Message-ID:
 <AM6PR03MB50806B93D8187527AE4A9B7E991A2@AM6PR03MB5080.eurprd03.prod.outlook.com>
Date: Thu, 16 Jan 2025 19:41:12 +0000
From: Juntong Deng <juntong.deng@...look.com>
To: ast@...nel.org,
	daniel@...earbox.net,
	john.fastabend@...il.com,
	andrii@...nel.org,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
	song@...nel.org,
	yonghong.song@...ux.dev,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com,
	jolsa@...nel.org,
	memxor@...il.com,
	tj@...nel.org,
	void@...ifault.com
Cc: bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH bpf-next 7/7] sched_ext: Add proof-of-concept test case

This patch adds a proof-of-concept test case scx_simple_cap_test, which
is based on scx_simple.

Add scx_bpf_dsq_move_to_local to enqueue, which will be rejected by
the verifier.

Signed-off-by: Juntong Deng <juntong.deng@...look.com>
---
 tools/sched_ext/Makefile                  |   2 +-
 tools/sched_ext/scx_simple_cap_test.bpf.c | 159 ++++++++++++++++++++++
 tools/sched_ext/scx_simple_cap_test.c     | 107 +++++++++++++++
 3 files changed, 267 insertions(+), 1 deletion(-)
 create mode 100644 tools/sched_ext/scx_simple_cap_test.bpf.c
 create mode 100644 tools/sched_ext/scx_simple_cap_test.c

diff --git a/tools/sched_ext/Makefile b/tools/sched_ext/Makefile
index ca3815e572d8..0e87aa77594b 100644
--- a/tools/sched_ext/Makefile
+++ b/tools/sched_ext/Makefile
@@ -176,7 +176,7 @@ $(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_qmap scx_central scx_flatcg
+c-sched-targets = scx_simple scx_qmap scx_central scx_flatcg scx_simple_cap_test
 
 $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 	$(BINDIR)/%: \
diff --git a/tools/sched_ext/scx_simple_cap_test.bpf.c b/tools/sched_ext/scx_simple_cap_test.bpf.c
new file mode 100644
index 000000000000..6bcf4dcbfcb4
--- /dev/null
+++ b/tools/sched_ext/scx_simple_cap_test.bpf.c
@@ -0,0 +1,159 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A simple scheduler.
+ *
+ * By default, it operates as a simple global weighted vtime scheduler and can
+ * be switched to FIFO scheduling. It also demonstrates the following niceties.
+ *
+ * - Statistics tracking how many tasks are queued to local and global dsq's.
+ * - Termination notification for userspace.
+ *
+ * While very simple, this scheduler should work reasonably well on CPUs with a
+ * uniform L3 cache topology. While preemption is not implemented, the fact that
+ * the scheduling queue is shared across all CPUs means that whatever is at the
+ * front of the queue is likely to be executed fairly quickly given enough
+ * number of CPUs. The FIFO scheduling mode may be beneficial to some workloads
+ * but comes with the usual problems with FIFO scheduling where saturating
+ * threads can easily drown out interactive ones.
+ *
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@...nel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@...a.com>
+ */
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+const volatile bool fifo_sched;
+
+static u64 vtime_now;
+UEI_DEFINE(uei);
+
+/*
+ * Built-in DSQs such as SCX_DSQ_GLOBAL cannot be used as priority queues
+ * (meaning, cannot be dispatched to with scx_bpf_dsq_insert_vtime()). We
+ * therefore create a separate DSQ with ID 0 that we dispatch to and consume
+ * from. If scx_simple only supported global FIFO scheduling, then we could just
+ * use SCX_DSQ_GLOBAL.
+ */
+#define SHARED_DSQ 0
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+	__uint(key_size, sizeof(u32));
+	__uint(value_size, sizeof(u64));
+	__uint(max_entries, 2);			/* [local, global] */
+} stats SEC(".maps");
+
+static void stat_inc(u32 idx)
+{
+	u64 *cnt_p = bpf_map_lookup_elem(&stats, &idx);
+	if (cnt_p)
+		(*cnt_p)++;
+}
+
+static inline bool vtime_before(u64 a, u64 b)
+{
+	return (s64)(a - b) < 0;
+}
+
+s32 BPF_STRUCT_OPS(simple_select_cpu, struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+	bool is_idle = false;
+	s32 cpu;
+
+	cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &is_idle);
+	if (is_idle) {
+		stat_inc(0);	/* count local queueing */
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
+	}
+
+	return cpu;
+}
+
+void BPF_STRUCT_OPS(simple_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	stat_inc(1);	/* count global queueing */
+
+	/* bpf capabilities test! */
+	scx_bpf_dsq_move_to_local(SHARED_DSQ);
+
+	if (fifo_sched) {
+		scx_bpf_dsq_insert(p, SHARED_DSQ, SCX_SLICE_DFL, enq_flags);
+	} else {
+		u64 vtime = p->scx.dsq_vtime;
+
+		/*
+		 * Limit the amount of budget that an idling task can accumulate
+		 * to one slice.
+		 */
+		if (vtime_before(vtime, vtime_now - SCX_SLICE_DFL))
+			vtime = vtime_now - SCX_SLICE_DFL;
+
+		scx_bpf_dsq_insert_vtime(p, SHARED_DSQ, SCX_SLICE_DFL, vtime,
+					 enq_flags);
+	}
+}
+
+void BPF_STRUCT_OPS(simple_dispatch, s32 cpu, struct task_struct *prev)
+{
+	scx_bpf_dsq_move_to_local(SHARED_DSQ);
+}
+
+void BPF_STRUCT_OPS(simple_running, struct task_struct *p)
+{
+	if (fifo_sched)
+		return;
+
+	/*
+	 * Global vtime 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(vtime_now, p->scx.dsq_vtime))
+		vtime_now = p->scx.dsq_vtime;
+}
+
+void BPF_STRUCT_OPS(simple_stopping, struct task_struct *p, bool runnable)
+{
+	if (fifo_sched)
+		return;
+
+	/*
+	 * 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.
+	 */
+	p->scx.dsq_vtime += (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
+}
+
+void BPF_STRUCT_OPS(simple_enable, struct task_struct *p)
+{
+	p->scx.dsq_vtime = vtime_now;
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(simple_init)
+{
+	return scx_bpf_create_dsq(SHARED_DSQ, -1);
+}
+
+void BPF_STRUCT_OPS(simple_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SCX_OPS_DEFINE(simple_ops,
+	       .select_cpu		= (void *)simple_select_cpu,
+	       .enqueue			= (void *)simple_enqueue,
+	       .dispatch		= (void *)simple_dispatch,
+	       .running			= (void *)simple_running,
+	       .stopping		= (void *)simple_stopping,
+	       .enable			= (void *)simple_enable,
+	       .init			= (void *)simple_init,
+	       .exit			= (void *)simple_exit,
+	       .name			= "simple");
diff --git a/tools/sched_ext/scx_simple_cap_test.c b/tools/sched_ext/scx_simple_cap_test.c
new file mode 100644
index 000000000000..c4a0a5b1e0cf
--- /dev/null
+++ b/tools/sched_ext/scx_simple_cap_test.c
@@ -0,0 +1,107 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@...nel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@...a.com>
+ */
+#include <stdio.h>
+#include <unistd.h>
+#include <signal.h>
+#include <libgen.h>
+#include <bpf/bpf.h>
+#include <scx/common.h>
+#include "scx_simple_cap_test.bpf.skel.h"
+
+const char help_fmt[] =
+"A simple sched_ext scheduler.\n"
+"\n"
+"See the top-level comment in .bpf.c for more details.\n"
+"\n"
+"Usage: %s [-f] [-v]\n"
+"\n"
+"  -f            Use FIFO scheduling instead of weighted vtime scheduling\n"
+"  -v            Print libbpf debug messages\n"
+"  -h            Display this help and exit\n";
+
+static bool verbose;
+static volatile int exit_req;
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	if (level == LIBBPF_DEBUG && !verbose)
+		return 0;
+	return vfprintf(stderr, format, args);
+}
+
+static void sigint_handler(int simple)
+{
+	exit_req = 1;
+}
+
+static void read_stats(struct scx_simple_cap_test *skel, __u64 *stats)
+{
+	int nr_cpus = libbpf_num_possible_cpus();
+	__u64 cnts[2][nr_cpus];
+	__u32 idx;
+
+	memset(stats, 0, sizeof(stats[0]) * 2);
+
+	for (idx = 0; idx < 2; idx++) {
+		int ret, cpu;
+
+		ret = bpf_map_lookup_elem(bpf_map__fd(skel->maps.stats),
+					  &idx, cnts[idx]);
+		if (ret < 0)
+			continue;
+		for (cpu = 0; cpu < nr_cpus; cpu++)
+			stats[idx] += cnts[idx][cpu];
+	}
+}
+
+int main(int argc, char **argv)
+{
+	struct scx_simple_cap_test *skel;
+	struct bpf_link *link;
+	__u32 opt;
+	__u64 ecode;
+
+	libbpf_set_print(libbpf_print_fn);
+	signal(SIGINT, sigint_handler);
+	signal(SIGTERM, sigint_handler);
+restart:
+	skel = SCX_OPS_OPEN(simple_ops, scx_simple_cap_test);
+
+	while ((opt = getopt(argc, argv, "fvh")) != -1) {
+		switch (opt) {
+		case 'f':
+			skel->rodata->fifo_sched = true;
+			break;
+		case 'v':
+			verbose = true;
+			break;
+		default:
+			fprintf(stderr, help_fmt, basename(argv[0]));
+			return opt != 'h';
+		}
+	}
+
+	SCX_OPS_LOAD(skel, simple_ops, scx_simple_cap_test, uei);
+	link = SCX_OPS_ATTACH(skel, simple_ops, scx_simple_cap_test);
+
+	while (!exit_req && !UEI_EXITED(skel, uei)) {
+		__u64 stats[2];
+
+		read_stats(skel, stats);
+		printf("local=%llu global=%llu\n", stats[0], stats[1]);
+		fflush(stdout);
+		sleep(1);
+	}
+
+	bpf_link__destroy(link);
+	ecode = UEI_REPORT(skel, uei);
+	scx_simple_cap_test__destroy(skel);
+
+	if (UEI_ECODE_RESTART(ecode))
+		goto restart;
+	return 0;
+}
-- 
2.39.5

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