Re: [PATCH bpf-next v9 5/5] selftests/bpf: add a selftest for cgroup hierarchical stats collection

[Hao Luo]

On Tue, Aug 23, 2022 at 8:01 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>
> Add a selftest that tests the whole workflow for collecting,
> aggregating (flushing), and displaying cgroup hierarchical stats.
>
> TL;DR:
> - Userspace program creates a cgroup hierarchy and induces memcg reclaim
>   in parts of it.
> - Whenever reclaim happens, vmscan_start and vmscan_end update
>   per-cgroup percpu readings, and tell rstat which (cgroup, cpu) pairs
>   have updates.
> - When userspace tries to read the stats, vmscan_dump calls rstat to flush
>   the stats, and outputs the stats in text format to userspace (similar
>   to cgroupfs stats).
> - rstat calls vmscan_flush once for every (cgroup, cpu) pair that has
>   updates, vmscan_flush aggregates cpu readings and propagates updates
>   to parents.
> - Userspace program makes sure the stats are aggregated and read
>   correctly.
>
> Detailed explanation:
> - The test loads tracing bpf programs, vmscan_start and vmscan_end, to
>   measure the latency of cgroup reclaim. Per-cgroup readings are stored in
>   percpu maps for efficiency. When a cgroup reading is updated on a cpu,
>   cgroup_rstat_updated(cgroup, cpu) is called to add the cgroup to the
>   rstat updated tree on that cpu.
>
> - A cgroup_iter program, vmscan_dump, is loaded and pinned to a file, for
>   each cgroup. Reading this file invokes the program, which calls
>   cgroup_rstat_flush(cgroup) to ask rstat to propagate the updates for all
>   cpus and cgroups that have updates in this cgroup's subtree. Afterwards,
>   the stats are exposed to the user. vmscan_dump returns 1 to terminate
>   iteration early, so that we only expose stats for one cgroup per read.
>
> - An ftrace program, vmscan_flush, is also loaded and attached to
>   bpf_rstat_flush. When rstat flushing is ongoing, vmscan_flush is invoked
>   once for each (cgroup, cpu) pair that has updates. cgroups are popped
>   from the rstat tree in a bottom-up fashion, so calls will always be
>   made for cgroups that have updates before their parents. The program
>   aggregates percpu readings to a total per-cgroup reading, and also
>   propagates them to the parent cgroup. After rstat flushing is over, all
>   cgroups will have correct updated hierarchical readings (including all
>   cpus and all their descendants).
>
> - Finally, the test creates a cgroup hierarchy and induces memcg reclaim
>   in parts of it, and makes sure that the stats collection, aggregation,
>   and reading workflow works as expected.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---

I saw this test failed on CI on s390x [0], because of using kfunc, and
on s390x, "JIT does not support calling kernel function". Is there
anything I can do about it?

[0] https://github.com/kernel-patches/bpf/runs/8003692391?check_suite_focus=true

>  .../prog_tests/cgroup_hierarchical_stats.c    | 357 ++++++++++++++++++
>  .../bpf/progs/cgroup_hierarchical_stats.c     | 226 +++++++++++
>  2 files changed, 583 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
>  create mode 100644 tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
> new file mode 100644
> index 000000000000..101a6d70b863
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
> @@ -0,0 +1,357 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Functions to manage eBPF programs attached to cgroup subsystems
> + *
> + * Copyright 2022 Google LLC.
> + */
> +#include <asm-generic/errno.h>
> +#include <errno.h>
> +#include <sys/types.h>
> +#include <sys/mount.h>
> +#include <sys/stat.h>
> +#include <unistd.h>
> +
> +#include <test_progs.h>
> +#include <bpf/libbpf.h>
> +#include <bpf/bpf.h>
> +
> +#include "cgroup_helpers.h"
> +#include "cgroup_hierarchical_stats.skel.h"
> +
> +#define PAGE_SIZE 4096
> +#define MB(x) (x << 20)
> +
> +#define BPFFS_ROOT "/sys/fs/bpf/"
> +#define BPFFS_VMSCAN BPFFS_ROOT"vmscan/"
> +
> +#define CG_ROOT_NAME "root"
> +#define CG_ROOT_ID 1
> +
> +#define CGROUP_PATH(p, n) {.path = p"/"n, .name = n}
> +
> +static struct {
> +       const char *path, *name;
> +       unsigned long long id;
> +       int fd;
> +} cgroups[] = {
> +       CGROUP_PATH("/", "test"),
> +       CGROUP_PATH("/test", "child1"),
> +       CGROUP_PATH("/test", "child2"),
> +       CGROUP_PATH("/test/child1", "child1_1"),
> +       CGROUP_PATH("/test/child1", "child1_2"),
> +       CGROUP_PATH("/test/child2", "child2_1"),
> +       CGROUP_PATH("/test/child2", "child2_2"),
> +};
> +
> +#define N_CGROUPS ARRAY_SIZE(cgroups)
> +#define N_NON_LEAF_CGROUPS 3
> +
> +static int root_cgroup_fd;
> +static bool mounted_bpffs;
> +
> +/* reads file at 'path' to 'buf', returns 0 on success. */
> +static int read_from_file(const char *path, char *buf, size_t size)
> +{
> +       int fd, len;
> +
> +       fd = open(path, O_RDONLY);
> +       if (fd < 0)
> +               return fd;
> +
> +       len = read(fd, buf, size);
> +       close(fd);
> +       if (len < 0)
> +               return len;
> +
> +       buf[len] = 0;
> +       return 0;
> +}
> +
> +/* mounts bpffs and mkdir for reading stats, returns 0 on success. */
> +static int setup_bpffs(void)
> +{
> +       int err;
> +
> +       /* Mount bpffs */
> +       err = mount("bpf", BPFFS_ROOT, "bpf", 0, NULL);
> +       mounted_bpffs = !err;
> +       if (ASSERT_FALSE(err && errno != EBUSY, "mount"))
> +               return err;
> +
> +       /* Create a directory to contain stat files in bpffs */
> +       err = mkdir(BPFFS_VMSCAN, 0755);
> +       if (!ASSERT_OK(err, "mkdir"))
> +               return err;
> +
> +       return 0;
> +}
> +
> +static void cleanup_bpffs(void)
> +{
> +       /* Remove created directory in bpffs */
> +       ASSERT_OK(rmdir(BPFFS_VMSCAN), "rmdir "BPFFS_VMSCAN);
> +
> +       /* Unmount bpffs, if it wasn't already mounted when we started */
> +       if (mounted_bpffs)
> +               return;
> +
> +       ASSERT_OK(umount(BPFFS_ROOT), "unmount bpffs");
> +}
> +
> +/* sets up cgroups, returns 0 on success. */
> +static int setup_cgroups(void)
> +{
> +       int i, fd, err;
> +
> +       err = setup_cgroup_environment();
> +       if (!ASSERT_OK(err, "setup_cgroup_environment"))
> +               return err;
> +
> +       root_cgroup_fd = get_root_cgroup();
> +       if (!ASSERT_GE(root_cgroup_fd, 0, "get_root_cgroup"))
> +               return root_cgroup_fd;
> +
> +       for (i = 0; i < N_CGROUPS; i++) {
> +               fd = create_and_get_cgroup(cgroups[i].path);
> +               if (!ASSERT_GE(fd, 0, "create_and_get_cgroup"))
> +                       return fd;
> +
> +               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;
> +}
> +
> +static void cleanup_cgroups(void)
> +{
> +       close(root_cgroup_fd);
> +       for (int i = 0; i < N_CGROUPS; i++)
> +               close(cgroups[i].fd);
> +       cleanup_cgroup_environment();
> +}
> +
> +/* Sets up cgroup hiearchary, returns 0 on success. */
> +static int setup_hierarchy(void)
> +{
> +       return setup_bpffs() || setup_cgroups();
> +}
> +
> +static void destroy_hierarchy(void)
> +{
> +       cleanup_cgroups();
> +       cleanup_bpffs();
> +}
> +
> +static int reclaimer(const char *cgroup_path, size_t size)
> +{
> +       static char size_buf[128];
> +       char *buf, *ptr;
> +       int err;
> +
> +       /* 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.
> +        */
> +       for (i = N_NON_LEAF_CGROUPS; i < N_CGROUPS; i++) {
> +               pid_t pid;
> +
> +               /* Create reclaimer child */
> +               pid = fork();
> +               if (pid == 0) {
> +                       status = reclaimer(cgroups[i].path, MB(5));
> +                       exit(status);
> +               }
> +
> +               /* 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)
> +{
> +       unsigned long long vmscan = 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);
> +       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");
> +
> +       /* 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;
> +}
> +
> +static void check_vmscan_stats(void)
> +{
> +       unsigned long long vmscan_readings[N_CGROUPS], vmscan_root;
> +       int i;
> +
> +       for (i = 0; i < N_CGROUPS; i++) {
> +               vmscan_readings[i] = get_cgroup_vmscan_delay(cgroups[i].id,
> +                                                            cgroups[i].name);
> +       }
> +
> +       /* Read stats for root too */
> +       vmscan_root = get_cgroup_vmscan_delay(CG_ROOT_ID, CG_ROOT_NAME);
> +
> +       /* Check that child1 == child1_1 + child1_2 */
> +       ASSERT_EQ(vmscan_readings[1], vmscan_readings[3] + vmscan_readings[4],
> +                 "child1_vmscan");
> +       /* Check that child2 == child2_1 + child2_2 */
> +       ASSERT_EQ(vmscan_readings[2], vmscan_readings[5] + vmscan_readings[6],
> +                 "child2_vmscan");
> +       /* Check that test == child1 + child2 */
> +       ASSERT_EQ(vmscan_readings[0], vmscan_readings[1] + vmscan_readings[2],
> +                 "test_vmscan");
> +       /* Check that root >= test */
> +       ASSERT_GE(vmscan_root, vmscan_readings[1], "root_vmscan");
> +}
> +
> +/* Creates iter link and pins in bpffs, returns 0 on success, -errno on failure.
> + */
> +static int setup_cgroup_iter(struct cgroup_hierarchical_stats *obj,
> +                            int cgroup_fd, const char *file_name)
> +{
> +       DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
> +       union bpf_iter_link_info linfo = {};
> +       struct bpf_link *link;
> +       static char path[128];
> +       int err;
> +
> +       /*
> +        * Create an iter link, parameterized by cgroup_fd. We only want to
> +        * traverse one cgroup, so set the traversal order to "self".
> +        */
> +       linfo.cgroup.cgroup_fd = cgroup_fd;
> +       linfo.cgroup.order = BPF_ITER_SELF_ONLY;
> +       opts.link_info = &linfo;
> +       opts.link_info_len = sizeof(linfo);
> +       link = bpf_program__attach_iter(obj->progs.dump_vmscan, &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);
> +       err = bpf_link__pin(link, path);
> +       ASSERT_OK(err, "pin cgroup_iter");
> +
> +       /* Remove the link, leaving only the ref held by the pinned file */
> +       bpf_link__destroy(link);
> +       return err;
> +}
> +
> +/* Sets up programs for collecting stats, returns 0 on success. */
> +static int setup_progs(struct cgroup_hierarchical_stats **skel)
> +{
> +       int i, err;
> +
> +       *skel = cgroup_hierarchical_stats__open_and_load();
> +       if (!ASSERT_OK_PTR(*skel, "open_and_load"))
> +               return 1;
> +
> +       /* Attach cgroup_iter program that will dump the stats to cgroups */
> +       for (i = 0; i < N_CGROUPS; i++) {
> +               err = setup_cgroup_iter(*skel, cgroups[i].fd, cgroups[i].name);
> +               if (!ASSERT_OK(err, "setup_cgroup_iter"))
> +                       return err;
> +       }
> +
> +       /* Also dump stats for root */
> +       err = setup_cgroup_iter(*skel, root_cgroup_fd, CG_ROOT_NAME);
> +       if (!ASSERT_OK(err, "setup_cgroup_iter"))
> +               return err;
> +
> +       bpf_program__set_autoattach((*skel)->progs.dump_vmscan, false);
> +       err = cgroup_hierarchical_stats__attach(*skel);
> +       if (!ASSERT_OK(err, "attach"))
> +               return err;
> +
> +       return 0;
> +}
> +
> +static void destroy_progs(struct cgroup_hierarchical_stats *skel)
> +{
> +       static char path[128];
> +       int i;
> +
> +       for (i = 0; i < N_CGROUPS; i++) {
> +               /* Delete files in bpffs that cgroup_iters are pinned in */
> +               snprintf(path, 128, "%s%s", BPFFS_VMSCAN,
> +                        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);
> +       ASSERT_OK(remove(path), "remove cgroup_iter root pin");
> +       cgroup_hierarchical_stats__destroy(skel);
> +}
> +
> +void test_cgroup_hierarchical_stats(void)
> +{
> +       struct cgroup_hierarchical_stats *skel = NULL;
> +
> +       if (setup_hierarchy())
> +               goto hierarchy_cleanup;
> +       if (setup_progs(&skel))
> +               goto cleanup;
> +       if (induce_vmscan())
> +               goto cleanup;
> +       check_vmscan_stats();
> +cleanup:
> +       destroy_progs(skel);
> +hierarchy_cleanup:
> +       destroy_hierarchy();
> +}
> diff --git a/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
> new file mode 100644
> index 000000000000..8ab4253a1592
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
> @@ -0,0 +1,226 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Functions to manage eBPF programs attached to cgroup subsystems
> + *
> + * Copyright 2022 Google LLC.
> + */
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include <bpf/bpf_core_read.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 {
> +       /* Previous percpu state, to figure out if we have new updates */
> +       __u64 prev;
> +       /* Current percpu state */
> +       __u64 state;
> +};
> +
> +struct vmscan {
> +       /* State propagated through children, pending aggregation */
> +       __u64 pending;
> +       /* Total state, including all cpus and all children */
> +       __u64 state;
> +};
> +
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERCPU_HASH);
> +       __uint(max_entries, 100);
> +       __type(key, __u64);
> +       __type(value, struct vmscan_percpu);
> +} pcpu_cgroup_vmscan_elapsed SEC(".maps");
> +
> +struct {
> +       __uint(type, BPF_MAP_TYPE_HASH);
> +       __uint(max_entries, 100);
> +       __type(key, __u64);
> +       __type(value, struct vmscan);
> +} cgroup_vmscan_elapsed 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)
> +{
> +       struct vmscan_percpu pcpu_init = {.state = state, .prev = 0};
> +
> +       return bpf_map_update_elem(&pcpu_cgroup_vmscan_elapsed, &cg_id,
> +                                  &pcpu_init, BPF_NOEXIST);
> +}
> +
> +static int create_vmscan_elem(__u64 cg_id, __u64 state, __u64 pending)
> +{
> +       struct vmscan init = {.state = state, .pending = pending};
> +
> +       return bpf_map_update_elem(&cgroup_vmscan_elapsed, &cg_id,
> +                                  &init, BPF_NOEXIST);
> +}
> +
> +SEC("tp_btf/mm_vmscan_memcg_reclaim_begin")
> +int BPF_PROG(vmscan_start, int order, gfp_t gfp_flags)
> +{
> +       struct task_struct *task = bpf_get_current_task_btf();
> +       __u64 *start_time_ptr;
> +
> +       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)
> +               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());
> +       return 0;
> +}
> +
> +SEC("fentry/bpf_rstat_flush")
> +int BPF_PROG(vmscan_flush, struct cgroup *cgrp, struct cgroup *parent, int cpu)
> +{
> +       struct vmscan_percpu *pcpu_stat;
> +       struct vmscan *total_stat, *parent_stat;
> +       __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;
> +       }
> +
> +       total_stat = bpf_map_lookup_elem(&cgroup_vmscan_elapsed, &cg_id);
> +       if (!total_stat) {
> +               if (create_vmscan_elem(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;
> +       }
> +
> +       /* Propagate changes to this cgroup's total */
> +       total_stat->state += delta;
> +
> +update_parent:
> +       /* Skip if there are no changes to propagate, or no parent */
> +       if (!delta || !parent_cg_id)
> +               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;
> +       else
> +               create_vmscan_elem(parent_cg_id, 0, delta);
> +       return 0;
> +}
> +
> +SEC("iter.s/cgroup")
> +int BPF_PROG(dump_vmscan, struct bpf_iter_meta *meta, struct cgroup *cgrp)
> +{
> +       struct seq_file *seq = meta->seq;
> +       struct vmscan *total_stat;
> +       __u64 cg_id = cgrp ? cgroup_id(cgrp) : 0;
> +
> +       /* Do nothing for the terminal call */
> +       if (!cg_id)
> +               return 1;
> +
> +       /* 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",
> +                              cg_id);
> +       } else {
> +               BPF_SEQ_PRINTF(seq, "cg_id: %llu, total_vmscan_delay: %llu\n",
> +                              cg_id, total_stat->state);
> +       }
> +
> +       /*
> +        * We only dump stats for one cgroup here, so return 1 to stop
> +        * iteration after the first cgroup.
> +        */
> +       return 1;
> +}
> --
> 2.37.1.595.g718a3a8f04-goog
>