--- /dev/null
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Arena-based task data scheduler. This is a variation of scx_simple
+ * that uses a combined allocator and indexing structure to organize
+ * task data. Task context allocation is done when a task enters the
+ * scheduler, while freeing is done when it exits. Task contexts are
+ * retrieved from task-local storage, pointing to the allocated memory.
+ *
+ * The main purpose of this scheduler is to demostrate arena memory
+ * management.
+ *
+ * Copyright (c) 2024-2025 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024-2025 Emil Tsalapatis <etsal@meta.com>
+ * Copyright (c) 2024-2025 Tejun Heo <tj@kernel.org>
+ *
+ */
+#include <scx/common.bpf.h>
+#include <scx/bpf_arena_common.bpf.h>
+
+#include "scx_sdt.h"
+
+char _license[] SEC("license") = "GPL";
+
+UEI_DEFINE(uei);
+
+struct {
+ __uint(type, BPF_MAP_TYPE_ARENA);
+ __uint(map_flags, BPF_F_MMAPABLE);
+#if defined(__TARGET_ARCH_arm64) || defined(__aarch64__)
+ __uint(max_entries, 1 << 16); /* number of pages */
+ __ulong(map_extra, (1ull << 32)); /* start of mmap() region */
+#else
+ __uint(max_entries, 1 << 20); /* number of pages */
+ __ulong(map_extra, (1ull << 44)); /* start of mmap() region */
+#endif
+} arena __weak SEC(".maps");
+
+#define SHARED_DSQ 0
+
+#define DEFINE_SDT_STAT(metric) \
+static inline void \
+stat_inc_##metric(struct scx_stats __arena *stats) \
+{ \
+ cast_kern(stats); \
+ stats->metric += 1; \
+} \
+__u64 stat_##metric; \
+
+DEFINE_SDT_STAT(enqueue);
+DEFINE_SDT_STAT(init);
+DEFINE_SDT_STAT(exit);
+DEFINE_SDT_STAT(select_idle_cpu);
+DEFINE_SDT_STAT(select_busy_cpu);
+
+/*
+ * Necessary for cond_break/can_loop's semantics. According to kernel commit
+ * 011832b, the loop counter variable must be seen as imprecise and bounded
+ * by the verifier. Initializing it from a constant (e.g., i = 0;), then,
+ * makes it precise and prevents may_goto from helping with converging the
+ * loop. For these loops we must initialize the loop counter from a variable
+ * whose value the verifier cannot reason about when checking the program, so
+ * that the loop counter's value is imprecise.
+ */
+static __u64 zero = 0;
+
+/*
+ * XXX Hack to get the verifier to find the arena for sdt_exit_task.
+ * As of 6.12-rc5, The verifier associates arenas with programs by
+ * checking LD.IMM instruction operands for an arena and populating
+ * the program state with the first instance it finds. This requires
+ * accessing our global arena variable, but scx methods do not necessarily
+ * do so while still using pointers from that arena. Insert a bpf_printk
+ * statement that triggers at most once to generate an LD.IMM instruction
+ * to access the arena and help the verifier.
+ */
+static volatile bool scx_arena_verify_once;
+
+__hidden void scx_arena_subprog_init(void)
+{
+ if (scx_arena_verify_once)
+ return;
+
+ bpf_printk("%s: arena pointer %p", __func__, &arena);
+ scx_arena_verify_once = true;
+}
+
+
+private(LOCK) struct bpf_spin_lock alloc_lock;
+private(POOL_LOCK) struct bpf_spin_lock alloc_pool_lock;
+
+/* allocation pools */
+struct sdt_pool desc_pool;
+struct sdt_pool chunk_pool;
+
+/* Protected by alloc_lock. */
+struct scx_alloc_stats alloc_stats;
+
+
+/* Allocate element from the pool. Must be called with a then pool lock held. */
+static
+void __arena *scx_alloc_from_pool(struct sdt_pool *pool)
+{
+ __u64 elem_size, max_elems;
+ void __arena *slab;
+ void __arena *ptr;
+
+ elem_size = pool->elem_size;
+ max_elems = pool->max_elems;
+
+ /* If the chunk is spent, get a new one. */
+ if (pool->idx >= max_elems) {
+ slab = bpf_arena_alloc_pages(&arena, NULL,
+ div_round_up(max_elems * elem_size, PAGE_SIZE), NUMA_NO_NODE, 0);
+ if (!slab)
+ return NULL;
+
+ pool->slab = slab;
+ pool->idx = 0;
+ }
+
+ ptr = (void __arena *)((__u64) pool->slab + elem_size * pool->idx);
+ pool->idx += 1;
+
+ return ptr;
+}
+
+/* Alloc desc and associated chunk. Called with the allocator spinlock held. */
+static sdt_desc_t *scx_alloc_chunk(void)
+{
+ struct sdt_chunk __arena *chunk;
+ sdt_desc_t *desc;
+ sdt_desc_t *out;
+
+ chunk = scx_alloc_from_pool(&chunk_pool);
+ if (!chunk)
+ return NULL;
+
+ desc = scx_alloc_from_pool(&desc_pool);
+ if (!desc) {
+ /*
+ * Effectively frees the previous chunk allocation.
+ * Index cannot be 0, so decrementing is always
+ * valid.
+ */
+ chunk_pool.idx -= 1;
+ return NULL;
+ }
+
+ out = desc;
+
+ desc->nr_free = SDT_TASK_ENTS_PER_CHUNK;
+ desc->chunk = chunk;
+
+ alloc_stats.chunk_allocs += 1;
+
+ return out;
+}
+
+static int pool_set_size(struct sdt_pool *pool, __u64 data_size, __u64 nr_pages)
+{
+ if (unlikely(data_size % 8))
+ return -EINVAL;
+
+ if (unlikely(nr_pages == 0))
+ return -EINVAL;
+
+ pool->elem_size = data_size;
+ pool->max_elems = (PAGE_SIZE * nr_pages) / pool->elem_size;
+ /* Populate the pool slab on the first allocation. */
+ pool->idx = pool->max_elems;
+
+ return 0;
+}
+
+/* Initialize both the base pool allocators and the root chunk of the index. */
+__hidden int
+scx_alloc_init(struct scx_allocator *alloc, __u64 data_size)
+{
+ size_t min_chunk_size;
+ int ret;
+
+ _Static_assert(sizeof(struct sdt_chunk) <= PAGE_SIZE,
+ "chunk size must fit into a page");
+
+ ret = pool_set_size(&chunk_pool, sizeof(struct sdt_chunk), 1);
+ if (ret != 0)
+ return ret;
+
+ ret = pool_set_size(&desc_pool, sizeof(struct sdt_desc), 1);
+ if (ret != 0)
+ return ret;
+
+ /* Wrap data into a descriptor and word align. */
+ data_size += sizeof(struct sdt_data);
+ data_size = round_up(data_size, 8);
+
+ /*
+ * Ensure we allocate large enough chunks from the arena to avoid excessive
+ * internal fragmentation when turning chunks it into structs.
+ */
+ min_chunk_size = div_round_up(SDT_TASK_MIN_ELEM_PER_ALLOC * data_size, PAGE_SIZE);
+ ret = pool_set_size(&alloc->pool, data_size, min_chunk_size);
+ if (ret != 0)
+ return ret;
+
+ bpf_spin_lock(&alloc_lock);
+ alloc->root = scx_alloc_chunk();
+ bpf_spin_unlock(&alloc_lock);
+ if (!alloc->root)
+ return -ENOMEM;
+
+ return 0;
+}
+
+static
+int set_idx_state(sdt_desc_t *desc, __u64 pos, bool state)
+{
+ __u64 __arena *allocated = desc->allocated;
+ __u64 bit;
+
+ if (unlikely(pos >= SDT_TASK_ENTS_PER_CHUNK))
+ return -EINVAL;
+
+ bit = (__u64)1 << (pos % 64);
+
+ if (state)
+ allocated[pos / 64] |= bit;
+ else
+ allocated[pos / 64] &= ~bit;
+
+ return 0;
+}
+
+static __noinline
+int mark_nodes_avail(sdt_desc_t *lv_desc[SDT_TASK_LEVELS], __u64 lv_pos[SDT_TASK_LEVELS])
+{
+ sdt_desc_t *desc;
+ __u64 u, level;
+ int ret;
+
+ for (u = zero; u < SDT_TASK_LEVELS && can_loop; u++) {
+ level = SDT_TASK_LEVELS - 1 - u;
+
+ /* Only propagate upwards if we are the parent's only free chunk. */
+ desc = lv_desc[level];
+
+ ret = set_idx_state(desc, lv_pos[level], false);
+ if (unlikely(ret != 0))
+ return ret;
+
+ desc->nr_free += 1;
+ if (desc->nr_free > 1)
+ return 0;
+ }
+
+ return 0;
+}
+
+/*
+ * Free the allocated struct with the given index. Called with the
+ * allocator lock taken.
+ */
+__hidden
+int scx_alloc_free_idx(struct scx_allocator *alloc, __u64 idx)
+{
+ const __u64 mask = (1 << SDT_TASK_ENTS_PER_PAGE_SHIFT) - 1;
+ sdt_desc_t *lv_desc[SDT_TASK_LEVELS];
+ sdt_desc_t * __arena *desc_children;
+ struct sdt_chunk __arena *chunk;
+ sdt_desc_t *desc;
+ struct sdt_data __arena *data;
+ __u64 level, shift, pos;
+ __u64 lv_pos[SDT_TASK_LEVELS];
+ int ret;
+ int i;
+
+ if (!alloc)
+ return 0;
+
+ desc = alloc->root;
+ if (unlikely(!desc))
+ return -EINVAL;
+
+ /* To appease the verifier. */
+ for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+ lv_desc[level] = NULL;
+ lv_pos[level] = 0;
+ }
+
+ /* Find the leaf node containing the index. */
+ for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+ shift = (SDT_TASK_LEVELS - 1 - level) * SDT_TASK_ENTS_PER_PAGE_SHIFT;
+ pos = (idx >> shift) & mask;
+
+ lv_desc[level] = desc;
+ lv_pos[level] = pos;
+
+ if (level == SDT_TASK_LEVELS - 1)
+ break;
+
+ chunk = desc->chunk;
+
+ desc_children = (sdt_desc_t * __arena *)chunk->descs;
+ desc = desc_children[pos];
+
+ if (unlikely(!desc))
+ return -EINVAL;
+ }
+
+ chunk = desc->chunk;
+
+ pos = idx & mask;
+ data = chunk->data[pos];
+ if (likely(data)) {
+ data[pos] = (struct sdt_data) {
+ .tid.genn = data->tid.genn + 1,
+ };
+
+ /* Zero out one word at a time. */
+ for (i = zero; i < alloc->pool.elem_size / 8 && can_loop; i++) {
+ data->payload[i] = 0;
+ }
+ }
+
+ ret = mark_nodes_avail(lv_desc, lv_pos);
+ if (unlikely(ret != 0))
+ return ret;
+
+ alloc_stats.active_allocs -= 1;
+ alloc_stats.free_ops += 1;
+
+ return 0;
+}
+
+static inline
+int ffs(__u64 word)
+{
+ unsigned int num = 0;
+
+ if ((word & 0xffffffff) == 0) {
+ num += 32;
+ word >>= 32;
+ }
+
+ if ((word & 0xffff) == 0) {
+ num += 16;
+ word >>= 16;
+ }
+
+ if ((word & 0xff) == 0) {
+ num += 8;
+ word >>= 8;
+ }
+
+ if ((word & 0xf) == 0) {
+ num += 4;
+ word >>= 4;
+ }
+
+ if ((word & 0x3) == 0) {
+ num += 2;
+ word >>= 2;
+ }
+
+ if ((word & 0x1) == 0) {
+ num += 1;
+ word >>= 1;
+ }
+
+ return num;
+}
+
+
+/* find the first empty slot */
+__hidden
+__u64 chunk_find_empty(sdt_desc_t __arg_arena *desc)
+{
+ __u64 freeslots;
+ __u64 i;
+
+ for (i = 0; i < SDT_TASK_CHUNK_BITMAP_U64S; i++) {
+ freeslots = ~desc->allocated[i];
+ if (freeslots == (__u64)0)
+ continue;
+
+ return (i * 64) + ffs(freeslots);
+ }
+
+ return SDT_TASK_ENTS_PER_CHUNK;
+}
+
+/*
+ * Find and return an available idx on the allocator.
+ * Called with the task spinlock held.
+ */
+static sdt_desc_t * desc_find_empty(sdt_desc_t *desc, __u64 *idxp)
+{
+ sdt_desc_t *lv_desc[SDT_TASK_LEVELS];
+ sdt_desc_t * __arena *desc_children;
+ struct sdt_chunk __arena *chunk;
+ sdt_desc_t *tmp;
+ __u64 lv_pos[SDT_TASK_LEVELS];
+ __u64 u, pos, level;
+ __u64 idx = 0;
+ int ret;
+
+ for (level = zero; level < SDT_TASK_LEVELS && can_loop; level++) {
+ pos = chunk_find_empty(desc);
+
+ /* If we error out, something has gone very wrong. */
+ if (unlikely(pos > SDT_TASK_ENTS_PER_CHUNK))
+ return NULL;
+
+ if (pos == SDT_TASK_ENTS_PER_CHUNK)
+ return NULL;
+
+ idx <<= SDT_TASK_ENTS_PER_PAGE_SHIFT;
+ idx |= pos;
+
+ /* Log the levels to complete allocation. */
+ lv_desc[level] = desc;
+ lv_pos[level] = pos;
+
+ /* The rest of the loop is for internal node traversal. */
+ if (level == SDT_TASK_LEVELS - 1)
+ break;
+
+ /* Allocate an internal node if necessary. */
+ chunk = desc->chunk;
+ desc_children = (sdt_desc_t * __arena *)chunk->descs;
+
+ desc = desc_children[pos];
+ if (!desc) {
+ desc = scx_alloc_chunk();
+ if (!desc)
+ return NULL;
+
+ desc_children[pos] = desc;
+ }
+ }
+
+ /*
+ * Finding the descriptor along with any internal node
+ * allocations was successful. Update all levels with
+ * the new allocation.
+ */
+ bpf_for(u, 0, SDT_TASK_LEVELS) {
+ level = SDT_TASK_LEVELS - 1 - u;
+ tmp = lv_desc[level];
+
+ ret = set_idx_state(tmp, lv_pos[level], true);
+ if (ret != 0)
+ break;
+
+ tmp->nr_free -= 1;
+ if (tmp->nr_free > 0)
+ break;
+ }
+
+ *idxp = idx;
+
+ return desc;
+}
+
+__hidden
+void __arena *scx_alloc(struct scx_allocator *alloc)
+{
+ struct sdt_data __arena *data = NULL;
+ struct sdt_chunk __arena *chunk;
+ sdt_desc_t *desc;
+ __u64 idx, pos;
+
+ if (!alloc)
+ return NULL;
+
+ bpf_spin_lock(&alloc_lock);
+
+ /* We unlock if we encounter an error in the function. */
+ desc = desc_find_empty(alloc->root, &idx);
+ if (unlikely(desc == NULL)) {
+ bpf_spin_unlock(&alloc_lock);
+ return NULL;
+ }
+
+ chunk = desc->chunk;
+
+ /* Populate the leaf node if necessary. */
+ pos = idx & (SDT_TASK_ENTS_PER_CHUNK - 1);
+ data = chunk->data[pos];
+ if (!data) {
+ data = scx_alloc_from_pool(&alloc->pool);
+ if (!data) {
+ scx_alloc_free_idx(alloc, idx);
+ bpf_spin_unlock(&alloc_lock);
+ return NULL;
+ }
+ }
+
+ chunk->data[pos] = data;
+
+ /* The data counts as a chunk */
+ alloc_stats.data_allocs += 1;
+ alloc_stats.alloc_ops += 1;
+ alloc_stats.active_allocs += 1;
+
+ data->tid.idx = idx;
+
+ bpf_spin_unlock(&alloc_lock);
+
+ return data;
+}
+
+/*
+ * Task BPF map entry recording the task's assigned ID and pointing to the data
+ * area allocated in arena.
+ */
+struct scx_task_map_val {
+ union sdt_id tid;
+ __u64 tptr;
+ struct sdt_data __arena *data;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+ __uint(map_flags, BPF_F_NO_PREALLOC);
+ __type(key, int);
+ __type(value, struct scx_task_map_val);
+} scx_task_map SEC(".maps");
+
+static struct scx_allocator scx_task_allocator;
+
+__hidden
+void __arena *scx_task_alloc(struct task_struct *p)
+{
+ struct sdt_data __arena *data = NULL;
+ struct scx_task_map_val *mval;
+
+ mval = bpf_task_storage_get(&scx_task_map, p, 0,
+ BPF_LOCAL_STORAGE_GET_F_CREATE);
+ if (!mval)
+ return NULL;
+
+ data = scx_alloc(&scx_task_allocator);
+ if (unlikely(!data))
+ return NULL;
+
+ mval->tid = data->tid;
+ mval->tptr = (__u64) p;
+ mval->data = data;
+
+ return (void __arena *)data->payload;
+}
+
+__hidden
+int scx_task_init(__u64 data_size)
+{
+ return scx_alloc_init(&scx_task_allocator, data_size);
+}
+
+__hidden
+void __arena *scx_task_data(struct task_struct *p)
+{
+ struct sdt_data __arena *data;
+ struct scx_task_map_val *mval;
+
+ scx_arena_subprog_init();
+
+ mval = bpf_task_storage_get(&scx_task_map, p, 0, 0);
+ if (!mval)
+ return NULL;
+
+ data = mval->data;
+
+ return (void __arena *)data->payload;
+}
+
+__hidden
+void scx_task_free(struct task_struct *p)
+{
+ struct scx_task_map_val *mval;
+
+ scx_arena_subprog_init();
+
+ mval = bpf_task_storage_get(&scx_task_map, p, 0, 0);
+ if (!mval)
+ return;
+
+ bpf_spin_lock(&alloc_lock);
+ scx_alloc_free_idx(&scx_task_allocator, mval->tid.idx);
+ bpf_spin_unlock(&alloc_lock);
+
+ bpf_task_storage_delete(&scx_task_map, p);
+}
+
+static inline void
+scx_stat_global_update(struct scx_stats __arena *stats)
+{
+ cast_kern(stats);
+ __sync_fetch_and_add(&stat_enqueue, stats->enqueue);
+ __sync_fetch_and_add(&stat_init, stats->init);
+ __sync_fetch_and_add(&stat_exit, stats->exit);
+ __sync_fetch_and_add(&stat_select_idle_cpu, stats->select_idle_cpu);
+ __sync_fetch_and_add(&stat_select_busy_cpu, stats->select_busy_cpu);
+}
+
+s32 BPF_STRUCT_OPS(sdt_select_cpu, struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+ struct scx_stats __arena *stats;
+ bool is_idle = false;
+ s32 cpu;
+
+ stats = scx_task_data(p);
+ if (!stats) {
+ scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+ return 0;
+ }
+
+ cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &is_idle);
+ if (is_idle) {
+ stat_inc_select_idle_cpu(stats);
+ scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
+ } else {
+ stat_inc_select_busy_cpu(stats);
+ }
+
+ return cpu;
+}
+
+void BPF_STRUCT_OPS(sdt_enqueue, struct task_struct *p, u64 enq_flags)
+{
+ struct scx_stats __arena *stats;
+
+ stats = scx_task_data(p);
+ if (!stats) {
+ scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+ return;
+ }
+
+ stat_inc_enqueue(stats);
+
+ scx_bpf_dsq_insert(p, SHARED_DSQ, SCX_SLICE_DFL, enq_flags);
+}
+
+void BPF_STRUCT_OPS(sdt_dispatch, s32 cpu, struct task_struct *prev)
+{
+ scx_bpf_dsq_move_to_local(SHARED_DSQ);
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(sdt_init_task, struct task_struct *p,
+ struct scx_init_task_args *args)
+{
+ struct scx_stats __arena *stats;
+
+ stats = scx_task_alloc(p);
+ if (!stats) {
+ scx_bpf_error("arena allocator out of memory");
+ return -ENOMEM;
+ }
+
+ stats->pid = p->pid;
+
+ stat_inc_init(stats);
+
+ return 0;
+}
+
+void BPF_STRUCT_OPS(sdt_exit_task, struct task_struct *p,
+ struct scx_exit_task_args *args)
+{
+ struct scx_stats __arena *stats;
+
+ stats = scx_task_data(p);
+ if (!stats) {
+ scx_bpf_error("%s: no stats for pid %d", __func__, p->pid);
+ return;
+ }
+
+ stat_inc_exit(stats);
+ scx_stat_global_update(stats);
+
+ scx_task_free(p);
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(sdt_init)
+{
+ int ret;
+
+ ret = scx_task_init(sizeof(struct scx_stats));
+ if (ret < 0) {
+ scx_bpf_error("%s: failed with %d", __func__, ret);
+ return ret;
+ }
+
+ return scx_bpf_create_dsq(SHARED_DSQ, -1);
+}
+
+void BPF_STRUCT_OPS(sdt_exit, struct scx_exit_info *ei)
+{
+ UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(sdt_ops,
+ .select_cpu = (void *)sdt_select_cpu,
+ .enqueue = (void *)sdt_enqueue,
+ .dispatch = (void *)sdt_dispatch,
+ .init_task = (void *)sdt_init_task,
+ .exit_task = (void *)sdt_exit_task,
+ .init = (void *)sdt_init,
+ .exit = (void *)sdt_exit,
+ .name = "sdt");
projects / thirdparty / kernel / linux.git / commitdiff
