(cpu);
> + struct cpumask *idle_smt = get_idle_smtmask(node);
>
> if (idle) {
> /*
> - * idle_masks.smt handling is racy but that's fine as
> - * it's only for optimization and self-correcting.
> + * idle_smt handling is racy but that's fine as it's
> + * only for optimization and self-correcting.
> */
> for_each_cpu(cpu, smt) {
> - if (!cpumask_test_cpu(cpu, idle_masks.cpu))
> + if (!cpumask_test_cpu(cpu, idle_cpu))
> return;
> }
> - cpumask_or(idle_masks.smt, idle_masks.smt, smt);
> + cpumask_or(idle_smt, idle_smt, smt);
> } else {
> - cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
> + cpumask_andnot(idle_smt, idle_smt, smt);
> }
> }
> #endif
> @@ -491,8 +626,23 @@ void __scx_update_idle(struct rq *rq, bool idle)
>
> #else /* !CONFIG_SMP */
>
> +static struct cpumask *get_idle_cpumask(int node)
> +{
> + return cpu_none_mask;
> +}
> +
> +static struct cpumask *get_idle_smtmask(int node)
> +{
> + return cpu_none_mask;
> +}
> +
> static bool test_and_clear_cpu_idle(int cpu) { return false; }
> -static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags) { return -EBUSY; }
> +
> +static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> +{
> + return -EBUSY;
> +}
> +
> static void reset_idle_masks(void) {}
>
> #endif /* CONFIG_SMP */
> @@ -546,11 +696,12 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask(void)
> if (!check_builtin_idle_enabled())
> return cpu_none_mask;
>
> -#ifdef CONFIG_SMP
> - return idle_masks.cpu;
> -#else
> - return cpu_none_mask;
> -#endif
> + if (static_branch_unlikely(&scx_builtin_idle_per_node)) {
> + scx_ops_error("SCX_OPS_BUILTIN_IDLE_PER_NODE enabled");
> + return cpu_none_mask;
> + }
> +
> + return get_idle_cpumask(NUMA_FLAT_NODE);
> }
>
> /**
> @@ -565,14 +716,15 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask(void)
> if (!check_builtin_idle_enabled())
> return cpu_none_mask;
>
> -#ifdef CONFIG_SMP
> + if (static_branch_unlikely(&scx_builtin_idle_per_node)) {
> + scx_ops_error("SCX_OPS_BUILTIN_IDLE_PER_NODE enabled");
> + return cpu_none_mask;
> + }
> +
> if (sched_smt_active())
> - return idle_masks.smt;
> + return get_idle_smtmask(NUMA_FLAT_NODE);
> else
> - return idle_masks.cpu;
> -#else
> - return cpu_none_mask;
> -#endif
> + return get_idle_cpumask(NUMA_FLAT_NODE);
> }
>
> /**
> @@ -635,7 +787,7 @@ __bpf_kfunc s32 scx_bpf_pick_idle_cpu(const struct cpumask *cpus_allowed,
> if (!check_builtin_idle_enabled())
> return -EBUSY;
>
> - return scx_pick_idle_cpu(cpus_allowed, flags);
> + return scx_pick_idle_cpu(cpus_allowed, NUMA_NO_NODE, flags);
> }
>
> /**
> @@ -658,7 +810,7 @@ __bpf_kfunc s32 scx_bpf_pick_any_cpu(const struct cpumask *cpus_allowed,
> s32 cpu;
>
> if (static_branch_likely(&scx_builtin_idle_enabled)) {
> - cpu = scx_pick_idle_cpu(cpus_allowed, flags);
> + cpu = scx_pick_idle_cpu(cpus_allowed, NUMA_NO_NODE, flags);
> if (cpu >= 0)
> return cpu;
> }
> --
> 2.47.1