1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
4 *
5 * Built-in idle CPU tracking policy.
6 *
7 * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
8 * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
9 * Copyright (c) 2022 David Vernet <dvernet@meta.com>
10 * Copyright (c) 2024 Andrea Righi <arighi@nvidia.com>
11 */
12 #include "ext_idle.h"
13
14 /* Enable/disable built-in idle CPU selection policy */
15 static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
16
17 /* Enable/disable per-node idle cpumasks */
18 static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_per_node);
19
20 /* Enable/disable LLC aware optimizations */
21 static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
22
23 /* Enable/disable NUMA aware optimizations */
24 static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
25
26 /*
27 * cpumasks to track idle CPUs within each NUMA node.
28 *
29 * If SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled, a single global cpumask
30 * from is used to track all the idle CPUs in the system.
31 */
32 struct scx_idle_cpus {
33 cpumask_var_t cpu;
34 cpumask_var_t smt;
35 };
36
37 /*
38 * Global host-wide idle cpumasks (used when SCX_OPS_BUILTIN_IDLE_PER_NODE
39 * is not enabled).
40 */
41 static struct scx_idle_cpus scx_idle_global_masks;
42
43 /*
44 * Per-node idle cpumasks.
45 */
46 static struct scx_idle_cpus **scx_idle_node_masks;
47
48 /*
49 * Local per-CPU cpumasks (used to generate temporary idle cpumasks).
50 */
51 static DEFINE_PER_CPU(cpumask_var_t, local_idle_cpumask);
52 static DEFINE_PER_CPU(cpumask_var_t, local_llc_idle_cpumask);
53 static DEFINE_PER_CPU(cpumask_var_t, local_numa_idle_cpumask);
54
55 /*
56 * Return the idle masks associated to a target @node.
57 *
58 * NUMA_NO_NODE identifies the global idle cpumask.
59 */
idle_cpumask(int node)60 static struct scx_idle_cpus *idle_cpumask(int node)
61 {
62 return node == NUMA_NO_NODE ? &scx_idle_global_masks : scx_idle_node_masks[node];
63 }
64
65 /*
66 * Returns the NUMA node ID associated with a @cpu, or NUMA_NO_NODE if
67 * per-node idle cpumasks are disabled.
68 */
scx_cpu_node_if_enabled(int cpu)69 static int scx_cpu_node_if_enabled(int cpu)
70 {
71 if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
72 return NUMA_NO_NODE;
73
74 return cpu_to_node(cpu);
75 }
76
scx_idle_test_and_clear_cpu(int cpu)77 static bool scx_idle_test_and_clear_cpu(int cpu)
78 {
79 int node = scx_cpu_node_if_enabled(cpu);
80 struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
81
82 #ifdef CONFIG_SCHED_SMT
83 /*
84 * SMT mask should be cleared whether we can claim @cpu or not. The SMT
85 * cluster is not wholly idle either way. This also prevents
86 * scx_pick_idle_cpu() from getting caught in an infinite loop.
87 */
88 if (sched_smt_active()) {
89 const struct cpumask *smt = cpu_smt_mask(cpu);
90 struct cpumask *idle_smts = idle_cpumask(node)->smt;
91
92 /*
93 * If offline, @cpu is not its own sibling and
94 * scx_pick_idle_cpu() can get caught in an infinite loop as
95 * @cpu is never cleared from the idle SMT mask. Ensure that
96 * @cpu is eventually cleared.
97 *
98 * NOTE: Use cpumask_intersects() and cpumask_test_cpu() to
99 * reduce memory writes, which may help alleviate cache
100 * coherence pressure.
101 */
102 if (cpumask_intersects(smt, idle_smts))
103 cpumask_andnot(idle_smts, idle_smts, smt);
104 else if (cpumask_test_cpu(cpu, idle_smts))
105 __cpumask_clear_cpu(cpu, idle_smts);
106 }
107 #endif
108
109 return cpumask_test_and_clear_cpu(cpu, idle_cpus);
110 }
111
112 /*
113 * Pick an idle CPU in a specific NUMA node.
114 */
pick_idle_cpu_in_node(const struct cpumask * cpus_allowed,int node,u64 flags)115 static s32 pick_idle_cpu_in_node(const struct cpumask *cpus_allowed, int node, u64 flags)
116 {
117 int cpu;
118
119 retry:
120 if (sched_smt_active()) {
121 cpu = cpumask_any_and_distribute(idle_cpumask(node)->smt, cpus_allowed);
122 if (cpu < nr_cpu_ids)
123 goto found;
124
125 if (flags & SCX_PICK_IDLE_CORE)
126 return -EBUSY;
127 }
128
129 cpu = cpumask_any_and_distribute(idle_cpumask(node)->cpu, cpus_allowed);
130 if (cpu >= nr_cpu_ids)
131 return -EBUSY;
132
133 found:
134 if (scx_idle_test_and_clear_cpu(cpu))
135 return cpu;
136 else
137 goto retry;
138 }
139
140 #ifdef CONFIG_NUMA
141 /*
142 * Tracks nodes that have not yet been visited when searching for an idle
143 * CPU across all available nodes.
144 */
145 static DEFINE_PER_CPU(nodemask_t, per_cpu_unvisited);
146
147 /*
148 * Search for an idle CPU across all nodes, excluding @node.
149 */
pick_idle_cpu_from_online_nodes(const struct cpumask * cpus_allowed,int node,u64 flags)150 static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
151 {
152 nodemask_t *unvisited;
153 s32 cpu = -EBUSY;
154
155 preempt_disable();
156 unvisited = this_cpu_ptr(&per_cpu_unvisited);
157
158 /*
159 * Restrict the search to the online nodes (excluding the current
160 * node that has been visited already).
161 */
162 nodes_copy(*unvisited, node_states[N_ONLINE]);
163 node_clear(node, *unvisited);
164
165 /*
166 * Traverse all nodes in order of increasing distance, starting
167 * from @node.
168 *
169 * This loop is O(N^2), with N being the amount of NUMA nodes,
170 * which might be quite expensive in large NUMA systems. However,
171 * this complexity comes into play only when a scheduler enables
172 * SCX_OPS_BUILTIN_IDLE_PER_NODE and it's requesting an idle CPU
173 * without specifying a target NUMA node, so it shouldn't be a
174 * bottleneck is most cases.
175 *
176 * As a future optimization we may want to cache the list of nodes
177 * in a per-node array, instead of actually traversing them every
178 * time.
179 */
180 for_each_node_numadist(node, *unvisited) {
181 cpu = pick_idle_cpu_in_node(cpus_allowed, node, flags);
182 if (cpu >= 0)
183 break;
184 }
185 preempt_enable();
186
187 return cpu;
188 }
189 #else
190 static inline s32
pick_idle_cpu_from_online_nodes(const struct cpumask * cpus_allowed,int node,u64 flags)191 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
192 {
193 return -EBUSY;
194 }
195 #endif
196
197 /*
198 * Find an idle CPU in the system, starting from @node.
199 */
scx_pick_idle_cpu(const struct cpumask * cpus_allowed,int node,u64 flags)200 static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
201 {
202 s32 cpu;
203
204 /*
205 * Always search in the starting node first (this is an
206 * optimization that can save some cycles even when the search is
207 * not limited to a single node).
208 */
209 cpu = pick_idle_cpu_in_node(cpus_allowed, node, flags);
210 if (cpu >= 0)
211 return cpu;
212
213 /*
214 * Stop the search if we are using only a single global cpumask
215 * (NUMA_NO_NODE) or if the search is restricted to the first node
216 * only.
217 */
218 if (node == NUMA_NO_NODE || flags & SCX_PICK_IDLE_IN_NODE)
219 return -EBUSY;
220
221 /*
222 * Extend the search to the other online nodes.
223 */
224 return pick_idle_cpu_from_online_nodes(cpus_allowed, node, flags);
225 }
226
227 /*
228 * Return the amount of CPUs in the same LLC domain of @cpu (or zero if the LLC
229 * domain is not defined).
230 */
llc_weight(s32 cpu)231 static unsigned int llc_weight(s32 cpu)
232 {
233 struct sched_domain *sd;
234
235 sd = rcu_dereference(per_cpu(sd_llc, cpu));
236 if (!sd)
237 return 0;
238
239 return sd->span_weight;
240 }
241
242 /*
243 * Return the cpumask representing the LLC domain of @cpu (or NULL if the LLC
244 * domain is not defined).
245 */
llc_span(s32 cpu)246 static struct cpumask *llc_span(s32 cpu)
247 {
248 struct sched_domain *sd;
249
250 sd = rcu_dereference(per_cpu(sd_llc, cpu));
251 if (!sd)
252 return NULL;
253
254 return sched_domain_span(sd);
255 }
256
257 /*
258 * Return the amount of CPUs in the same NUMA domain of @cpu (or zero if the
259 * NUMA domain is not defined).
260 */
numa_weight(s32 cpu)261 static unsigned int numa_weight(s32 cpu)
262 {
263 struct sched_domain *sd;
264 struct sched_group *sg;
265
266 sd = rcu_dereference(per_cpu(sd_numa, cpu));
267 if (!sd)
268 return 0;
269 sg = sd->groups;
270 if (!sg)
271 return 0;
272
273 return sg->group_weight;
274 }
275
276 /*
277 * Return the cpumask representing the NUMA domain of @cpu (or NULL if the NUMA
278 * domain is not defined).
279 */
numa_span(s32 cpu)280 static struct cpumask *numa_span(s32 cpu)
281 {
282 struct sched_domain *sd;
283 struct sched_group *sg;
284
285 sd = rcu_dereference(per_cpu(sd_numa, cpu));
286 if (!sd)
287 return NULL;
288 sg = sd->groups;
289 if (!sg)
290 return NULL;
291
292 return sched_group_span(sg);
293 }
294
295 /*
296 * Return true if the LLC domains do not perfectly overlap with the NUMA
297 * domains, false otherwise.
298 */
llc_numa_mismatch(void)299 static bool llc_numa_mismatch(void)
300 {
301 int cpu;
302
303 /*
304 * We need to scan all online CPUs to verify whether their scheduling
305 * domains overlap.
306 *
307 * While it is rare to encounter architectures with asymmetric NUMA
308 * topologies, CPU hotplugging or virtualized environments can result
309 * in asymmetric configurations.
310 *
311 * For example:
312 *
313 * NUMA 0:
314 * - LLC 0: cpu0..cpu7
315 * - LLC 1: cpu8..cpu15 [offline]
316 *
317 * NUMA 1:
318 * - LLC 0: cpu16..cpu23
319 * - LLC 1: cpu24..cpu31
320 *
321 * In this case, if we only check the first online CPU (cpu0), we might
322 * incorrectly assume that the LLC and NUMA domains are fully
323 * overlapping, which is incorrect (as NUMA 1 has two distinct LLC
324 * domains).
325 */
326 for_each_online_cpu(cpu)
327 if (llc_weight(cpu) != numa_weight(cpu))
328 return true;
329
330 return false;
331 }
332
333 /*
334 * Initialize topology-aware scheduling.
335 *
336 * Detect if the system has multiple LLC or multiple NUMA domains and enable
337 * cache-aware / NUMA-aware scheduling optimizations in the default CPU idle
338 * selection policy.
339 *
340 * Assumption: the kernel's internal topology representation assumes that each
341 * CPU belongs to a single LLC domain, and that each LLC domain is entirely
342 * contained within a single NUMA node.
343 */
scx_idle_update_selcpu_topology(struct sched_ext_ops * ops)344 void scx_idle_update_selcpu_topology(struct sched_ext_ops *ops)
345 {
346 bool enable_llc = false, enable_numa = false;
347 unsigned int nr_cpus;
348 s32 cpu = cpumask_first(cpu_online_mask);
349
350 /*
351 * Enable LLC domain optimization only when there are multiple LLC
352 * domains among the online CPUs. If all online CPUs are part of a
353 * single LLC domain, the idle CPU selection logic can choose any
354 * online CPU without bias.
355 *
356 * Note that it is sufficient to check the LLC domain of the first
357 * online CPU to determine whether a single LLC domain includes all
358 * CPUs.
359 */
360 rcu_read_lock();
361 nr_cpus = llc_weight(cpu);
362 if (nr_cpus > 0) {
363 if (nr_cpus < num_online_cpus())
364 enable_llc = true;
365 pr_debug("sched_ext: LLC=%*pb weight=%u\n",
366 cpumask_pr_args(llc_span(cpu)), llc_weight(cpu));
367 }
368
369 /*
370 * Enable NUMA optimization only when there are multiple NUMA domains
371 * among the online CPUs and the NUMA domains don't perfectly overlaps
372 * with the LLC domains.
373 *
374 * If all CPUs belong to the same NUMA node and the same LLC domain,
375 * enabling both NUMA and LLC optimizations is unnecessary, as checking
376 * for an idle CPU in the same domain twice is redundant.
377 *
378 * If SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled ignore the NUMA
379 * optimization, as we would naturally select idle CPUs within
380 * specific NUMA nodes querying the corresponding per-node cpumask.
381 */
382 if (!(ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE)) {
383 nr_cpus = numa_weight(cpu);
384 if (nr_cpus > 0) {
385 if (nr_cpus < num_online_cpus() && llc_numa_mismatch())
386 enable_numa = true;
387 pr_debug("sched_ext: NUMA=%*pb weight=%u\n",
388 cpumask_pr_args(numa_span(cpu)), nr_cpus);
389 }
390 }
391 rcu_read_unlock();
392
393 pr_debug("sched_ext: LLC idle selection %s\n",
394 str_enabled_disabled(enable_llc));
395 pr_debug("sched_ext: NUMA idle selection %s\n",
396 str_enabled_disabled(enable_numa));
397
398 if (enable_llc)
399 static_branch_enable_cpuslocked(&scx_selcpu_topo_llc);
400 else
401 static_branch_disable_cpuslocked(&scx_selcpu_topo_llc);
402 if (enable_numa)
403 static_branch_enable_cpuslocked(&scx_selcpu_topo_numa);
404 else
405 static_branch_disable_cpuslocked(&scx_selcpu_topo_numa);
406 }
407
408 /*
409 * Return true if @p can run on all possible CPUs, false otherwise.
410 */
task_affinity_all(const struct task_struct * p)411 static inline bool task_affinity_all(const struct task_struct *p)
412 {
413 return p->nr_cpus_allowed >= num_possible_cpus();
414 }
415
416 /*
417 * Built-in CPU idle selection policy:
418 *
419 * 1. Prioritize full-idle cores:
420 * - always prioritize CPUs from fully idle cores (both logical CPUs are
421 * idle) to avoid interference caused by SMT.
422 *
423 * 2. Reuse the same CPU:
424 * - prefer the last used CPU to take advantage of cached data (L1, L2) and
425 * branch prediction optimizations.
426 *
427 * 3. Pick a CPU within the same LLC (Last-Level Cache):
428 * - if the above conditions aren't met, pick a CPU that shares the same
429 * LLC, if the LLC domain is a subset of @cpus_allowed, to maintain
430 * cache locality.
431 *
432 * 4. Pick a CPU within the same NUMA node, if enabled:
433 * - choose a CPU from the same NUMA node, if the node cpumask is a
434 * subset of @cpus_allowed, to reduce memory access latency.
435 *
436 * 5. Pick any idle CPU within the @cpus_allowed domain.
437 *
438 * Step 3 and 4 are performed only if the system has, respectively,
439 * multiple LLCs / multiple NUMA nodes (see scx_selcpu_topo_llc and
440 * scx_selcpu_topo_numa) and they don't contain the same subset of CPUs.
441 *
442 * If %SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled, the search will always
443 * begin in @prev_cpu's node and proceed to other nodes in order of
444 * increasing distance.
445 *
446 * Return the picked CPU if idle, or a negative value otherwise.
447 *
448 * NOTE: tasks that can only run on 1 CPU are excluded by this logic, because
449 * we never call ops.select_cpu() for them, see select_task_rq().
450 */
scx_select_cpu_dfl(struct task_struct * p,s32 prev_cpu,u64 wake_flags,const struct cpumask * cpus_allowed,u64 flags)451 s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
452 const struct cpumask *cpus_allowed, u64 flags)
453 {
454 const struct cpumask *llc_cpus = NULL, *numa_cpus = NULL;
455 const struct cpumask *allowed = cpus_allowed ?: p->cpus_ptr;
456 int node = scx_cpu_node_if_enabled(prev_cpu);
457 bool is_prev_allowed;
458 s32 cpu;
459
460 preempt_disable();
461
462 /*
463 * Check whether @prev_cpu is still within the allowed set. If not,
464 * we can still try selecting a nearby CPU.
465 */
466 is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
467
468 /*
469 * Determine the subset of CPUs usable by @p within @cpus_allowed.
470 */
471 if (allowed != p->cpus_ptr) {
472 struct cpumask *local_cpus = this_cpu_cpumask_var_ptr(local_idle_cpumask);
473
474 if (task_affinity_all(p)) {
475 allowed = cpus_allowed;
476 } else if (cpumask_and(local_cpus, cpus_allowed, p->cpus_ptr)) {
477 allowed = local_cpus;
478 } else {
479 cpu = -EBUSY;
480 goto out_enable;
481 }
482 }
483
484 /*
485 * This is necessary to protect llc_cpus.
486 */
487 rcu_read_lock();
488
489 /*
490 * Determine the subset of CPUs that the task can use in its
491 * current LLC and node.
492 *
493 * If the task can run on all CPUs, use the node and LLC cpumasks
494 * directly.
495 */
496 if (static_branch_maybe(CONFIG_NUMA, &scx_selcpu_topo_numa)) {
497 struct cpumask *local_cpus = this_cpu_cpumask_var_ptr(local_numa_idle_cpumask);
498 const struct cpumask *cpus = numa_span(prev_cpu);
499
500 if (allowed == p->cpus_ptr && task_affinity_all(p))
501 numa_cpus = cpus;
502 else if (cpus && cpumask_and(local_cpus, allowed, cpus))
503 numa_cpus = local_cpus;
504 }
505
506 if (static_branch_maybe(CONFIG_SCHED_MC, &scx_selcpu_topo_llc)) {
507 struct cpumask *local_cpus = this_cpu_cpumask_var_ptr(local_llc_idle_cpumask);
508 const struct cpumask *cpus = llc_span(prev_cpu);
509
510 if (allowed == p->cpus_ptr && task_affinity_all(p))
511 llc_cpus = cpus;
512 else if (cpus && cpumask_and(local_cpus, allowed, cpus))
513 llc_cpus = local_cpus;
514 }
515
516 /*
517 * If WAKE_SYNC, try to migrate the wakee to the waker's CPU.
518 */
519 if (wake_flags & SCX_WAKE_SYNC) {
520 int waker_node;
521
522 /*
523 * If the waker's CPU is cache affine and prev_cpu is idle,
524 * then avoid a migration.
525 */
526 cpu = smp_processor_id();
527 if (is_prev_allowed && cpus_share_cache(cpu, prev_cpu) &&
528 scx_idle_test_and_clear_cpu(prev_cpu)) {
529 cpu = prev_cpu;
530 goto out_unlock;
531 }
532
533 /*
534 * If the waker's local DSQ is empty, and the system is under
535 * utilized, try to wake up @p to the local DSQ of the waker.
536 *
537 * Checking only for an empty local DSQ is insufficient as it
538 * could give the wakee an unfair advantage when the system is
539 * oversaturated.
540 *
541 * Checking only for the presence of idle CPUs is also
542 * insufficient as the local DSQ of the waker could have tasks
543 * piled up on it even if there is an idle core elsewhere on
544 * the system.
545 */
546 waker_node = scx_cpu_node_if_enabled(cpu);
547 if (!(current->flags & PF_EXITING) &&
548 cpu_rq(cpu)->scx.local_dsq.nr == 0 &&
549 (!(flags & SCX_PICK_IDLE_IN_NODE) || (waker_node == node)) &&
550 !cpumask_empty(idle_cpumask(waker_node)->cpu)) {
551 if (cpumask_test_cpu(cpu, allowed))
552 goto out_unlock;
553 }
554 }
555
556 /*
557 * If CPU has SMT, any wholly idle CPU is likely a better pick than
558 * partially idle @prev_cpu.
559 */
560 if (sched_smt_active()) {
561 /*
562 * Keep using @prev_cpu if it's part of a fully idle core.
563 */
564 if (is_prev_allowed &&
565 cpumask_test_cpu(prev_cpu, idle_cpumask(node)->smt) &&
566 scx_idle_test_and_clear_cpu(prev_cpu)) {
567 cpu = prev_cpu;
568 goto out_unlock;
569 }
570
571 /*
572 * Search for any fully idle core in the same LLC domain.
573 */
574 if (llc_cpus) {
575 cpu = pick_idle_cpu_in_node(llc_cpus, node, SCX_PICK_IDLE_CORE);
576 if (cpu >= 0)
577 goto out_unlock;
578 }
579
580 /*
581 * Search for any fully idle core in the same NUMA node.
582 */
583 if (numa_cpus) {
584 cpu = pick_idle_cpu_in_node(numa_cpus, node, SCX_PICK_IDLE_CORE);
585 if (cpu >= 0)
586 goto out_unlock;
587 }
588
589 /*
590 * Search for any full-idle core usable by the task.
591 *
592 * If the node-aware idle CPU selection policy is enabled
593 * (%SCX_OPS_BUILTIN_IDLE_PER_NODE), the search will always
594 * begin in prev_cpu's node and proceed to other nodes in
595 * order of increasing distance.
596 */
597 cpu = scx_pick_idle_cpu(allowed, node, flags | SCX_PICK_IDLE_CORE);
598 if (cpu >= 0)
599 goto out_unlock;
600
601 /*
602 * Give up if we're strictly looking for a full-idle SMT
603 * core.
604 */
605 if (flags & SCX_PICK_IDLE_CORE) {
606 cpu = -EBUSY;
607 goto out_unlock;
608 }
609 }
610
611 /*
612 * Use @prev_cpu if it's idle.
613 */
614 if (is_prev_allowed && scx_idle_test_and_clear_cpu(prev_cpu)) {
615 cpu = prev_cpu;
616 goto out_unlock;
617 }
618
619 /*
620 * Search for any idle CPU in the same LLC domain.
621 */
622 if (llc_cpus) {
623 cpu = pick_idle_cpu_in_node(llc_cpus, node, 0);
624 if (cpu >= 0)
625 goto out_unlock;
626 }
627
628 /*
629 * Search for any idle CPU in the same NUMA node.
630 */
631 if (numa_cpus) {
632 cpu = pick_idle_cpu_in_node(numa_cpus, node, 0);
633 if (cpu >= 0)
634 goto out_unlock;
635 }
636
637 /*
638 * Search for any idle CPU usable by the task.
639 *
640 * If the node-aware idle CPU selection policy is enabled
641 * (%SCX_OPS_BUILTIN_IDLE_PER_NODE), the search will always begin
642 * in prev_cpu's node and proceed to other nodes in order of
643 * increasing distance.
644 */
645 cpu = scx_pick_idle_cpu(allowed, node, flags);
646
647 out_unlock:
648 rcu_read_unlock();
649 out_enable:
650 preempt_enable();
651
652 return cpu;
653 }
654
655 /*
656 * Initialize global and per-node idle cpumasks.
657 */
scx_idle_init_masks(void)658 void scx_idle_init_masks(void)
659 {
660 int i;
661
662 /* Allocate global idle cpumasks */
663 BUG_ON(!alloc_cpumask_var(&scx_idle_global_masks.cpu, GFP_KERNEL));
664 BUG_ON(!alloc_cpumask_var(&scx_idle_global_masks.smt, GFP_KERNEL));
665
666 /* Allocate per-node idle cpumasks (use nr_node_ids for non-contiguous NUMA nodes) */
667 scx_idle_node_masks = kzalloc_objs(*scx_idle_node_masks, nr_node_ids);
668 BUG_ON(!scx_idle_node_masks);
669
670 for_each_node(i) {
671 scx_idle_node_masks[i] = kzalloc_node(sizeof(**scx_idle_node_masks),
672 GFP_KERNEL, i);
673 BUG_ON(!scx_idle_node_masks[i]);
674
675 BUG_ON(!alloc_cpumask_var_node(&scx_idle_node_masks[i]->cpu, GFP_KERNEL, i));
676 BUG_ON(!alloc_cpumask_var_node(&scx_idle_node_masks[i]->smt, GFP_KERNEL, i));
677 }
678
679 /* Allocate local per-cpu idle cpumasks */
680 for_each_possible_cpu(i) {
681 BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_idle_cpumask, i),
682 GFP_KERNEL, cpu_to_node(i)));
683 BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_llc_idle_cpumask, i),
684 GFP_KERNEL, cpu_to_node(i)));
685 BUG_ON(!alloc_cpumask_var_node(&per_cpu(local_numa_idle_cpumask, i),
686 GFP_KERNEL, cpu_to_node(i)));
687 }
688 }
689
update_builtin_idle(int cpu,bool idle)690 static void update_builtin_idle(int cpu, bool idle)
691 {
692 int node = scx_cpu_node_if_enabled(cpu);
693 struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
694
695 assign_cpu(cpu, idle_cpus, idle);
696
697 #ifdef CONFIG_SCHED_SMT
698 if (sched_smt_active()) {
699 const struct cpumask *smt = cpu_smt_mask(cpu);
700 struct cpumask *idle_smts = idle_cpumask(node)->smt;
701
702 if (idle) {
703 /*
704 * idle_smt handling is racy but that's fine as it's
705 * only for optimization and self-correcting.
706 */
707 if (!cpumask_subset(smt, idle_cpus))
708 return;
709 cpumask_or(idle_smts, idle_smts, smt);
710 } else {
711 cpumask_andnot(idle_smts, idle_smts, smt);
712 }
713 }
714 #endif
715 }
716
717 /*
718 * Update the idle state of a CPU to @idle.
719 *
720 * If @do_notify is true, ops.update_idle() is invoked to notify the scx
721 * scheduler of an actual idle state transition (idle to busy or vice
722 * versa). If @do_notify is false, only the idle state in the idle masks is
723 * refreshed without invoking ops.update_idle().
724 *
725 * This distinction is necessary, because an idle CPU can be "reserved" and
726 * awakened via scx_bpf_pick_idle_cpu() + scx_bpf_kick_cpu(), marking it as
727 * busy even if no tasks are dispatched. In this case, the CPU may return
728 * to idle without a true state transition. Refreshing the idle masks
729 * without invoking ops.update_idle() ensures accurate idle state tracking
730 * while avoiding unnecessary updates and maintaining balanced state
731 * transitions.
732 */
__scx_update_idle(struct rq * rq,bool idle,bool do_notify)733 void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
734 {
735 struct scx_sched *sch = scx_root;
736 int cpu = cpu_of(rq);
737
738 lockdep_assert_rq_held(rq);
739
740 /*
741 * Update the idle masks:
742 * - for real idle transitions (do_notify == true)
743 * - for idle-to-idle transitions (indicated by the previous task
744 * being the idle thread, managed by pick_task_idle())
745 *
746 * Skip updating idle masks if the previous task is not the idle
747 * thread, since set_next_task_idle() has already handled it when
748 * transitioning from a task to the idle thread (calling this
749 * function with do_notify == true).
750 *
751 * In this way we can avoid updating the idle masks twice,
752 * unnecessarily.
753 */
754 if (static_branch_likely(&scx_builtin_idle_enabled))
755 if (do_notify || is_idle_task(rq->curr))
756 update_builtin_idle(cpu, idle);
757
758 /*
759 * Trigger ops.update_idle() only when transitioning from a task to
760 * the idle thread and vice versa.
761 *
762 * Idle transitions are indicated by do_notify being set to true,
763 * managed by put_prev_task_idle()/set_next_task_idle().
764 *
765 * This must come after builtin idle update so that BPF schedulers can
766 * create interlocking between ops.update_idle() and ops.enqueue() -
767 * either enqueue() sees the idle bit or update_idle() sees the task
768 * that enqueue() queued.
769 */
770 if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
771 SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
772 }
773
reset_idle_masks(struct sched_ext_ops * ops)774 static void reset_idle_masks(struct sched_ext_ops *ops)
775 {
776 int node;
777
778 /*
779 * Consider all online cpus idle. Should converge to the actual state
780 * quickly.
781 */
782 if (!(ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE)) {
783 cpumask_copy(idle_cpumask(NUMA_NO_NODE)->cpu, cpu_online_mask);
784 cpumask_copy(idle_cpumask(NUMA_NO_NODE)->smt, cpu_online_mask);
785 return;
786 }
787
788 for_each_node(node) {
789 const struct cpumask *node_mask = cpumask_of_node(node);
790
791 cpumask_and(idle_cpumask(node)->cpu, cpu_online_mask, node_mask);
792 cpumask_and(idle_cpumask(node)->smt, cpu_online_mask, node_mask);
793 }
794 }
795
scx_idle_enable(struct sched_ext_ops * ops)796 void scx_idle_enable(struct sched_ext_ops *ops)
797 {
798 if (!ops->update_idle || (ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE))
799 static_branch_enable_cpuslocked(&scx_builtin_idle_enabled);
800 else
801 static_branch_disable_cpuslocked(&scx_builtin_idle_enabled);
802
803 if (ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE)
804 static_branch_enable_cpuslocked(&scx_builtin_idle_per_node);
805 else
806 static_branch_disable_cpuslocked(&scx_builtin_idle_per_node);
807
808 reset_idle_masks(ops);
809 }
810
scx_idle_disable(void)811 void scx_idle_disable(void)
812 {
813 static_branch_disable(&scx_builtin_idle_enabled);
814 static_branch_disable(&scx_builtin_idle_per_node);
815 }
816
817 /********************************************************************************
818 * Helpers that can be called from the BPF scheduler.
819 */
820
validate_node(struct scx_sched * sch,int node)821 static int validate_node(struct scx_sched *sch, int node)
822 {
823 if (!static_branch_likely(&scx_builtin_idle_per_node)) {
824 scx_error(sch, "per-node idle tracking is disabled");
825 return -EOPNOTSUPP;
826 }
827
828 /* Return no entry for NUMA_NO_NODE (not a critical scx error) */
829 if (node == NUMA_NO_NODE)
830 return -ENOENT;
831
832 /* Make sure node is in a valid range */
833 if (node < 0 || node >= nr_node_ids) {
834 scx_error(sch, "invalid node %d", node);
835 return -EINVAL;
836 }
837
838 /* Make sure the node is part of the set of possible nodes */
839 if (!node_possible(node)) {
840 scx_error(sch, "unavailable node %d", node);
841 return -EINVAL;
842 }
843
844 return node;
845 }
846
847 __bpf_kfunc_start_defs();
848
check_builtin_idle_enabled(struct scx_sched * sch)849 static bool check_builtin_idle_enabled(struct scx_sched *sch)
850 {
851 if (static_branch_likely(&scx_builtin_idle_enabled))
852 return true;
853
854 scx_error(sch, "built-in idle tracking is disabled");
855 return false;
856 }
857
858 /*
859 * Determine whether @p is a migration-disabled task in the context of BPF
860 * code.
861 *
862 * We can't simply check whether @p->migration_disabled is set in a
863 * sched_ext callback, because the BPF prolog (__bpf_prog_enter) may disable
864 * migration for the current task while running BPF code.
865 *
866 * Since the BPF prolog calls migrate_disable() only when CONFIG_PREEMPT_RCU
867 * is enabled (via rcu_read_lock_dont_migrate()), migration_disabled == 1 for
868 * the current task is ambiguous only in that case: it could be from the BPF
869 * prolog rather than a real migrate_disable() call.
870 *
871 * Without CONFIG_PREEMPT_RCU, the BPF prolog never calls migrate_disable(),
872 * so migration_disabled == 1 always means the task is truly
873 * migration-disabled.
874 *
875 * Therefore, when migration_disabled == 1 and CONFIG_PREEMPT_RCU is enabled,
876 * check whether @p is the current task or not: if it is, then migration was
877 * not disabled before entering the callback, otherwise migration was disabled.
878 *
879 * Returns true if @p is migration-disabled, false otherwise.
880 */
is_bpf_migration_disabled(const struct task_struct * p)881 static bool is_bpf_migration_disabled(const struct task_struct *p)
882 {
883 if (p->migration_disabled == 1) {
884 if (IS_ENABLED(CONFIG_PREEMPT_RCU))
885 return p != current;
886 return true;
887 }
888 return p->migration_disabled;
889 }
890
select_cpu_from_kfunc(struct scx_sched * sch,struct task_struct * p,s32 prev_cpu,u64 wake_flags,const struct cpumask * allowed,u64 flags)891 static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,
892 s32 prev_cpu, u64 wake_flags,
893 const struct cpumask *allowed, u64 flags)
894 {
895 struct rq *rq;
896 struct rq_flags rf;
897 s32 cpu;
898
899 if (!ops_cpu_valid(sch, prev_cpu, NULL))
900 return -EINVAL;
901
902 if (!check_builtin_idle_enabled(sch))
903 return -EBUSY;
904
905 /*
906 * If called from an unlocked context, acquire the task's rq lock,
907 * so that we can safely access p->cpus_ptr and p->nr_cpus_allowed.
908 *
909 * Otherwise, allow to use this kfunc only from ops.select_cpu()
910 * and ops.select_enqueue().
911 */
912 if (scx_kf_allowed_if_unlocked()) {
913 rq = task_rq_lock(p, &rf);
914 } else {
915 if (!scx_kf_allowed(sch, SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE))
916 return -EPERM;
917 rq = scx_locked_rq();
918 }
919
920 /*
921 * Validate locking correctness to access p->cpus_ptr and
922 * p->nr_cpus_allowed: if we're holding an rq lock, we're safe;
923 * otherwise, assert that p->pi_lock is held.
924 */
925 if (!rq)
926 lockdep_assert_held(&p->pi_lock);
927
928 /*
929 * This may also be called from ops.enqueue(), so we need to handle
930 * per-CPU tasks as well. For these tasks, we can skip all idle CPU
931 * selection optimizations and simply check whether the previously
932 * used CPU is idle and within the allowed cpumask.
933 */
934 if (p->nr_cpus_allowed == 1 || is_bpf_migration_disabled(p)) {
935 if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
936 scx_idle_test_and_clear_cpu(prev_cpu))
937 cpu = prev_cpu;
938 else
939 cpu = -EBUSY;
940 } else {
941 cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags,
942 allowed ?: p->cpus_ptr, flags);
943 }
944
945 if (scx_kf_allowed_if_unlocked())
946 task_rq_unlock(rq, p, &rf);
947
948 return cpu;
949 }
950
951 /**
952 * scx_bpf_cpu_node - Return the NUMA node the given @cpu belongs to, or
953 * trigger an error if @cpu is invalid
954 * @cpu: target CPU
955 */
scx_bpf_cpu_node(s32 cpu)956 __bpf_kfunc int scx_bpf_cpu_node(s32 cpu)
957 {
958 struct scx_sched *sch;
959
960 guard(rcu)();
961
962 sch = rcu_dereference(scx_root);
963 if (unlikely(!sch) || !ops_cpu_valid(sch, cpu, NULL))
964 return NUMA_NO_NODE;
965 return cpu_to_node(cpu);
966 }
967
968 /**
969 * scx_bpf_select_cpu_dfl - The default implementation of ops.select_cpu()
970 * @p: task_struct to select a CPU for
971 * @prev_cpu: CPU @p was on previously
972 * @wake_flags: %SCX_WAKE_* flags
973 * @is_idle: out parameter indicating whether the returned CPU is idle
974 *
975 * Can be called from ops.select_cpu(), ops.enqueue(), or from an unlocked
976 * context such as a BPF test_run() call, as long as built-in CPU selection
977 * is enabled: ops.update_idle() is missing or %SCX_OPS_KEEP_BUILTIN_IDLE
978 * is set.
979 *
980 * Returns the picked CPU with *@is_idle indicating whether the picked CPU is
981 * currently idle and thus a good candidate for direct dispatching.
982 */
scx_bpf_select_cpu_dfl(struct task_struct * p,s32 prev_cpu,u64 wake_flags,bool * is_idle)983 __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
984 u64 wake_flags, bool *is_idle)
985 {
986 struct scx_sched *sch;
987 s32 cpu;
988
989 guard(rcu)();
990
991 sch = rcu_dereference(scx_root);
992 if (unlikely(!sch))
993 return -ENODEV;
994
995 cpu = select_cpu_from_kfunc(sch, p, prev_cpu, wake_flags, NULL, 0);
996 if (cpu >= 0) {
997 *is_idle = true;
998 return cpu;
999 }
1000 *is_idle = false;
1001 return prev_cpu;
1002 }
1003
1004 struct scx_bpf_select_cpu_and_args {
1005 /* @p and @cpus_allowed can't be packed together as KF_RCU is not transitive */
1006 s32 prev_cpu;
1007 u64 wake_flags;
1008 u64 flags;
1009 };
1010
1011 /**
1012 * __scx_bpf_select_cpu_and - Arg-wrapped CPU selection with cpumask
1013 * @p: task_struct to select a CPU for
1014 * @cpus_allowed: cpumask of allowed CPUs
1015 * @args: struct containing the rest of the arguments
1016 * @args->prev_cpu: CPU @p was on previously
1017 * @args->wake_flags: %SCX_WAKE_* flags
1018 * @args->flags: %SCX_PICK_IDLE* flags
1019 *
1020 * Wrapper kfunc that takes arguments via struct to work around BPF's 5 argument
1021 * limit. BPF programs should use scx_bpf_select_cpu_and() which is provided
1022 * as an inline wrapper in common.bpf.h.
1023 *
1024 * Can be called from ops.select_cpu(), ops.enqueue(), or from an unlocked
1025 * context such as a BPF test_run() call, as long as built-in CPU selection
1026 * is enabled: ops.update_idle() is missing or %SCX_OPS_KEEP_BUILTIN_IDLE
1027 * is set.
1028 *
1029 * @p, @args->prev_cpu and @args->wake_flags match ops.select_cpu().
1030 *
1031 * Returns the selected idle CPU, which will be automatically awakened upon
1032 * returning from ops.select_cpu() and can be used for direct dispatch, or
1033 * a negative value if no idle CPU is available.
1034 */
1035 __bpf_kfunc s32
__scx_bpf_select_cpu_and(struct task_struct * p,const struct cpumask * cpus_allowed,struct scx_bpf_select_cpu_and_args * args)1036 __scx_bpf_select_cpu_and(struct task_struct *p, const struct cpumask *cpus_allowed,
1037 struct scx_bpf_select_cpu_and_args *args)
1038 {
1039 struct scx_sched *sch;
1040
1041 guard(rcu)();
1042
1043 sch = rcu_dereference(scx_root);
1044 if (unlikely(!sch))
1045 return -ENODEV;
1046
1047 return select_cpu_from_kfunc(sch, p, args->prev_cpu, args->wake_flags,
1048 cpus_allowed, args->flags);
1049 }
1050
1051 /*
1052 * COMPAT: Will be removed in v6.22.
1053 */
scx_bpf_select_cpu_and(struct task_struct * p,s32 prev_cpu,u64 wake_flags,const struct cpumask * cpus_allowed,u64 flags)1054 __bpf_kfunc s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
1055 const struct cpumask *cpus_allowed, u64 flags)
1056 {
1057 struct scx_sched *sch;
1058
1059 guard(rcu)();
1060
1061 sch = rcu_dereference(scx_root);
1062 if (unlikely(!sch))
1063 return -ENODEV;
1064
1065 return select_cpu_from_kfunc(sch, p, prev_cpu, wake_flags,
1066 cpus_allowed, flags);
1067 }
1068
1069 /**
1070 * scx_bpf_get_idle_cpumask_node - Get a referenced kptr to the
1071 * idle-tracking per-CPU cpumask of a target NUMA node.
1072 * @node: target NUMA node
1073 *
1074 * Returns an empty cpumask if idle tracking is not enabled, if @node is
1075 * not valid, or running on a UP kernel. In this case the actual error will
1076 * be reported to the BPF scheduler via scx_error().
1077 */
scx_bpf_get_idle_cpumask_node(int node)1078 __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
1079 {
1080 struct scx_sched *sch;
1081
1082 guard(rcu)();
1083
1084 sch = rcu_dereference(scx_root);
1085 if (unlikely(!sch))
1086 return cpu_none_mask;
1087
1088 node = validate_node(sch, node);
1089 if (node < 0)
1090 return cpu_none_mask;
1091
1092 return idle_cpumask(node)->cpu;
1093 }
1094
1095 /**
1096 * scx_bpf_get_idle_cpumask - Get a referenced kptr to the idle-tracking
1097 * per-CPU cpumask.
1098 *
1099 * Returns an empty mask if idle tracking is not enabled, or running on a
1100 * UP kernel.
1101 */
scx_bpf_get_idle_cpumask(void)1102 __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask(void)
1103 {
1104 struct scx_sched *sch;
1105
1106 guard(rcu)();
1107
1108 sch = rcu_dereference(scx_root);
1109 if (unlikely(!sch))
1110 return cpu_none_mask;
1111
1112 if (static_branch_unlikely(&scx_builtin_idle_per_node)) {
1113 scx_error(sch, "SCX_OPS_BUILTIN_IDLE_PER_NODE enabled");
1114 return cpu_none_mask;
1115 }
1116
1117 if (!check_builtin_idle_enabled(sch))
1118 return cpu_none_mask;
1119
1120 return idle_cpumask(NUMA_NO_NODE)->cpu;
1121 }
1122
1123 /**
1124 * scx_bpf_get_idle_smtmask_node - Get a referenced kptr to the
1125 * idle-tracking, per-physical-core cpumask of a target NUMA node. Can be
1126 * used to determine if an entire physical core is free.
1127 * @node: target NUMA node
1128 *
1129 * Returns an empty cpumask if idle tracking is not enabled, if @node is
1130 * not valid, or running on a UP kernel. In this case the actual error will
1131 * be reported to the BPF scheduler via scx_error().
1132 */
scx_bpf_get_idle_smtmask_node(int node)1133 __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
1134 {
1135 struct scx_sched *sch;
1136
1137 guard(rcu)();
1138
1139 sch = rcu_dereference(scx_root);
1140 if (unlikely(!sch))
1141 return cpu_none_mask;
1142
1143 node = validate_node(sch, node);
1144 if (node < 0)
1145 return cpu_none_mask;
1146
1147 if (sched_smt_active())
1148 return idle_cpumask(node)->smt;
1149 else
1150 return idle_cpumask(node)->cpu;
1151 }
1152
1153 /**
1154 * scx_bpf_get_idle_smtmask - Get a referenced kptr to the idle-tracking,
1155 * per-physical-core cpumask. Can be used to determine if an entire physical
1156 * core is free.
1157 *
1158 * Returns an empty mask if idle tracking is not enabled, or running on a
1159 * UP kernel.
1160 */
scx_bpf_get_idle_smtmask(void)1161 __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask(void)
1162 {
1163 struct scx_sched *sch;
1164
1165 guard(rcu)();
1166
1167 sch = rcu_dereference(scx_root);
1168 if (unlikely(!sch))
1169 return cpu_none_mask;
1170
1171 if (static_branch_unlikely(&scx_builtin_idle_per_node)) {
1172 scx_error(sch, "SCX_OPS_BUILTIN_IDLE_PER_NODE enabled");
1173 return cpu_none_mask;
1174 }
1175
1176 if (!check_builtin_idle_enabled(sch))
1177 return cpu_none_mask;
1178
1179 if (sched_smt_active())
1180 return idle_cpumask(NUMA_NO_NODE)->smt;
1181 else
1182 return idle_cpumask(NUMA_NO_NODE)->cpu;
1183 }
1184
1185 /**
1186 * scx_bpf_put_idle_cpumask - Release a previously acquired referenced kptr to
1187 * either the percpu, or SMT idle-tracking cpumask.
1188 * @idle_mask: &cpumask to use
1189 */
scx_bpf_put_idle_cpumask(const struct cpumask * idle_mask)1190 __bpf_kfunc void scx_bpf_put_idle_cpumask(const struct cpumask *idle_mask)
1191 {
1192 /*
1193 * Empty function body because we aren't actually acquiring or releasing
1194 * a reference to a global idle cpumask, which is read-only in the
1195 * caller and is never released. The acquire / release semantics here
1196 * are just used to make the cpumask a trusted pointer in the caller.
1197 */
1198 }
1199
1200 /**
1201 * scx_bpf_test_and_clear_cpu_idle - Test and clear @cpu's idle state
1202 * @cpu: cpu to test and clear idle for
1203 *
1204 * Returns %true if @cpu was idle and its idle state was successfully cleared.
1205 * %false otherwise.
1206 *
1207 * Unavailable if ops.update_idle() is implemented and
1208 * %SCX_OPS_KEEP_BUILTIN_IDLE is not set.
1209 */
scx_bpf_test_and_clear_cpu_idle(s32 cpu)1210 __bpf_kfunc bool scx_bpf_test_and_clear_cpu_idle(s32 cpu)
1211 {
1212 struct scx_sched *sch;
1213
1214 guard(rcu)();
1215
1216 sch = rcu_dereference(scx_root);
1217 if (unlikely(!sch))
1218 return false;
1219
1220 if (!check_builtin_idle_enabled(sch))
1221 return false;
1222
1223 if (!ops_cpu_valid(sch, cpu, NULL))
1224 return false;
1225
1226 return scx_idle_test_and_clear_cpu(cpu);
1227 }
1228
1229 /**
1230 * scx_bpf_pick_idle_cpu_node - Pick and claim an idle cpu from @node
1231 * @cpus_allowed: Allowed cpumask
1232 * @node: target NUMA node
1233 * @flags: %SCX_PICK_IDLE_* flags
1234 *
1235 * Pick and claim an idle cpu in @cpus_allowed from the NUMA node @node.
1236 *
1237 * Returns the picked idle cpu number on success, or -%EBUSY if no matching
1238 * cpu was found.
1239 *
1240 * The search starts from @node and proceeds to other online NUMA nodes in
1241 * order of increasing distance (unless SCX_PICK_IDLE_IN_NODE is specified,
1242 * in which case the search is limited to the target @node).
1243 *
1244 * Always returns an error if ops.update_idle() is implemented and
1245 * %SCX_OPS_KEEP_BUILTIN_IDLE is not set, or if
1246 * %SCX_OPS_BUILTIN_IDLE_PER_NODE is not set.
1247 */
scx_bpf_pick_idle_cpu_node(const struct cpumask * cpus_allowed,int node,u64 flags)1248 __bpf_kfunc s32 scx_bpf_pick_idle_cpu_node(const struct cpumask *cpus_allowed,
1249 int node, u64 flags)
1250 {
1251 struct scx_sched *sch;
1252
1253 guard(rcu)();
1254
1255 sch = rcu_dereference(scx_root);
1256 if (unlikely(!sch))
1257 return -ENODEV;
1258
1259 node = validate_node(sch, node);
1260 if (node < 0)
1261 return node;
1262
1263 return scx_pick_idle_cpu(cpus_allowed, node, flags);
1264 }
1265
1266 /**
1267 * scx_bpf_pick_idle_cpu - Pick and claim an idle cpu
1268 * @cpus_allowed: Allowed cpumask
1269 * @flags: %SCX_PICK_IDLE_CPU_* flags
1270 *
1271 * Pick and claim an idle cpu in @cpus_allowed. Returns the picked idle cpu
1272 * number on success. -%EBUSY if no matching cpu was found.
1273 *
1274 * Idle CPU tracking may race against CPU scheduling state transitions. For
1275 * example, this function may return -%EBUSY as CPUs are transitioning into the
1276 * idle state. If the caller then assumes that there will be dispatch events on
1277 * the CPUs as they were all busy, the scheduler may end up stalling with CPUs
1278 * idling while there are pending tasks. Use scx_bpf_pick_any_cpu() and
1279 * scx_bpf_kick_cpu() to guarantee that there will be at least one dispatch
1280 * event in the near future.
1281 *
1282 * Unavailable if ops.update_idle() is implemented and
1283 * %SCX_OPS_KEEP_BUILTIN_IDLE is not set.
1284 *
1285 * Always returns an error if %SCX_OPS_BUILTIN_IDLE_PER_NODE is set, use
1286 * scx_bpf_pick_idle_cpu_node() instead.
1287 */
scx_bpf_pick_idle_cpu(const struct cpumask * cpus_allowed,u64 flags)1288 __bpf_kfunc s32 scx_bpf_pick_idle_cpu(const struct cpumask *cpus_allowed,
1289 u64 flags)
1290 {
1291 struct scx_sched *sch;
1292
1293 guard(rcu)();
1294
1295 sch = rcu_dereference(scx_root);
1296 if (unlikely(!sch))
1297 return -ENODEV;
1298
1299 if (static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
1300 scx_error(sch, "per-node idle tracking is enabled");
1301 return -EBUSY;
1302 }
1303
1304 if (!check_builtin_idle_enabled(sch))
1305 return -EBUSY;
1306
1307 return scx_pick_idle_cpu(cpus_allowed, NUMA_NO_NODE, flags);
1308 }
1309
1310 /**
1311 * scx_bpf_pick_any_cpu_node - Pick and claim an idle cpu if available
1312 * or pick any CPU from @node
1313 * @cpus_allowed: Allowed cpumask
1314 * @node: target NUMA node
1315 * @flags: %SCX_PICK_IDLE_CPU_* flags
1316 *
1317 * Pick and claim an idle cpu in @cpus_allowed. If none is available, pick any
1318 * CPU in @cpus_allowed. Guaranteed to succeed and returns the picked idle cpu
1319 * number if @cpus_allowed is not empty. -%EBUSY is returned if @cpus_allowed is
1320 * empty.
1321 *
1322 * The search starts from @node and proceeds to other online NUMA nodes in
1323 * order of increasing distance (unless %SCX_PICK_IDLE_IN_NODE is specified,
1324 * in which case the search is limited to the target @node, regardless of
1325 * the CPU idle state).
1326 *
1327 * If ops.update_idle() is implemented and %SCX_OPS_KEEP_BUILTIN_IDLE is not
1328 * set, this function can't tell which CPUs are idle and will always pick any
1329 * CPU.
1330 */
scx_bpf_pick_any_cpu_node(const struct cpumask * cpus_allowed,int node,u64 flags)1331 __bpf_kfunc s32 scx_bpf_pick_any_cpu_node(const struct cpumask *cpus_allowed,
1332 int node, u64 flags)
1333 {
1334 struct scx_sched *sch;
1335 s32 cpu;
1336
1337 guard(rcu)();
1338
1339 sch = rcu_dereference(scx_root);
1340 if (unlikely(!sch))
1341 return -ENODEV;
1342
1343 node = validate_node(sch, node);
1344 if (node < 0)
1345 return node;
1346
1347 cpu = scx_pick_idle_cpu(cpus_allowed, node, flags);
1348 if (cpu >= 0)
1349 return cpu;
1350
1351 if (flags & SCX_PICK_IDLE_IN_NODE)
1352 cpu = cpumask_any_and_distribute(cpumask_of_node(node), cpus_allowed);
1353 else
1354 cpu = cpumask_any_distribute(cpus_allowed);
1355 if (cpu < nr_cpu_ids)
1356 return cpu;
1357 else
1358 return -EBUSY;
1359 }
1360
1361 /**
1362 * scx_bpf_pick_any_cpu - Pick and claim an idle cpu if available or pick any CPU
1363 * @cpus_allowed: Allowed cpumask
1364 * @flags: %SCX_PICK_IDLE_CPU_* flags
1365 *
1366 * Pick and claim an idle cpu in @cpus_allowed. If none is available, pick any
1367 * CPU in @cpus_allowed. Guaranteed to succeed and returns the picked idle cpu
1368 * number if @cpus_allowed is not empty. -%EBUSY is returned if @cpus_allowed is
1369 * empty.
1370 *
1371 * If ops.update_idle() is implemented and %SCX_OPS_KEEP_BUILTIN_IDLE is not
1372 * set, this function can't tell which CPUs are idle and will always pick any
1373 * CPU.
1374 *
1375 * Always returns an error if %SCX_OPS_BUILTIN_IDLE_PER_NODE is set, use
1376 * scx_bpf_pick_any_cpu_node() instead.
1377 */
scx_bpf_pick_any_cpu(const struct cpumask * cpus_allowed,u64 flags)1378 __bpf_kfunc s32 scx_bpf_pick_any_cpu(const struct cpumask *cpus_allowed,
1379 u64 flags)
1380 {
1381 struct scx_sched *sch;
1382 s32 cpu;
1383
1384 guard(rcu)();
1385
1386 sch = rcu_dereference(scx_root);
1387 if (unlikely(!sch))
1388 return -ENODEV;
1389
1390 if (static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
1391 scx_error(sch, "per-node idle tracking is enabled");
1392 return -EBUSY;
1393 }
1394
1395 if (static_branch_likely(&scx_builtin_idle_enabled)) {
1396 cpu = scx_pick_idle_cpu(cpus_allowed, NUMA_NO_NODE, flags);
1397 if (cpu >= 0)
1398 return cpu;
1399 }
1400
1401 cpu = cpumask_any_distribute(cpus_allowed);
1402 if (cpu < nr_cpu_ids)
1403 return cpu;
1404 else
1405 return -EBUSY;
1406 }
1407
1408 __bpf_kfunc_end_defs();
1409
1410 BTF_KFUNCS_START(scx_kfunc_ids_idle)
1411 BTF_ID_FLAGS(func, scx_bpf_cpu_node)
1412 BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask_node, KF_ACQUIRE)
1413 BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask, KF_ACQUIRE)
1414 BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask_node, KF_ACQUIRE)
1415 BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask, KF_ACQUIRE)
1416 BTF_ID_FLAGS(func, scx_bpf_put_idle_cpumask, KF_RELEASE)
1417 BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle)
1418 BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu_node, KF_RCU)
1419 BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu, KF_RCU)
1420 BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu_node, KF_RCU)
1421 BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu, KF_RCU)
1422 BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_RCU)
1423 BTF_ID_FLAGS(func, scx_bpf_select_cpu_and, KF_RCU)
1424 BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_RCU)
1425 BTF_KFUNCS_END(scx_kfunc_ids_idle)
1426
1427 static const struct btf_kfunc_id_set scx_kfunc_set_idle = {
1428 .owner = THIS_MODULE,
1429 .set = &scx_kfunc_ids_idle,
1430 };
1431
scx_idle_init(void)1432 int scx_idle_init(void)
1433 {
1434 int ret;
1435
1436 ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &scx_kfunc_set_idle) ||
1437 register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &scx_kfunc_set_idle) ||
1438 register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &scx_kfunc_set_idle);
1439
1440 return ret;
1441 }
1442