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-rw-r--r--tools/sched_ext/scx_flatcg.bpf.c957
1 files changed, 957 insertions, 0 deletions
diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
new file mode 100644
index 000000000000..b722baf6da4b
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+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -0,0 +1,957 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A demo sched_ext flattened cgroup hierarchy scheduler. It implements
+ * hierarchical weight-based cgroup CPU control by flattening the cgroup
+ * hierarchy into a single layer by compounding the active weight share at each
+ * level. Consider the following hierarchy with weights in parentheses:
+ *
+ * R + A (100) + B (100)
+ * | \ C (100)
+ * \ D (200)
+ *
+ * Ignoring the root and threaded cgroups, only B, C and D can contain tasks.
+ * Let's say all three have runnable tasks. The total share that each of these
+ * three cgroups is entitled to can be calculated by compounding its share at
+ * each level.
+ *
+ * For example, B is competing against C and in that competition its share is
+ * 100/(100+100) == 1/2. At its parent level, A is competing against D and A's
+ * share in that competition is 100/(200+100) == 1/3. B's eventual share in the
+ * system can be calculated by multiplying the two shares, 1/2 * 1/3 == 1/6. C's
+ * eventual shaer is the same at 1/6. D is only competing at the top level and
+ * its share is 200/(100+200) == 2/3.
+ *
+ * So, instead of hierarchically scheduling level-by-level, we can consider it
+ * as B, C and D competing each other with respective share of 1/6, 1/6 and 2/3
+ * and keep updating the eventual shares as the cgroups' runnable states change.
+ *
+ * This flattening of hierarchy can bring a substantial performance gain when
+ * the cgroup hierarchy is nested multiple levels. in a simple benchmark using
+ * wrk[8] on apache serving a CGI script calculating sha1sum of a small file, it
+ * outperforms CFS by ~3% with CPU controller disabled and by ~10% with two
+ * apache instances competing with 2:1 weight ratio nested four level deep.
+ *
+ * However, the gain comes at the cost of not being able to properly handle
+ * thundering herd of cgroups. For example, if many cgroups which are nested
+ * behind a low priority parent cgroup wake up around the same time, they may be
+ * able to consume more CPU cycles than they are entitled to. In many use cases,
+ * this isn't a real concern especially given the performance gain. Also, there
+ * are ways to mitigate the problem further by e.g. introducing an extra
+ * scheduling layer on cgroup delegation boundaries.
+ *
+ * The scheduler first picks the cgroup to run and then schedule the tasks
+ * within by using nested weighted vtime scheduling by default. The
+ * cgroup-internal scheduling can be switched to FIFO with the -f option.
+ */
+#include <scx/common.bpf.h>
+#include "scx_flatcg.h"
+
+/*
+ * Maximum amount of retries to find a valid cgroup.
+ */
+enum {
+ FALLBACK_DSQ = 0,
+ CGROUP_MAX_RETRIES = 1024,
+};
+
+char _license[] SEC("license") = "GPL";
+
+const volatile u32 nr_cpus = 32; /* !0 for veristat, set during init */
+const volatile u64 cgrp_slice_ns = SCX_SLICE_DFL;
+const volatile bool fifo_sched;
+
+u64 cvtime_now;
+UEI_DEFINE(uei);
+
+struct {
+ __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+ __type(key, u32);
+ __type(value, u64);
+ __uint(max_entries, FCG_NR_STATS);
+} stats SEC(".maps");
+
+static void stat_inc(enum fcg_stat_idx idx)
+{
+ u32 idx_v = idx;
+
+ u64 *cnt_p = bpf_map_lookup_elem(&stats, &idx_v);
+ if (cnt_p)
+ (*cnt_p)++;
+}
+
+struct fcg_cpu_ctx {
+ u64 cur_cgid;
+ u64 cur_at;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+ __type(key, u32);
+ __type(value, struct fcg_cpu_ctx);
+ __uint(max_entries, 1);
+} cpu_ctx SEC(".maps");
+
+struct {
+ __uint(type, BPF_MAP_TYPE_CGRP_STORAGE);
+ __uint(map_flags, BPF_F_NO_PREALLOC);
+ __type(key, int);
+ __type(value, struct fcg_cgrp_ctx);
+} cgrp_ctx SEC(".maps");
+
+struct cgv_node {
+ struct bpf_rb_node rb_node;
+ __u64 cvtime;
+ __u64 cgid;
+};
+
+private(CGV_TREE) struct bpf_spin_lock cgv_tree_lock;
+private(CGV_TREE) struct bpf_rb_root cgv_tree __contains(cgv_node, rb_node);
+
+struct cgv_node_stash {
+ struct cgv_node __kptr *node;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_HASH);
+ __uint(max_entries, 16384);
+ __type(key, __u64);
+ __type(value, struct cgv_node_stash);
+} cgv_node_stash SEC(".maps");
+
+struct fcg_task_ctx {
+ u64 bypassed_at;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+ __uint(map_flags, BPF_F_NO_PREALLOC);
+ __type(key, int);
+ __type(value, struct fcg_task_ctx);
+} task_ctx SEC(".maps");
+
+/* gets inc'd on weight tree changes to expire the cached hweights */
+u64 hweight_gen = 1;
+
+static u64 div_round_up(u64 dividend, u64 divisor)
+{
+ return (dividend + divisor - 1) / divisor;
+}
+
+static bool vtime_before(u64 a, u64 b)
+{
+ return (s64)(a - b) < 0;
+}
+
+static bool cgv_node_less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
+{
+ struct cgv_node *cgc_a, *cgc_b;
+
+ cgc_a = container_of(a, struct cgv_node, rb_node);
+ cgc_b = container_of(b, struct cgv_node, rb_node);
+
+ return cgc_a->cvtime < cgc_b->cvtime;
+}
+
+static struct fcg_cpu_ctx *find_cpu_ctx(void)
+{
+ struct fcg_cpu_ctx *cpuc;
+ u32 idx = 0;
+
+ cpuc = bpf_map_lookup_elem(&cpu_ctx, &idx);
+ if (!cpuc) {
+ scx_bpf_error("cpu_ctx lookup failed");
+ return NULL;
+ }
+ return cpuc;
+}
+
+static struct fcg_cgrp_ctx *find_cgrp_ctx(struct cgroup *cgrp)
+{
+ struct fcg_cgrp_ctx *cgc;
+
+ cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0, 0);
+ if (!cgc) {
+ scx_bpf_error("cgrp_ctx lookup failed for cgid %llu", cgrp->kn->id);
+ return NULL;
+ }
+ return cgc;
+}
+
+static struct fcg_cgrp_ctx *find_ancestor_cgrp_ctx(struct cgroup *cgrp, int level)
+{
+ struct fcg_cgrp_ctx *cgc;
+
+ cgrp = bpf_cgroup_ancestor(cgrp, level);
+ if (!cgrp) {
+ scx_bpf_error("ancestor cgroup lookup failed");
+ return NULL;
+ }
+
+ cgc = find_cgrp_ctx(cgrp);
+ if (!cgc)
+ scx_bpf_error("ancestor cgrp_ctx lookup failed");
+ bpf_cgroup_release(cgrp);
+ return cgc;
+}
+
+static void cgrp_refresh_hweight(struct cgroup *cgrp, struct fcg_cgrp_ctx *cgc)
+{
+ int level;
+
+ if (!cgc->nr_active) {
+ stat_inc(FCG_STAT_HWT_SKIP);
+ return;
+ }
+
+ if (cgc->hweight_gen == hweight_gen) {
+ stat_inc(FCG_STAT_HWT_CACHE);
+ return;
+ }
+
+ stat_inc(FCG_STAT_HWT_UPDATES);
+ bpf_for(level, 0, cgrp->level + 1) {
+ struct fcg_cgrp_ctx *cgc;
+ bool is_active;
+
+ cgc = find_ancestor_cgrp_ctx(cgrp, level);
+ if (!cgc)
+ break;
+
+ if (!level) {
+ cgc->hweight = FCG_HWEIGHT_ONE;
+ cgc->hweight_gen = hweight_gen;
+ } else {
+ struct fcg_cgrp_ctx *pcgc;
+
+ pcgc = find_ancestor_cgrp_ctx(cgrp, level - 1);
+ if (!pcgc)
+ break;
+
+ /*
+ * We can be opportunistic here and not grab the
+ * cgv_tree_lock and deal with the occasional races.
+ * However, hweight updates are already cached and
+ * relatively low-frequency. Let's just do the
+ * straightforward thing.
+ */
+ bpf_spin_lock(&cgv_tree_lock);
+ is_active = cgc->nr_active;
+ if (is_active) {
+ cgc->hweight_gen = pcgc->hweight_gen;
+ cgc->hweight =
+ div_round_up(pcgc->hweight * cgc->weight,
+ pcgc->child_weight_sum);
+ }
+ bpf_spin_unlock(&cgv_tree_lock);
+
+ if (!is_active) {
+ stat_inc(FCG_STAT_HWT_RACE);
+ break;
+ }
+ }
+ }
+}
+
+static void cgrp_cap_budget(struct cgv_node *cgv_node, struct fcg_cgrp_ctx *cgc)
+{
+ u64 delta, cvtime, max_budget;
+
+ /*
+ * A node which is on the rbtree can't be pointed to from elsewhere yet
+ * and thus can't be updated and repositioned. Instead, we collect the
+ * vtime deltas separately and apply it asynchronously here.
+ */
+ delta = __sync_fetch_and_sub(&cgc->cvtime_delta, cgc->cvtime_delta);
+ cvtime = cgv_node->cvtime + delta;
+
+ /*
+ * Allow a cgroup to carry the maximum budget proportional to its
+ * hweight such that a full-hweight cgroup can immediately take up half
+ * of the CPUs at the most while staying at the front of the rbtree.
+ */
+ max_budget = (cgrp_slice_ns * nr_cpus * cgc->hweight) /
+ (2 * FCG_HWEIGHT_ONE);
+ if (vtime_before(cvtime, cvtime_now - max_budget))
+ cvtime = cvtime_now - max_budget;
+
+ cgv_node->cvtime = cvtime;
+}
+
+static void cgrp_enqueued(struct cgroup *cgrp, struct fcg_cgrp_ctx *cgc)
+{
+ struct cgv_node_stash *stash;
+ struct cgv_node *cgv_node;
+ u64 cgid = cgrp->kn->id;
+
+ /* paired with cmpxchg in try_pick_next_cgroup() */
+ if (__sync_val_compare_and_swap(&cgc->queued, 0, 1)) {
+ stat_inc(FCG_STAT_ENQ_SKIP);
+ return;
+ }
+
+ stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
+ if (!stash) {
+ scx_bpf_error("cgv_node lookup failed for cgid %llu", cgid);
+ return;
+ }
+
+ /* NULL if the node is already on the rbtree */
+ cgv_node = bpf_kptr_xchg(&stash->node, NULL);
+ if (!cgv_node) {
+ stat_inc(FCG_STAT_ENQ_RACE);
+ return;
+ }
+
+ bpf_spin_lock(&cgv_tree_lock);
+ cgrp_cap_budget(cgv_node, cgc);
+ bpf_rbtree_add(&cgv_tree, &cgv_node->rb_node, cgv_node_less);
+ bpf_spin_unlock(&cgv_tree_lock);
+}
+
+static void set_bypassed_at(struct task_struct *p, struct fcg_task_ctx *taskc)
+{
+ /*
+ * Tell fcg_stopping() that this bypassed the regular scheduling path
+ * and should be force charged to the cgroup. 0 is used to indicate that
+ * the task isn't bypassing, so if the current runtime is 0, go back by
+ * one nanosecond.
+ */
+ taskc->bypassed_at = p->se.sum_exec_runtime ?: (u64)-1;
+}
+
+s32 BPF_STRUCT_OPS(fcg_select_cpu, struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+ struct fcg_task_ctx *taskc;
+ bool is_idle = false;
+ s32 cpu;
+
+ cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &is_idle);
+
+ taskc = bpf_task_storage_get(&task_ctx, p, 0, 0);
+ if (!taskc) {
+ scx_bpf_error("task_ctx lookup failed");
+ return cpu;
+ }
+
+ /*
+ * If select_cpu_dfl() is recommending local enqueue, the target CPU is
+ * idle. Follow it and charge the cgroup later in fcg_stopping() after
+ * the fact.
+ */
+ if (is_idle) {
+ set_bypassed_at(p, taskc);
+ stat_inc(FCG_STAT_LOCAL);
+ scx_bpf_dispatch(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
+ }
+
+ return cpu;
+}
+
+void BPF_STRUCT_OPS(fcg_enqueue, struct task_struct *p, u64 enq_flags)
+{
+ struct fcg_task_ctx *taskc;
+ struct cgroup *cgrp;
+ struct fcg_cgrp_ctx *cgc;
+
+ taskc = bpf_task_storage_get(&task_ctx, p, 0, 0);
+ if (!taskc) {
+ scx_bpf_error("task_ctx lookup failed");
+ return;
+ }
+
+ /*
+ * Use the direct dispatching and force charging to deal with tasks with
+ * custom affinities so that we don't have to worry about per-cgroup
+ * dq's containing tasks that can't be executed from some CPUs.
+ */
+ if (p->nr_cpus_allowed != nr_cpus) {
+ set_bypassed_at(p, taskc);
+
+ /*
+ * The global dq is deprioritized as we don't want to let tasks
+ * to boost themselves by constraining its cpumask. The
+ * deprioritization is rather severe, so let's not apply that to
+ * per-cpu kernel threads. This is ham-fisted. We probably wanna
+ * implement per-cgroup fallback dq's instead so that we have
+ * more control over when tasks with custom cpumask get issued.
+ */
+ if (p->nr_cpus_allowed == 1 && (p->flags & PF_KTHREAD)) {
+ stat_inc(FCG_STAT_LOCAL);
+ scx_bpf_dispatch(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, enq_flags);
+ } else {
+ stat_inc(FCG_STAT_GLOBAL);
+ scx_bpf_dispatch(p, FALLBACK_DSQ, SCX_SLICE_DFL, enq_flags);
+ }
+ return;
+ }
+
+ cgrp = __COMPAT_scx_bpf_task_cgroup(p);
+ cgc = find_cgrp_ctx(cgrp);
+ if (!cgc)
+ goto out_release;
+
+ if (fifo_sched) {
+ scx_bpf_dispatch(p, cgrp->kn->id, SCX_SLICE_DFL, enq_flags);
+ } else {
+ u64 tvtime = p->scx.dsq_vtime;
+
+ /*
+ * Limit the amount of budget that an idling task can accumulate
+ * to one slice.
+ */
+ if (vtime_before(tvtime, cgc->tvtime_now - SCX_SLICE_DFL))
+ tvtime = cgc->tvtime_now - SCX_SLICE_DFL;
+
+ scx_bpf_dispatch_vtime(p, cgrp->kn->id, SCX_SLICE_DFL,
+ tvtime, enq_flags);
+ }
+
+ cgrp_enqueued(cgrp, cgc);
+out_release:
+ bpf_cgroup_release(cgrp);
+}
+
+/*
+ * Walk the cgroup tree to update the active weight sums as tasks wake up and
+ * sleep. The weight sums are used as the base when calculating the proportion a
+ * given cgroup or task is entitled to at each level.
+ */
+static void update_active_weight_sums(struct cgroup *cgrp, bool runnable)
+{
+ struct fcg_cgrp_ctx *cgc;
+ bool updated = false;
+ int idx;
+
+ cgc = find_cgrp_ctx(cgrp);
+ if (!cgc)
+ return;
+
+ /*
+ * In most cases, a hot cgroup would have multiple threads going to
+ * sleep and waking up while the whole cgroup stays active. In leaf
+ * cgroups, ->nr_runnable which is updated with __sync operations gates
+ * ->nr_active updates, so that we don't have to grab the cgv_tree_lock
+ * repeatedly for a busy cgroup which is staying active.
+ */
+ if (runnable) {
+ if (__sync_fetch_and_add(&cgc->nr_runnable, 1))
+ return;
+ stat_inc(FCG_STAT_ACT);
+ } else {
+ if (__sync_sub_and_fetch(&cgc->nr_runnable, 1))
+ return;
+ stat_inc(FCG_STAT_DEACT);
+ }
+
+ /*
+ * If @cgrp is becoming runnable, its hweight should be refreshed after
+ * it's added to the weight tree so that enqueue has the up-to-date
+ * value. If @cgrp is becoming quiescent, the hweight should be
+ * refreshed before it's removed from the weight tree so that the usage
+ * charging which happens afterwards has access to the latest value.
+ */
+ if (!runnable)
+ cgrp_refresh_hweight(cgrp, cgc);
+
+ /* propagate upwards */
+ bpf_for(idx, 0, cgrp->level) {
+ int level = cgrp->level - idx;
+ struct fcg_cgrp_ctx *cgc, *pcgc = NULL;
+ bool propagate = false;
+
+ cgc = find_ancestor_cgrp_ctx(cgrp, level);
+ if (!cgc)
+ break;
+ if (level) {
+ pcgc = find_ancestor_cgrp_ctx(cgrp, level - 1);
+ if (!pcgc)
+ break;
+ }
+
+ /*
+ * We need the propagation protected by a lock to synchronize
+ * against weight changes. There's no reason to drop the lock at
+ * each level but bpf_spin_lock() doesn't want any function
+ * calls while locked.
+ */
+ bpf_spin_lock(&cgv_tree_lock);
+
+ if (runnable) {
+ if (!cgc->nr_active++) {
+ updated = true;
+ if (pcgc) {
+ propagate = true;
+ pcgc->child_weight_sum += cgc->weight;
+ }
+ }
+ } else {
+ if (!--cgc->nr_active) {
+ updated = true;
+ if (pcgc) {
+ propagate = true;
+ pcgc->child_weight_sum -= cgc->weight;
+ }
+ }
+ }
+
+ bpf_spin_unlock(&cgv_tree_lock);
+
+ if (!propagate)
+ break;
+ }
+
+ if (updated)
+ __sync_fetch_and_add(&hweight_gen, 1);
+
+ if (runnable)
+ cgrp_refresh_hweight(cgrp, cgc);
+}
+
+void BPF_STRUCT_OPS(fcg_runnable, struct task_struct *p, u64 enq_flags)
+{
+ struct cgroup *cgrp;
+
+ cgrp = __COMPAT_scx_bpf_task_cgroup(p);
+ update_active_weight_sums(cgrp, true);
+ bpf_cgroup_release(cgrp);
+}
+
+void BPF_STRUCT_OPS(fcg_running, struct task_struct *p)
+{
+ struct cgroup *cgrp;
+ struct fcg_cgrp_ctx *cgc;
+
+ if (fifo_sched)
+ return;
+
+ cgrp = __COMPAT_scx_bpf_task_cgroup(p);
+ cgc = find_cgrp_ctx(cgrp);
+ if (cgc) {
+ /*
+ * @cgc->tvtime_now always progresses forward as tasks start
+ * executing. The test and update can be performed concurrently
+ * from multiple CPUs and thus racy. Any error should be
+ * contained and temporary. Let's just live with it.
+ */
+ if (vtime_before(cgc->tvtime_now, p->scx.dsq_vtime))
+ cgc->tvtime_now = p->scx.dsq_vtime;
+ }
+ bpf_cgroup_release(cgrp);
+}
+
+void BPF_STRUCT_OPS(fcg_stopping, struct task_struct *p, bool runnable)
+{
+ struct fcg_task_ctx *taskc;
+ struct cgroup *cgrp;
+ struct fcg_cgrp_ctx *cgc;
+
+ /*
+ * Scale the execution time by the inverse of the weight and charge.
+ *
+ * Note that the default yield implementation yields by setting
+ * @p->scx.slice to zero and the following would treat the yielding task
+ * as if it has consumed all its slice. If this penalizes yielding tasks
+ * too much, determine the execution time by taking explicit timestamps
+ * instead of depending on @p->scx.slice.
+ */
+ if (!fifo_sched)
+ p->scx.dsq_vtime +=
+ (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
+
+ taskc = bpf_task_storage_get(&task_ctx, p, 0, 0);
+ if (!taskc) {
+ scx_bpf_error("task_ctx lookup failed");
+ return;
+ }
+
+ if (!taskc->bypassed_at)
+ return;
+
+ cgrp = __COMPAT_scx_bpf_task_cgroup(p);
+ cgc = find_cgrp_ctx(cgrp);
+ if (cgc) {
+ __sync_fetch_and_add(&cgc->cvtime_delta,
+ p->se.sum_exec_runtime - taskc->bypassed_at);
+ taskc->bypassed_at = 0;
+ }
+ bpf_cgroup_release(cgrp);
+}
+
+void BPF_STRUCT_OPS(fcg_quiescent, struct task_struct *p, u64 deq_flags)
+{
+ struct cgroup *cgrp;
+
+ cgrp = __COMPAT_scx_bpf_task_cgroup(p);
+ update_active_weight_sums(cgrp, false);
+ bpf_cgroup_release(cgrp);
+}
+
+void BPF_STRUCT_OPS(fcg_cgroup_set_weight, struct cgroup *cgrp, u32 weight)
+{
+ struct fcg_cgrp_ctx *cgc, *pcgc = NULL;
+
+ cgc = find_cgrp_ctx(cgrp);
+ if (!cgc)
+ return;
+
+ if (cgrp->level) {
+ pcgc = find_ancestor_cgrp_ctx(cgrp, cgrp->level - 1);
+ if (!pcgc)
+ return;
+ }
+
+ bpf_spin_lock(&cgv_tree_lock);
+ if (pcgc && cgc->nr_active)
+ pcgc->child_weight_sum += (s64)weight - cgc->weight;
+ cgc->weight = weight;
+ bpf_spin_unlock(&cgv_tree_lock);
+}
+
+static bool try_pick_next_cgroup(u64 *cgidp)
+{
+ struct bpf_rb_node *rb_node;
+ struct cgv_node_stash *stash;
+ struct cgv_node *cgv_node;
+ struct fcg_cgrp_ctx *cgc;
+ struct cgroup *cgrp;
+ u64 cgid;
+
+ /* pop the front cgroup and wind cvtime_now accordingly */
+ bpf_spin_lock(&cgv_tree_lock);
+
+ rb_node = bpf_rbtree_first(&cgv_tree);
+ if (!rb_node) {
+ bpf_spin_unlock(&cgv_tree_lock);
+ stat_inc(FCG_STAT_PNC_NO_CGRP);
+ *cgidp = 0;
+ return true;
+ }
+
+ rb_node = bpf_rbtree_remove(&cgv_tree, rb_node);
+ bpf_spin_unlock(&cgv_tree_lock);
+
+ if (!rb_node) {
+ /*
+ * This should never happen. bpf_rbtree_first() was called
+ * above while the tree lock was held, so the node should
+ * always be present.
+ */
+ scx_bpf_error("node could not be removed");
+ return true;
+ }
+
+ cgv_node = container_of(rb_node, struct cgv_node, rb_node);
+ cgid = cgv_node->cgid;
+
+ if (vtime_before(cvtime_now, cgv_node->cvtime))
+ cvtime_now = cgv_node->cvtime;
+
+ /*
+ * If lookup fails, the cgroup's gone. Free and move on. See
+ * fcg_cgroup_exit().
+ */
+ cgrp = bpf_cgroup_from_id(cgid);
+ if (!cgrp) {
+ stat_inc(FCG_STAT_PNC_GONE);
+ goto out_free;
+ }
+
+ cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0, 0);
+ if (!cgc) {
+ bpf_cgroup_release(cgrp);
+ stat_inc(FCG_STAT_PNC_GONE);
+ goto out_free;
+ }
+
+ if (!scx_bpf_consume(cgid)) {
+ bpf_cgroup_release(cgrp);
+ stat_inc(FCG_STAT_PNC_EMPTY);
+ goto out_stash;
+ }
+
+ /*
+ * Successfully consumed from the cgroup. This will be our current
+ * cgroup for the new slice. Refresh its hweight.
+ */
+ cgrp_refresh_hweight(cgrp, cgc);
+
+ bpf_cgroup_release(cgrp);
+
+ /*
+ * As the cgroup may have more tasks, add it back to the rbtree. Note
+ * that here we charge the full slice upfront and then exact later
+ * according to the actual consumption. This prevents lowpri thundering
+ * herd from saturating the machine.
+ */
+ bpf_spin_lock(&cgv_tree_lock);
+ cgv_node->cvtime += cgrp_slice_ns * FCG_HWEIGHT_ONE / (cgc->hweight ?: 1);
+ cgrp_cap_budget(cgv_node, cgc);
+ bpf_rbtree_add(&cgv_tree, &cgv_node->rb_node, cgv_node_less);
+ bpf_spin_unlock(&cgv_tree_lock);
+
+ *cgidp = cgid;
+ stat_inc(FCG_STAT_PNC_NEXT);
+ return true;
+
+out_stash:
+ stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
+ if (!stash) {
+ stat_inc(FCG_STAT_PNC_GONE);
+ goto out_free;
+ }
+
+ /*
+ * Paired with cmpxchg in cgrp_enqueued(). If they see the following
+ * transition, they'll enqueue the cgroup. If they are earlier, we'll
+ * see their task in the dq below and requeue the cgroup.
+ */
+ __sync_val_compare_and_swap(&cgc->queued, 1, 0);
+
+ if (scx_bpf_dsq_nr_queued(cgid)) {
+ bpf_spin_lock(&cgv_tree_lock);
+ bpf_rbtree_add(&cgv_tree, &cgv_node->rb_node, cgv_node_less);
+ bpf_spin_unlock(&cgv_tree_lock);
+ stat_inc(FCG_STAT_PNC_RACE);
+ } else {
+ cgv_node = bpf_kptr_xchg(&stash->node, cgv_node);
+ if (cgv_node) {
+ scx_bpf_error("unexpected !NULL cgv_node stash");
+ goto out_free;
+ }
+ }
+
+ return false;
+
+out_free:
+ bpf_obj_drop(cgv_node);
+ return false;
+}
+
+void BPF_STRUCT_OPS(fcg_dispatch, s32 cpu, struct task_struct *prev)
+{
+ struct fcg_cpu_ctx *cpuc;
+ struct fcg_cgrp_ctx *cgc;
+ struct cgroup *cgrp;
+ u64 now = bpf_ktime_get_ns();
+ bool picked_next = false;
+
+ cpuc = find_cpu_ctx();
+ if (!cpuc)
+ return;
+
+ if (!cpuc->cur_cgid)
+ goto pick_next_cgroup;
+
+ if (vtime_before(now, cpuc->cur_at + cgrp_slice_ns)) {
+ if (scx_bpf_consume(cpuc->cur_cgid)) {
+ stat_inc(FCG_STAT_CNS_KEEP);
+ return;
+ }
+ stat_inc(FCG_STAT_CNS_EMPTY);
+ } else {
+ stat_inc(FCG_STAT_CNS_EXPIRE);
+ }
+
+ /*
+ * The current cgroup is expiring. It was already charged a full slice.
+ * Calculate the actual usage and accumulate the delta.
+ */
+ cgrp = bpf_cgroup_from_id(cpuc->cur_cgid);
+ if (!cgrp) {
+ stat_inc(FCG_STAT_CNS_GONE);
+ goto pick_next_cgroup;
+ }
+
+ cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0, 0);
+ if (cgc) {
+ /*
+ * We want to update the vtime delta and then look for the next
+ * cgroup to execute but the latter needs to be done in a loop
+ * and we can't keep the lock held. Oh well...
+ */
+ bpf_spin_lock(&cgv_tree_lock);
+ __sync_fetch_and_add(&cgc->cvtime_delta,
+ (cpuc->cur_at + cgrp_slice_ns - now) *
+ FCG_HWEIGHT_ONE / (cgc->hweight ?: 1));
+ bpf_spin_unlock(&cgv_tree_lock);
+ } else {
+ stat_inc(FCG_STAT_CNS_GONE);
+ }
+
+ bpf_cgroup_release(cgrp);
+
+pick_next_cgroup:
+ cpuc->cur_at = now;
+
+ if (scx_bpf_consume(FALLBACK_DSQ)) {
+ cpuc->cur_cgid = 0;
+ return;
+ }
+
+ bpf_repeat(CGROUP_MAX_RETRIES) {
+ if (try_pick_next_cgroup(&cpuc->cur_cgid)) {
+ picked_next = true;
+ break;
+ }
+ }
+
+ /*
+ * This only happens if try_pick_next_cgroup() races against enqueue
+ * path for more than CGROUP_MAX_RETRIES times, which is extremely
+ * unlikely and likely indicates an underlying bug. There shouldn't be
+ * any stall risk as the race is against enqueue.
+ */
+ if (!picked_next)
+ stat_inc(FCG_STAT_PNC_FAIL);
+}
+
+s32 BPF_STRUCT_OPS(fcg_init_task, struct task_struct *p,
+ struct scx_init_task_args *args)
+{
+ struct fcg_task_ctx *taskc;
+ struct fcg_cgrp_ctx *cgc;
+
+ /*
+ * @p is new. Let's ensure that its task_ctx is available. We can sleep
+ * in this function and the following will automatically use GFP_KERNEL.
+ */
+ taskc = bpf_task_storage_get(&task_ctx, p, 0,
+ BPF_LOCAL_STORAGE_GET_F_CREATE);
+ if (!taskc)
+ return -ENOMEM;
+
+ taskc->bypassed_at = 0;
+
+ if (!(cgc = find_cgrp_ctx(args->cgroup)))
+ return -ENOENT;
+
+ p->scx.dsq_vtime = cgc->tvtime_now;
+
+ return 0;
+}
+
+int BPF_STRUCT_OPS_SLEEPABLE(fcg_cgroup_init, struct cgroup *cgrp,
+ struct scx_cgroup_init_args *args)
+{
+ struct fcg_cgrp_ctx *cgc;
+ struct cgv_node *cgv_node;
+ struct cgv_node_stash empty_stash = {}, *stash;
+ u64 cgid = cgrp->kn->id;
+ int ret;
+
+ /*
+ * Technically incorrect as cgroup ID is full 64bit while dsq ID is
+ * 63bit. Should not be a problem in practice and easy to spot in the
+ * unlikely case that it breaks.
+ */
+ ret = scx_bpf_create_dsq(cgid, -1);
+ if (ret)
+ return ret;
+
+ cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0,
+ BPF_LOCAL_STORAGE_GET_F_CREATE);
+ if (!cgc) {
+ ret = -ENOMEM;
+ goto err_destroy_dsq;
+ }
+
+ cgc->weight = args->weight;
+ cgc->hweight = FCG_HWEIGHT_ONE;
+
+ ret = bpf_map_update_elem(&cgv_node_stash, &cgid, &empty_stash,
+ BPF_NOEXIST);
+ if (ret) {
+ if (ret != -ENOMEM)
+ scx_bpf_error("unexpected stash creation error (%d)",
+ ret);
+ goto err_destroy_dsq;
+ }
+
+ stash = bpf_map_lookup_elem(&cgv_node_stash, &cgid);
+ if (!stash) {
+ scx_bpf_error("unexpected cgv_node stash lookup failure");
+ ret = -ENOENT;
+ goto err_destroy_dsq;
+ }
+
+ cgv_node = bpf_obj_new(struct cgv_node);
+ if (!cgv_node) {
+ ret = -ENOMEM;
+ goto err_del_cgv_node;
+ }
+
+ cgv_node->cgid = cgid;
+ cgv_node->cvtime = cvtime_now;
+
+ cgv_node = bpf_kptr_xchg(&stash->node, cgv_node);
+ if (cgv_node) {
+ scx_bpf_error("unexpected !NULL cgv_node stash");
+ ret = -EBUSY;
+ goto err_drop;
+ }
+
+ return 0;
+
+err_drop:
+ bpf_obj_drop(cgv_node);
+err_del_cgv_node:
+ bpf_map_delete_elem(&cgv_node_stash, &cgid);
+err_destroy_dsq:
+ scx_bpf_destroy_dsq(cgid);
+ return ret;
+}
+
+void BPF_STRUCT_OPS(fcg_cgroup_exit, struct cgroup *cgrp)
+{
+ u64 cgid = cgrp->kn->id;
+
+ /*
+ * For now, there's no way find and remove the cgv_node if it's on the
+ * cgv_tree. Let's drain them in the dispatch path as they get popped
+ * off the front of the tree.
+ */
+ bpf_map_delete_elem(&cgv_node_stash, &cgid);
+ scx_bpf_destroy_dsq(cgid);
+}
+
+void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
+ struct cgroup *from, struct cgroup *to)
+{
+ struct fcg_cgrp_ctx *from_cgc, *to_cgc;
+ s64 vtime_delta;
+
+ /* find_cgrp_ctx() triggers scx_ops_error() on lookup failures */
+ if (!(from_cgc = find_cgrp_ctx(from)) || !(to_cgc = find_cgrp_ctx(to)))
+ return;
+
+ vtime_delta = p->scx.dsq_vtime - from_cgc->tvtime_now;
+ p->scx.dsq_vtime = to_cgc->tvtime_now + vtime_delta;
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(fcg_init)
+{
+ return scx_bpf_create_dsq(FALLBACK_DSQ, -1);
+}
+
+void BPF_STRUCT_OPS(fcg_exit, struct scx_exit_info *ei)
+{
+ UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(flatcg_ops,
+ .select_cpu = (void *)fcg_select_cpu,
+ .enqueue = (void *)fcg_enqueue,
+ .dispatch = (void *)fcg_dispatch,
+ .runnable = (void *)fcg_runnable,
+ .running = (void *)fcg_running,
+ .stopping = (void *)fcg_stopping,
+ .quiescent = (void *)fcg_quiescent,
+ .init_task = (void *)fcg_init_task,
+ .cgroup_set_weight = (void *)fcg_cgroup_set_weight,
+ .cgroup_init = (void *)fcg_cgroup_init,
+ .cgroup_exit = (void *)fcg_cgroup_exit,
+ .cgroup_move = (void *)fcg_cgroup_move,
+ .init = (void *)fcg_init,
+ .exit = (void *)fcg_exit,
+ .flags = SCX_OPS_HAS_CGROUP_WEIGHT | SCX_OPS_ENQ_EXITING,
+ .name = "flatcg");