tools/sched_ext: add scx_userland scheduler

Add in the scx_userland scheduler that does vruntime-based
scheduling in userspace code and communicates scheduling
decisions to BPF by accessing and modifying globals through
the skeleton.

Cc: Tejun Heo <tj@kernel.org>
Cc: David Vernet <dvernet@meta.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: etsal

tools/sched_ext/Makefile

@@ -189,7 +189,7 @@ $(INCLUDE_DIR)/%.bpf.skel.h: $(SCXOBJ_DIR)/%.bpf.o $(INCLUDE_DIR)/vmlinux.h $(BP
 
 SCX_COMMON_DEPS := include/scx/common.h include/scx/user_exit_info.h | $(BINDIR)
 
-c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_flatcg
+c-sched-targets = scx_simple scx_cpu0 scx_qmap scx_central scx_flatcg scx_userland
 
 $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 	$(BINDIR)/%: \

tools/sched_ext/scx_userland.bpf.c

@@ -0,0 +1,344 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A minimal userland scheduler.
+ *
+ * In terms of scheduling, this provides two different types of behaviors:
+ * 1. A global FIFO scheduling order for _any_ tasks that have CPU affinity.
+ *    All such tasks are direct-dispatched from the kernel, and are never
+ *    enqueued in user space.
+ * 2. A primitive vruntime scheduler that is implemented in user space, for all
+ *    other tasks.
+ *
+ * Some parts of this example user space scheduler could be implemented more
+ * efficiently using more complex and sophisticated data structures. For
+ * example, rather than using BPF_MAP_TYPE_QUEUE's,
+ * BPF_MAP_TYPE_{USER_}RINGBUF's could be used for exchanging messages between
+ * user space and kernel space. Similarly, we use a simple vruntime-sorted list
+ * in user space, but an rbtree could be used instead.
+ *
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
+#include <scx/common.bpf.h>
+#include "scx_userland.h"
+
+/*
+ * Maximum amount of tasks enqueued/dispatched between kernel and user-space.
+ */
+#define MAX_ENQUEUED_TASKS 4096
+
+char _license[] SEC("license") = "GPL";
+
+const volatile s32 usersched_pid;
+
+/* !0 for veristat, set during init */
+const volatile u32 num_possible_cpus = 64;
+
+/* Stats that are printed by user space. */
+u64 nr_failed_enqueues, nr_kernel_enqueues, nr_user_enqueues;
+
+/*
+ * Number of tasks that are queued for scheduling.
+ *
+ * This number is incremented by the BPF component when a task is queued to the
+ * user-space scheduler and it must be decremented by the user-space scheduler
+ * when a task is consumed.
+ */
+volatile u64 nr_queued;
+
+/*
+ * Number of tasks that are waiting for scheduling.
+ *
+ * This number must be updated by the user-space scheduler to keep track if
+ * there is still some scheduling work to do.
+ */
+volatile u64 nr_scheduled;
+
+UEI_DEFINE(uei);
+
+/*
+ * The map containing tasks that are enqueued in user space from the kernel.
+ *
+ * This map is drained by the user space scheduler.
+ */
+struct {
+	__uint(type, BPF_MAP_TYPE_QUEUE);
+	__uint(max_entries, MAX_ENQUEUED_TASKS);
+	__type(value, struct scx_userland_enqueued_task);
+} enqueued SEC(".maps");
+
+/*
+ * The map containing tasks that are dispatched to the kernel from user space.
+ *
+ * Drained by the kernel in userland_dispatch().
+ */
+struct {
+	__uint(type, BPF_MAP_TYPE_QUEUE);
+	__uint(max_entries, MAX_ENQUEUED_TASKS);
+	__type(value, s32);
+} dispatched SEC(".maps");
+
+/* Per-task scheduling context */
+struct task_ctx {
+	bool force_local; /* Dispatch directly to local DSQ */
+};
+
+/* Map that contains task-local storage. */
+struct {
+	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, struct task_ctx);
+} task_ctx_stor SEC(".maps");
+
+/*
+ * Flag used to wake-up the user-space scheduler.
+ */
+static volatile u32 usersched_needed;
+
+/*
+ * Set user-space scheduler wake-up flag (equivalent to an atomic release
+ * operation).
+ */
+static void set_usersched_needed(void)
+{
+	__sync_fetch_and_or(&usersched_needed, 1);
+}
+
+/*
+ * Check and clear user-space scheduler wake-up flag (equivalent to an atomic
+ * acquire operation).
+ */
+static bool test_and_clear_usersched_needed(void)
+{
+	return __sync_fetch_and_and(&usersched_needed, 0) == 1;
+}
+
+static bool is_usersched_task(const struct task_struct *p)
+{
+	return p->pid == usersched_pid;
+}
+
+static bool keep_in_kernel(const struct task_struct *p)
+{
+	return p->nr_cpus_allowed < num_possible_cpus;
+}
+
+static struct task_struct *usersched_task(void)
+{
+	struct task_struct *p;
+
+	p = bpf_task_from_pid(usersched_pid);
+	/*
+	 * Should never happen -- the usersched task should always be managed
+	 * by sched_ext.
+	 */
+	if (!p)
+		scx_bpf_error("Failed to find usersched task %d", usersched_pid);
+
+	return p;
+}
+
+s32 BPF_STRUCT_OPS(userland_select_cpu, struct task_struct *p,
+		   s32 prev_cpu, u64 wake_flags)
+{
+	if (keep_in_kernel(p)) {
+		s32 cpu;
+		struct task_ctx *tctx;
+
+		tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+		if (!tctx) {
+			scx_bpf_error("Failed to look up task-local storage for %s", p->comm);
+			return -ESRCH;
+		}
+
+		if (p->nr_cpus_allowed == 1 ||
+		    scx_bpf_test_and_clear_cpu_idle(prev_cpu)) {
+			tctx->force_local = true;
+			return prev_cpu;
+		}
+
+		cpu = scx_bpf_pick_idle_cpu(p->cpus_ptr, 0);
+		if (cpu >= 0) {
+			tctx->force_local = true;
+			return cpu;
+		}
+	}
+
+	return prev_cpu;
+}
+
+static void dispatch_user_scheduler(void)
+{
+	struct task_struct *p;
+
+	p = usersched_task();
+	if (p) {
+		scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, 0);
+		bpf_task_release(p);
+	}
+}
+
+static void enqueue_task_in_user_space(struct task_struct *p, u64 enq_flags)
+{
+	struct scx_userland_enqueued_task task = {};
+
+	task.pid = p->pid;
+	task.sum_exec_runtime = p->se.sum_exec_runtime;
+	task.weight = p->scx.weight;
+
+	if (bpf_map_push_elem(&enqueued, &task, 0)) {
+		/*
+		 * If we fail to enqueue the task in user space, put it
+		 * directly on the global DSQ.
+		 */
+		__sync_fetch_and_add(&nr_failed_enqueues, 1);
+		scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
+	} else {
+		__sync_fetch_and_add(&nr_user_enqueues, 1);
+		set_usersched_needed();
+	}
+}
+
+void BPF_STRUCT_OPS(userland_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	if (keep_in_kernel(p)) {
+		u64 dsq_id = SCX_DSQ_GLOBAL;
+		struct task_ctx *tctx;
+
+		tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+		if (!tctx) {
+			scx_bpf_error("Failed to lookup task ctx for %s", p->comm);
+			return;
+		}
+
+		if (tctx->force_local)
+			dsq_id = SCX_DSQ_LOCAL;
+		tctx->force_local = false;
+		scx_bpf_dsq_insert(p, dsq_id, SCX_SLICE_DFL, enq_flags);
+		__sync_fetch_and_add(&nr_kernel_enqueues, 1);
+		return;
+	} else if (!is_usersched_task(p)) {
+		enqueue_task_in_user_space(p, enq_flags);
+	}
+}
+
+void BPF_STRUCT_OPS(userland_dispatch, s32 cpu, struct task_struct *prev)
+{
+	if (test_and_clear_usersched_needed())
+		dispatch_user_scheduler();
+
+	bpf_repeat(MAX_ENQUEUED_TASKS) {
+		s32 pid;
+		struct task_struct *p;
+
+		if (bpf_map_pop_elem(&dispatched, &pid))
+			break;
+
+		/*
+		 * The task could have exited by the time we get around to
+		 * dispatching it. Treat this as a normal occurrence, and simply
+		 * move onto the next iteration.
+		 */
+		p = bpf_task_from_pid(pid);
+		if (!p)
+			continue;
+
+		scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, 0);
+		bpf_task_release(p);
+	}
+}
+
+/*
+ * A CPU is about to change its idle state. If the CPU is going idle, ensure
+ * that the user-space scheduler has a chance to run if there is any remaining
+ * work to do.
+ */
+void BPF_STRUCT_OPS(userland_update_idle, s32 cpu, bool idle)
+{
+	/*
+	 * Don't do anything if we exit from and idle state, a CPU owner will
+	 * be assigned in .running().
+	 */
+	if (!idle)
+		return;
+	/*
+	 * A CPU is now available, notify the user-space scheduler that tasks
+	 * can be dispatched, if there is at least one task waiting to be
+	 * scheduled, either queued (accounted in nr_queued) or scheduled
+	 * (accounted in nr_scheduled).
+	 *
+	 * NOTE: nr_queued is incremented by the BPF component, more exactly in
+	 * enqueue(), when a task is sent to the user-space scheduler, then
+	 * the scheduler drains the queued tasks (updating nr_queued) and adds
+	 * them to its internal data structures / state; at this point tasks
+	 * become "scheduled" and the user-space scheduler will take care of
+	 * updating nr_scheduled accordingly; lastly tasks will be dispatched
+	 * and the user-space scheduler will update nr_scheduled again.
+	 *
+	 * Checking both counters allows to determine if there is still some
+	 * pending work to do for the scheduler: new tasks have been queued
+	 * since last check, or there are still tasks "queued" or "scheduled"
+	 * since the previous user-space scheduler run. If the counters are
+	 * both zero it is pointless to wake-up the scheduler (even if a CPU
+	 * becomes idle), because there is nothing to do.
+	 *
+	 * Keep in mind that update_idle() doesn't run concurrently with the
+	 * user-space scheduler (that is single-threaded): this function is
+	 * naturally serialized with the user-space scheduler code, therefore
+	 * this check here is also safe from a concurrency perspective.
+	 */
+	if (nr_queued || nr_scheduled) {
+		/*
+		 * Kick the CPU to make it immediately ready to accept
+		 * dispatched tasks.
+		 */
+		set_usersched_needed();
+		scx_bpf_kick_cpu(cpu, 0);
+	}
+}
+
+s32 BPF_STRUCT_OPS(userland_init_task, struct task_struct *p,
+		   struct scx_init_task_args *args)
+{
+	if (bpf_task_storage_get(&task_ctx_stor, p, 0,
+				 BPF_LOCAL_STORAGE_GET_F_CREATE))
+		return 0;
+	else
+		return -ENOMEM;
+}
+
+s32 BPF_STRUCT_OPS(userland_init)
+{
+	if (num_possible_cpus == 0) {
+		scx_bpf_error("User scheduler # CPUs uninitialized (%d)",
+			      num_possible_cpus);
+		return -EINVAL;
+	}
+
+	if (usersched_pid <= 0) {
+		scx_bpf_error("User scheduler pid uninitialized (%d)",
+			      usersched_pid);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+void BPF_STRUCT_OPS(userland_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(userland_ops,
+	       .select_cpu		= (void *)userland_select_cpu,
+	       .enqueue			= (void *)userland_enqueue,
+	       .dispatch		= (void *)userland_dispatch,
+	       .update_idle		= (void *)userland_update_idle,
+	       .init_task		= (void *)userland_init_task,
+	       .init			= (void *)userland_init,
+	       .exit			= (void *)userland_exit,
+	       .flags			= SCX_OPS_ENQ_LAST |
+					  SCX_OPS_KEEP_BUILTIN_IDLE,
+	       .name			= "userland");