@@ -1,5 +1,6 @@
scx_simple
scx_qmap
+scx_central
*.skel.h
*.subskel.h
/tools/
@@ -115,7 +115,7 @@ BPF_CFLAGS = -g -D__TARGET_ARCH_$(SRCARCH) \
-Wall -Wno-compare-distinct-pointer-types \
-O2 -mcpu=v3
-all: scx_simple scx_qmap
+all: scx_simple scx_qmap scx_central
# sort removes libbpf duplicates when not cross-building
MAKE_DIRS := $(sort $(BUILD_DIR)/libbpf $(HOST_BUILD_DIR)/libbpf \
@@ -174,10 +174,14 @@ scx_qmap: scx_qmap.c scx_qmap.skel.h user_exit_info.h
$(CC) $(CFLAGS) -c $< -o $@.o
$(CC) -o $@ $@.o $(HOST_BPFOBJ) $(LDFLAGS)
+scx_central: scx_central.c scx_central.skel.h user_exit_info.h
+ $(CC) $(CFLAGS) -c $< -o $@.o
+ $(CC) -o $@ $@.o $(HOST_BPFOBJ) $(LDFLAGS)
+
clean:
rm -rf $(SCRATCH_DIR) $(HOST_SCRATCH_DIR)
rm -f *.o *.bpf.o *.skel.h *.subskel.h
- rm -f scx_simple scx_qmap
+ rm -f scx_simple scx_qmap scx_central
.PHONY: all clean
new file mode 100644
@@ -0,0 +1,225 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A central FIFO sched_ext scheduler which demonstrates the followings:
+ *
+ * a. Making all scheduling decisions from one CPU:
+ *
+ * The central CPU is the only one making scheduling decisions. All other
+ * CPUs kick the central CPU when they run out of tasks to run.
+ *
+ * There is one global BPF queue and the central CPU schedules all CPUs by
+ * dispatching from the global queue to each CPU's local dsq from dispatch().
+ * This isn't the most straightforward. e.g. It'd be easier to bounce
+ * through per-CPU BPF queues. The current design is chosen to maximally
+ * utilize and verify various SCX mechanisms such as LOCAL_ON dispatching.
+ *
+ * b. Preemption
+ *
+ * SCX_KICK_PREEMPT is used to trigger scheduling and CPUs to move to the
+ * next tasks.
+ *
+ * This scheduler is designed to maximize usage of various SCX mechanisms. A
+ * more practical implementation would likely put the scheduling loop outside
+ * the central CPU's dispatch() path and add some form of priority mechanism.
+ *
+ * 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"
+
+char _license[] SEC("license") = "GPL";
+
+enum {
+ FALLBACK_DSQ_ID = 0,
+ MAX_CPUS = 4096,
+ MS_TO_NS = 1000LLU * 1000,
+ TIMER_INTERVAL_NS = 1 * MS_TO_NS,
+};
+
+const volatile bool switch_partial;
+const volatile s32 central_cpu;
+const volatile u32 nr_cpu_ids = 64; /* !0 for veristat, set during init */
+
+u64 nr_total, nr_locals, nr_queued, nr_lost_pids;
+u64 nr_dispatches, nr_mismatches, nr_retries;
+u64 nr_overflows;
+
+struct user_exit_info uei;
+
+struct {
+ __uint(type, BPF_MAP_TYPE_QUEUE);
+ __uint(max_entries, 4096);
+ __type(value, s32);
+} central_q SEC(".maps");
+
+/* can't use percpu map due to bad lookups */
+static bool cpu_gimme_task[MAX_CPUS];
+
+struct central_timer {
+ struct bpf_timer timer;
+};
+
+struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __uint(max_entries, 1);
+ __type(key, u32);
+ __type(value, struct central_timer);
+} central_timer SEC(".maps");
+
+s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
+ s32 prev_cpu, u64 wake_flags)
+{
+ /*
+ * Steer wakeups to the central CPU as much as possible to avoid
+ * disturbing other CPUs. It's safe to blindly return the central cpu as
+ * select_cpu() is a hint and if @p can't be on it, the kernel will
+ * automatically pick a fallback CPU.
+ */
+ return central_cpu;
+}
+
+void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
+{
+ s32 pid = p->pid;
+
+ __sync_fetch_and_add(&nr_total, 1);
+
+ if (bpf_map_push_elem(¢ral_q, &pid, 0)) {
+ __sync_fetch_and_add(&nr_overflows, 1);
+ scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, enq_flags);
+ return;
+ }
+
+ __sync_fetch_and_add(&nr_queued, 1);
+
+ if (!scx_bpf_task_running(p))
+ scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+}
+
+static bool dispatch_to_cpu(s32 cpu)
+{
+ struct task_struct *p;
+ s32 pid;
+
+ bpf_repeat(BPF_MAX_LOOPS) {
+ if (bpf_map_pop_elem(¢ral_q, &pid))
+ break;
+
+ __sync_fetch_and_sub(&nr_queued, 1);
+
+ p = bpf_task_from_pid(pid);
+ if (!p) {
+ __sync_fetch_and_add(&nr_lost_pids, 1);
+ continue;
+ }
+
+ /*
+ * If we can't run the task at the top, do the dumb thing and
+ * bounce it to the fallback dsq.
+ */
+ if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
+ __sync_fetch_and_add(&nr_mismatches, 1);
+ scx_bpf_dispatch(p, FALLBACK_DSQ_ID, SCX_SLICE_DFL, 0);
+ bpf_task_release(p);
+ continue;
+ }
+
+ /* dispatch to local and mark that @cpu doesn't need more */
+ scx_bpf_dispatch(p, SCX_DSQ_LOCAL_ON | cpu, SCX_SLICE_DFL, 0);
+
+ if (cpu != central_cpu)
+ scx_bpf_kick_cpu(cpu, 0);
+
+ bpf_task_release(p);
+ return true;
+ }
+
+ return false;
+}
+
+void BPF_STRUCT_OPS(central_dispatch, s32 cpu, struct task_struct *prev)
+{
+ if (cpu == central_cpu) {
+ /* dispatch for all other CPUs first */
+ __sync_fetch_and_add(&nr_dispatches, 1);
+
+ bpf_for(cpu, 0, nr_cpu_ids) {
+ bool *gimme;
+
+ if (!scx_bpf_dispatch_nr_slots())
+ break;
+
+ /* central's gimme is never set */
+ gimme = MEMBER_VPTR(cpu_gimme_task, [cpu]);
+ if (gimme && !*gimme)
+ continue;
+
+ if (dispatch_to_cpu(cpu))
+ *gimme = false;
+ }
+
+ /*
+ * Retry if we ran out of dispatch buffer slots as we might have
+ * skipped some CPUs and also need to dispatch for self. The ext
+ * core automatically retries if the local dsq is empty but we
+ * can't rely on that as we're dispatching for other CPUs too.
+ * Kick self explicitly to retry.
+ */
+ if (!scx_bpf_dispatch_nr_slots()) {
+ __sync_fetch_and_add(&nr_retries, 1);
+ scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+ return;
+ }
+
+ /* look for a task to run on the central CPU */
+ if (scx_bpf_consume(FALLBACK_DSQ_ID))
+ return;
+ dispatch_to_cpu(central_cpu);
+ } else {
+ bool *gimme;
+
+ if (scx_bpf_consume(FALLBACK_DSQ_ID))
+ return;
+
+ gimme = MEMBER_VPTR(cpu_gimme_task, [cpu]);
+ if (gimme)
+ *gimme = true;
+
+ /*
+ * Force dispatch on the scheduling CPU so that it finds a task
+ * to run for us.
+ */
+ scx_bpf_kick_cpu(central_cpu, SCX_KICK_PREEMPT);
+ }
+}
+
+int BPF_STRUCT_OPS_SLEEPABLE(central_init)
+{
+ if (!switch_partial)
+ scx_bpf_switch_all();
+
+ return scx_bpf_create_dsq(FALLBACK_DSQ_ID, -1);
+}
+
+void BPF_STRUCT_OPS(central_exit, struct scx_exit_info *ei)
+{
+ uei_record(&uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops central_ops = {
+ /*
+ * We are offloading all scheduling decisions to the central CPU and
+ * thus being the last task on a given CPU doesn't mean anything
+ * special. Enqueue the last tasks like any other tasks.
+ */
+ .flags = SCX_OPS_ENQ_LAST,
+
+ .select_cpu = (void *)central_select_cpu,
+ .enqueue = (void *)central_enqueue,
+ .dispatch = (void *)central_dispatch,
+ .init = (void *)central_init,
+ .exit = (void *)central_exit,
+ .name = "central",
+};
new file mode 100644
@@ -0,0 +1,93 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+#include <signal.h>
+#include <assert.h>
+#include <libgen.h>
+#include <bpf/bpf.h>
+#include "user_exit_info.h"
+#include "scx_central.skel.h"
+
+const char help_fmt[] =
+"A central FIFO sched_ext scheduler.\n"
+"\n"
+"See the top-level comment in .bpf.c for more details.\n"
+"\n"
+"Usage: %s [-c CPU] [-p]\n"
+"\n"
+" -c CPU Override the central CPU (default: 0)\n"
+" -p Switch only tasks on SCHED_EXT policy intead of all\n"
+" -h Display this help and exit\n";
+
+static volatile int exit_req;
+
+static void sigint_handler(int dummy)
+{
+ exit_req = 1;
+}
+
+int main(int argc, char **argv)
+{
+ struct scx_central *skel;
+ struct bpf_link *link;
+ u64 seq = 0;
+ s32 opt;
+
+ signal(SIGINT, sigint_handler);
+ signal(SIGTERM, sigint_handler);
+
+ libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+
+ skel = scx_central__open();
+ assert(skel);
+
+ skel->rodata->central_cpu = 0;
+ skel->rodata->nr_cpu_ids = libbpf_num_possible_cpus();
+
+ while ((opt = getopt(argc, argv, "c:ph")) != -1) {
+ switch (opt) {
+ case 'c':
+ skel->rodata->central_cpu = strtoul(optarg, NULL, 0);
+ break;
+ case 'p':
+ skel->rodata->switch_partial = true;
+ break;
+ default:
+ fprintf(stderr, help_fmt, basename(argv[0]));
+ return opt != 'h';
+ }
+ }
+
+ assert(!scx_central__load(skel));
+
+ link = bpf_map__attach_struct_ops(skel->maps.central_ops);
+ assert(link);
+
+ while (!exit_req && !uei_exited(&skel->bss->uei)) {
+ printf("[SEQ %lu]\n", seq++);
+ printf("total :%10lu local:%10lu queued:%10lu lost:%10lu\n",
+ skel->bss->nr_total,
+ skel->bss->nr_locals,
+ skel->bss->nr_queued,
+ skel->bss->nr_lost_pids);
+ printf(" dispatch:%10lu mismatch:%10lu retry:%10lu\n",
+ skel->bss->nr_dispatches,
+ skel->bss->nr_mismatches,
+ skel->bss->nr_retries);
+ printf("overflow:%10lu\n",
+ skel->bss->nr_overflows);
+ fflush(stdout);
+ sleep(1);
+ }
+
+ bpf_link__destroy(link);
+ uei_print(&skel->bss->uei);
+ scx_central__destroy(skel);
+ return 0;
+}