selftests/bpf: Simplify cgroup_hierarchical_stats selftest

The cgroup_hierarchical_stats selftest is complicated. It has to be,
because it tests an entire workflow of recording, aggregating, and
dumping cgroup stats. However, some of the complexity is unnecessary.
The test now enables the memory controller in a cgroup hierarchy, invokes
reclaim, measure reclaim time, THEN uses that reclaim time to test the
stats collection and aggregation. We don't need to use such a
complicated stat, as the context in which the stat is collected is
orthogonal.

Simplify the test by using a simple stat instead of reclaim time, the
total number of times a process has ever entered a cgroup. This makes
the test simpler and removes the dependency on the memory controller and
the memory reclaim interface.

Signed-off-by: Yosry Ahmed <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: KP Singh <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c

···
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * Functions to manage eBPF programs attached to cgroup subsystems
+ * This test makes sure BPF stats collection using rstat works correctly.
+ * The test uses 3 BPF progs:
+ * (a) counter: This BPF prog is invoked every time we attach a process to a
+ *              cgroup and locklessly increments a percpu counter.
+ *              The program then calls cgroup_rstat_updated() to inform rstat
+ *              of an update on the (cpu, cgroup) pair.
+ *
+ * (b) flusher: This BPF prog is invoked when an rstat flush is ongoing, it
+ *              aggregates all percpu counters to a total counter, and also
+ *              propagates the changes to the ancestor cgroups.
+ *
+ * (c) dumper: This BPF prog is a cgroup_iter. It is used to output the total
+ *             counter of a cgroup through reading a file in userspace.
+ *
+ * The test sets up a cgroup hierarchy, and the above programs. It spawns a few
+ * processes in the leaf cgroups and makes sure all the counters are aggregated
+ * correctly.
  *
  * Copyright 2022 Google LLC.
  */
···
 #define PAGE_SIZE 4096
 #define MB(x) (x << 20)
 
+#define PROCESSES_PER_CGROUP 3
+
 #define BPFFS_ROOT "/sys/fs/bpf/"
-#define BPFFS_VMSCAN BPFFS_ROOT"vmscan/"
+#define BPFFS_ATTACH_COUNTERS BPFFS_ROOT "attach_counters/"
 
 #define CG_ROOT_NAME "root"
 #define CG_ROOT_ID 1
···
 		return err;
 
 	/* Create a directory to contain stat files in bpffs */
-	err = mkdir(BPFFS_VMSCAN, 0755);
+	err = mkdir(BPFFS_ATTACH_COUNTERS, 0755);
 	if (!ASSERT_OK(err, "mkdir"))
 		return err;
 
···
 static void cleanup_bpffs(void)
 {
 	/* Remove created directory in bpffs */
-	ASSERT_OK(rmdir(BPFFS_VMSCAN), "rmdir "BPFFS_VMSCAN);
+	ASSERT_OK(rmdir(BPFFS_ATTACH_COUNTERS), "rmdir "BPFFS_ATTACH_COUNTERS);
 
 	/* Unmount bpffs, if it wasn't already mounted when we started */
 	if (mounted_bpffs)
···
 
 		cgroups[i].fd = fd;
 		cgroups[i].id = get_cgroup_id(cgroups[i].path);
-
-		/*
-		 * Enable memcg controller for the entire hierarchy.
-		 * Note that stats are collected for all cgroups in a hierarchy
-		 * with memcg enabled anyway, but are only exposed for cgroups
-		 * that have memcg enabled.
-		 */
-		if (i < N_NON_LEAF_CGROUPS) {
-			err = enable_controllers(cgroups[i].path, "memory");
-			if (!ASSERT_OK(err, "enable_controllers"))
-				return err;
-		}
 	}
 	return 0;
 }
···
 	cleanup_bpffs();
 }
 
-static int reclaimer(const char *cgroup_path, size_t size)
+static int attach_processes(void)
 {
-	static char size_buf[128];
-	char *buf, *ptr;
-	int err;
+	int i, j, status;
 
-	/* Join cgroup in the parent process workdir */
-	if (join_parent_cgroup(cgroup_path))
-		return EACCES;
-
-	/* Allocate memory */
-	buf = malloc(size);
-	if (!buf)
-		return ENOMEM;
-
-	/* Write to memory to make sure it's actually allocated */
-	for (ptr = buf; ptr < buf + size; ptr += PAGE_SIZE)
-		*ptr = 1;
-
-	/* Try to reclaim memory */
-	snprintf(size_buf, 128, "%lu", size);
-	err = write_cgroup_file_parent(cgroup_path, "memory.reclaim", size_buf);
-
-	free(buf);
-	/* memory.reclaim returns EAGAIN if the amount is not fully reclaimed */
-	if (err && errno != EAGAIN)
-		return errno;
-
-	return 0;
-}
-
-static int induce_vmscan(void)
-{
-	int i, status;
-
-	/*
-	 * In every leaf cgroup, run a child process that allocates some memory
-	 * and attempts to reclaim some of it.
-	 */
+	/* In every leaf cgroup, attach 3 processes */
 	for (i = N_NON_LEAF_CGROUPS; i < N_CGROUPS; i++) {
-		pid_t pid;
+		for (j = 0; j < PROCESSES_PER_CGROUP; j++) {
+			pid_t pid;
 
-		/* Create reclaimer child */
-		pid = fork();
-		if (pid == 0) {
-			status = reclaimer(cgroups[i].path, MB(5));
-			exit(status);
+			/* Create child and attach to cgroup */
+			pid = fork();
+			if (pid == 0) {
+				if (join_parent_cgroup(cgroups[i].path))
+					exit(EACCES);
+				exit(0);
+			}
+
+			/* Cleanup child */
+			waitpid(pid, &status, 0);
+			if (!ASSERT_TRUE(WIFEXITED(status), "child process exited"))
+				return 1;
+			if (!ASSERT_EQ(WEXITSTATUS(status), 0,
+				       "child process exit code"))
+				return 1;
 		}
-
-		/* Cleanup reclaimer child */
-		waitpid(pid, &status, 0);
-		ASSERT_TRUE(WIFEXITED(status), "reclaimer exited");
-		ASSERT_EQ(WEXITSTATUS(status), 0, "reclaim exit code");
 	}
 	return 0;
 }
 
 static unsigned long long
-get_cgroup_vmscan_delay(unsigned long long cgroup_id, const char *file_name)
+get_attach_counter(unsigned long long cgroup_id, const char *file_name)
 {
-	unsigned long long vmscan = 0, id = 0;
+	unsigned long long attach_counter = 0, id = 0;
 	static char buf[128], path[128];
 
 	/* For every cgroup, read the file generated by cgroup_iter */
-	snprintf(path, 128, "%s%s", BPFFS_VMSCAN, file_name);
+	snprintf(path, 128, "%s%s", BPFFS_ATTACH_COUNTERS, file_name);
 	if (!ASSERT_OK(read_from_file(path, buf, 128), "read cgroup_iter"))
 		return 0;
 
 	/* Check the output file formatting */
-	ASSERT_EQ(sscanf(buf, "cg_id: %llu, total_vmscan_delay: %llu\n",
-			 &id, &vmscan), 2, "output format");
+	ASSERT_EQ(sscanf(buf, "cg_id: %llu, attach_counter: %llu\n",
+			 &id, &attach_counter), 2, "output format");
 
 	/* Check that the cgroup_id is displayed correctly */
 	ASSERT_EQ(id, cgroup_id, "cgroup_id");
-	/* Check that the vmscan reading is non-zero */
-	ASSERT_GT(vmscan, 0, "vmscan_reading");
-	return vmscan;
+	/* Check that the counter is non-zero */
+	ASSERT_GT(attach_counter, 0, "attach counter non-zero");
+	return attach_counter;
 }
 
-static void check_vmscan_stats(void)
+static void check_attach_counters(void)
 {
-	unsigned long long vmscan_readings[N_CGROUPS], vmscan_root;
+	unsigned long long attach_counters[N_CGROUPS], root_attach_counter;
 	int i;
 
-	for (i = 0; i < N_CGROUPS; i++) {
-		vmscan_readings[i] = get_cgroup_vmscan_delay(cgroups[i].id,
-							     cgroups[i].name);
-	}
+	for (i = 0; i < N_CGROUPS; i++)
+		attach_counters[i] = get_attach_counter(cgroups[i].id,
+							cgroups[i].name);
 
 	/* Read stats for root too */
-	vmscan_root = get_cgroup_vmscan_delay(CG_ROOT_ID, CG_ROOT_NAME);
+	root_attach_counter = get_attach_counter(CG_ROOT_ID, CG_ROOT_NAME);
+
+	/* Check that all leafs cgroups have an attach counter of 3 */
+	for (i = N_NON_LEAF_CGROUPS; i < N_CGROUPS; i++)
+		ASSERT_EQ(attach_counters[i], PROCESSES_PER_CGROUP,
+			  "leaf cgroup attach counter");
 
 	/* Check that child1 == child1_1 + child1_2 */
-	ASSERT_EQ(vmscan_readings[1], vmscan_readings[3] + vmscan_readings[4],
-		  "child1_vmscan");
+	ASSERT_EQ(attach_counters[1], attach_counters[3] + attach_counters[4],
+		  "child1_counter");
 	/* Check that child2 == child2_1 + child2_2 */
-	ASSERT_EQ(vmscan_readings[2], vmscan_readings[5] + vmscan_readings[6],
-		  "child2_vmscan");
+	ASSERT_EQ(attach_counters[2], attach_counters[5] + attach_counters[6],
+		  "child2_counter");
 	/* Check that test == child1 + child2 */
-	ASSERT_EQ(vmscan_readings[0], vmscan_readings[1] + vmscan_readings[2],
-		  "test_vmscan");
+	ASSERT_EQ(attach_counters[0], attach_counters[1] + attach_counters[2],
+		  "test_counter");
 	/* Check that root >= test */
-	ASSERT_GE(vmscan_root, vmscan_readings[1], "root_vmscan");
+	ASSERT_GE(root_attach_counter, attach_counters[1], "root_counter");
 }
 
 /* Creates iter link and pins in bpffs, returns 0 on success, -errno on failure.
···
 	linfo.cgroup.order = BPF_CGROUP_ITER_SELF_ONLY;
 	opts.link_info = &linfo;
 	opts.link_info_len = sizeof(linfo);
-	link = bpf_program__attach_iter(obj->progs.dump_vmscan, &opts);
+	link = bpf_program__attach_iter(obj->progs.dumper, &opts);
 	if (!ASSERT_OK_PTR(link, "attach_iter"))
 		return -EFAULT;
 
 	/* Pin the link to a bpffs file */
-	snprintf(path, 128, "%s%s", BPFFS_VMSCAN, file_name);
+	snprintf(path, 128, "%s%s", BPFFS_ATTACH_COUNTERS, file_name);
 	err = bpf_link__pin(link, path);
 	ASSERT_OK(err, "pin cgroup_iter");
 
···
 	if (!ASSERT_OK(err, "setup_cgroup_iter"))
 		return err;
 
-	bpf_program__set_autoattach((*skel)->progs.dump_vmscan, false);
+	bpf_program__set_autoattach((*skel)->progs.dumper, false);
 	err = cgroup_hierarchical_stats__attach(*skel);
 	if (!ASSERT_OK(err, "attach"))
 		return err;
···
 
 	for (i = 0; i < N_CGROUPS; i++) {
 		/* Delete files in bpffs that cgroup_iters are pinned in */
-		snprintf(path, 128, "%s%s", BPFFS_VMSCAN,
+		snprintf(path, 128, "%s%s", BPFFS_ATTACH_COUNTERS,
 			 cgroups[i].name);
 		ASSERT_OK(remove(path), "remove cgroup_iter pin");
 	}
 
 	/* Delete root file in bpffs */
-	snprintf(path, 128, "%s%s", BPFFS_VMSCAN, CG_ROOT_NAME);
+	snprintf(path, 128, "%s%s", BPFFS_ATTACH_COUNTERS, CG_ROOT_NAME);
 	ASSERT_OK(remove(path), "remove cgroup_iter root pin");
 	cgroup_hierarchical_stats__destroy(skel);
 }
···
 		goto hierarchy_cleanup;
 	if (setup_progs(&skel))
 		goto cleanup;
-	if (induce_vmscan())
+	if (attach_processes())
 		goto cleanup;
-	check_vmscan_stats();
+	check_attach_counters();
 cleanup:
 	destroy_progs(skel);
 hierarchy_cleanup:

tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c

···
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * Functions to manage eBPF programs attached to cgroup subsystems
- *
  * Copyright 2022 Google LLC.
  */
 #include "vmlinux.h"
···
 
 char _license[] SEC("license") = "GPL";
 
-/*
- * Start times are stored per-task, not per-cgroup, as multiple tasks in one
- * cgroup can perform reclaim concurrently.
- */
-struct {
-	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
-	__uint(map_flags, BPF_F_NO_PREALLOC);
-	__type(key, int);
-	__type(value, __u64);
-} vmscan_start_time SEC(".maps");
-
-struct vmscan_percpu {
+struct percpu_attach_counter {
 	/* Previous percpu state, to figure out if we have new updates */
 	__u64 prev;
 	/* Current percpu state */
 	__u64 state;
 };
 
-struct vmscan {
+struct attach_counter {
 	/* State propagated through children, pending aggregation */
 	__u64 pending;
 	/* Total state, including all cpus and all children */
···
 
 struct {
 	__uint(type, BPF_MAP_TYPE_PERCPU_HASH);
-	__uint(max_entries, 100);
+	__uint(max_entries, 1024);
 	__type(key, __u64);
-	__type(value, struct vmscan_percpu);
-} pcpu_cgroup_vmscan_elapsed SEC(".maps");
+	__type(value, struct percpu_attach_counter);
+} percpu_attach_counters SEC(".maps");
 
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
-	__uint(max_entries, 100);
+	__uint(max_entries, 1024);
 	__type(key, __u64);
-	__type(value, struct vmscan);
-} cgroup_vmscan_elapsed SEC(".maps");
+	__type(value, struct attach_counter);
+} attach_counters SEC(".maps");
 
 extern void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) __ksym;
 extern void cgroup_rstat_flush(struct cgroup *cgrp) __ksym;
-
-static struct cgroup *task_memcg(struct task_struct *task)
-{
-	int cgrp_id;
-
-#if __has_builtin(__builtin_preserve_enum_value)
-	cgrp_id = bpf_core_enum_value(enum cgroup_subsys_id, memory_cgrp_id);
-#else
-	cgrp_id = memory_cgrp_id;
-#endif
-	return task->cgroups->subsys[cgrp_id]->cgroup;
-}
 
 static uint64_t cgroup_id(struct cgroup *cgrp)
 {
 	return cgrp->kn->id;
 }
 
-static int create_vmscan_percpu_elem(__u64 cg_id, __u64 state)
+static int create_percpu_attach_counter(__u64 cg_id, __u64 state)
 {
-	struct vmscan_percpu pcpu_init = {.state = state, .prev = 0};
+	struct percpu_attach_counter pcpu_init = {.state = state, .prev = 0};
 
-	return bpf_map_update_elem(&pcpu_cgroup_vmscan_elapsed, &cg_id,
+	return bpf_map_update_elem(&percpu_attach_counters, &cg_id,
 				   &pcpu_init, BPF_NOEXIST);
 }
 
-static int create_vmscan_elem(__u64 cg_id, __u64 state, __u64 pending)
+static int create_attach_counter(__u64 cg_id, __u64 state, __u64 pending)
 {
-	struct vmscan init = {.state = state, .pending = pending};
+	struct attach_counter init = {.state = state, .pending = pending};
 
-	return bpf_map_update_elem(&cgroup_vmscan_elapsed, &cg_id,
+	return bpf_map_update_elem(&attach_counters, &cg_id,
 				   &init, BPF_NOEXIST);
 }
 
-SEC("tp_btf/mm_vmscan_memcg_reclaim_begin")
-int BPF_PROG(vmscan_start, int order, gfp_t gfp_flags)
+SEC("fentry/cgroup_attach_task")
+int BPF_PROG(counter, struct cgroup *dst_cgrp, struct task_struct *leader,
+	     bool threadgroup)
 {
-	struct task_struct *task = bpf_get_current_task_btf();
-	__u64 *start_time_ptr;
+	__u64 cg_id = cgroup_id(dst_cgrp);
+	struct percpu_attach_counter *pcpu_counter = bpf_map_lookup_elem(
+			&percpu_attach_counters,
+			&cg_id);
 
-	start_time_ptr = bpf_task_storage_get(&vmscan_start_time, task, 0,
-					      BPF_LOCAL_STORAGE_GET_F_CREATE);
-	if (start_time_ptr)
-		*start_time_ptr = bpf_ktime_get_ns();
-	return 0;
-}
-
-SEC("tp_btf/mm_vmscan_memcg_reclaim_end")
-int BPF_PROG(vmscan_end, unsigned long nr_reclaimed)
-{
-	struct vmscan_percpu *pcpu_stat;
-	struct task_struct *current = bpf_get_current_task_btf();
-	struct cgroup *cgrp;
-	__u64 *start_time_ptr;
-	__u64 current_elapsed, cg_id;
-	__u64 end_time = bpf_ktime_get_ns();
-
-	/*
-	 * cgrp is the first parent cgroup of current that has memcg enabled in
-	 * its subtree_control, or NULL if memcg is disabled in the entire tree.
-	 * In a cgroup hierarchy like this:
-	 *                               a
-	 *                              / \
-	 *                             b   c
-	 *  If "a" has memcg enabled, while "b" doesn't, then processes in "b"
-	 *  will accumulate their stats directly to "a". This makes sure that no
-	 *  stats are lost from processes in leaf cgroups that don't have memcg
-	 *  enabled, but only exposes stats for cgroups that have memcg enabled.
-	 */
-	cgrp = task_memcg(current);
-	if (!cgrp)
+	if (pcpu_counter)
+		pcpu_counter->state += 1;
+	else if (create_percpu_attach_counter(cg_id, 1))
 		return 0;
 
-	cg_id = cgroup_id(cgrp);
-	start_time_ptr = bpf_task_storage_get(&vmscan_start_time, current, 0,
-					      BPF_LOCAL_STORAGE_GET_F_CREATE);
-	if (!start_time_ptr)
-		return 0;
-
-	current_elapsed = end_time - *start_time_ptr;
-	pcpu_stat = bpf_map_lookup_elem(&pcpu_cgroup_vmscan_elapsed,
-					&cg_id);
-	if (pcpu_stat)
-		pcpu_stat->state += current_elapsed;
-	else if (create_vmscan_percpu_elem(cg_id, current_elapsed))
-		return 0;
-
-	cgroup_rstat_updated(cgrp, bpf_get_smp_processor_id());
+	cgroup_rstat_updated(dst_cgrp, bpf_get_smp_processor_id());
 	return 0;
 }
 
 SEC("fentry/bpf_rstat_flush")
-int BPF_PROG(vmscan_flush, struct cgroup *cgrp, struct cgroup *parent, int cpu)
+int BPF_PROG(flusher, struct cgroup *cgrp, struct cgroup *parent, int cpu)
 {
-	struct vmscan_percpu *pcpu_stat;
-	struct vmscan *total_stat, *parent_stat;
+	struct percpu_attach_counter *pcpu_counter;
+	struct attach_counter *total_counter, *parent_counter;
 	__u64 cg_id = cgroup_id(cgrp);
 	__u64 parent_cg_id = parent ? cgroup_id(parent) : 0;
-	__u64 *pcpu_vmscan;
 	__u64 state;
 	__u64 delta = 0;
 
 	/* Add CPU changes on this level since the last flush */
-	pcpu_stat = bpf_map_lookup_percpu_elem(&pcpu_cgroup_vmscan_elapsed,
-					       &cg_id, cpu);
-	if (pcpu_stat) {
-		state = pcpu_stat->state;
-		delta += state - pcpu_stat->prev;
-		pcpu_stat->prev = state;
+	pcpu_counter = bpf_map_lookup_percpu_elem(&percpu_attach_counters,
+						  &cg_id, cpu);
+	if (pcpu_counter) {
+		state = pcpu_counter->state;
+		delta += state - pcpu_counter->prev;
+		pcpu_counter->prev = state;
 	}
 
-	total_stat = bpf_map_lookup_elem(&cgroup_vmscan_elapsed, &cg_id);
-	if (!total_stat) {
-		if (create_vmscan_elem(cg_id, delta, 0))
+	total_counter = bpf_map_lookup_elem(&attach_counters, &cg_id);
+	if (!total_counter) {
+		if (create_attach_counter(cg_id, delta, 0))
 			return 0;
-
 		goto update_parent;
 	}
 
 	/* Collect pending stats from subtree */
-	if (total_stat->pending) {
-		delta += total_stat->pending;
-		total_stat->pending = 0;
+	if (total_counter->pending) {
+		delta += total_counter->pending;
+		total_counter->pending = 0;
 	}
 
 	/* Propagate changes to this cgroup's total */
-	total_stat->state += delta;
+	total_counter->state += delta;
 
 update_parent:
 	/* Skip if there are no changes to propagate, or no parent */
···
 		return 0;
 
 	/* Propagate changes to cgroup's parent */
-	parent_stat = bpf_map_lookup_elem(&cgroup_vmscan_elapsed,
-					  &parent_cg_id);
-	if (parent_stat)
-		parent_stat->pending += delta;
+	parent_counter = bpf_map_lookup_elem(&attach_counters,
+					     &parent_cg_id);
+	if (parent_counter)
+		parent_counter->pending += delta;
 	else
-		create_vmscan_elem(parent_cg_id, 0, delta);
+		create_attach_counter(parent_cg_id, 0, delta);
 	return 0;
 }
 
 SEC("iter.s/cgroup")
-int BPF_PROG(dump_vmscan, struct bpf_iter_meta *meta, struct cgroup *cgrp)
+int BPF_PROG(dumper, struct bpf_iter_meta *meta, struct cgroup *cgrp)
 {
 	struct seq_file *seq = meta->seq;
-	struct vmscan *total_stat;
+	struct attach_counter *total_counter;
 	__u64 cg_id = cgrp ? cgroup_id(cgrp) : 0;
 
 	/* Do nothing for the terminal call */
···
 	/* Flush the stats to make sure we get the most updated numbers */
 	cgroup_rstat_flush(cgrp);
 
-	total_stat = bpf_map_lookup_elem(&cgroup_vmscan_elapsed, &cg_id);
-	if (!total_stat) {
-		BPF_SEQ_PRINTF(seq, "cg_id: %llu, total_vmscan_delay: 0\n",
+	total_counter = bpf_map_lookup_elem(&attach_counters, &cg_id);
+	if (!total_counter) {
+		BPF_SEQ_PRINTF(seq, "cg_id: %llu, attach_counter: 0\n",
 			       cg_id);
 	} else {
-		BPF_SEQ_PRINTF(seq, "cg_id: %llu, total_vmscan_delay: %llu\n",
-			       cg_id, total_stat->state);
+		BPF_SEQ_PRINTF(seq, "cg_id: %llu, attach_counter: %llu\n",
+			       cg_id, total_counter->state);
 	}
-
-	/*
-	 * We only dump stats for one cgroup here, so return 1 to stop
-	 * iteration after the first cgroup.
-	 */
-	return 1;
+	return 0;
 }