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@xxxxxxxxxx> 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@xxxxxxxxxx> > */ > > +/* > + * 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