Date:   Fri, 20 May 2022 09:18:51 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup
 hierarchical stats collection

On Fri, May 20, 2022 at 9:09 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 5/19/22 6:21 PM, Yosry Ahmed wrote:
> > Add a selftest that tests the whole workflow for collecting,
> > aggregating, and display cgroup hierarchical stats.
> >
> > TL;DR:
> > - 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.
> > - rstat calls vmscan_flush once for every (cgroup, cpu) pair that has
> >    updates, vmscan_flush aggregates cpu readings and propagates updates
> >    to parents.
> >
> > Detailed explanation:
> > - The test loads tracing bpf programs, vmscan_start and vmscan_end, to
> >    measure the latency of cgroup reclaim. Per-cgroup ratings 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.
> >
> > - 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).
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > ---
> >   .../test_cgroup_hierarchical_stats.c          | 339 ++++++++++++++++++
> >   tools/testing/selftests/bpf/progs/bpf_iter.h  |   7 +
> >   .../selftests/bpf/progs/cgroup_vmscan.c       | 221 ++++++++++++
> >   3 files changed, 567 insertions(+)
> >   create mode 100644 tools/testing/selftests/bpf/prog_tests/test_cgroup_hierarchical_stats.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/cgroup_vmscan.c
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/test_cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/prog_tests/test_cgroup_hierarchical_stats.c
> > new file mode 100644
> > index 000000000000..e560c1f6291f
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/test_cgroup_hierarchical_stats.c
> > @@ -0,0 +1,339 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Functions to manage eBPF programs attached to cgroup subsystems
> > + *
> > + * Copyright 2022 Google LLC.
> > + */
> > +#include <errno.h>
> > +#include <sys/types.h>
> > +#include <sys/mount.h>
> > +#include <sys/stat.h>
> > +#include <unistd.h>
> > +
> > +#include <bpf/libbpf.h>
> > +#include <bpf/bpf.h>
> > +#include <test_progs.h>
> > +
> > +#include "cgroup_helpers.h"
> > +#include "cgroup_vmscan.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) {.name = #n, .path = #p"/"#n}
> > +
> > +static struct {
> > +     const char *name, *path;
> > +     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
> > +
> > +bool mounted_bpffs;
> > +static int duration;
> > +
> > +static int read_from_file(const char *path, char *buf, size_t size)
> > +{
> > +     int fd, len;
> > +
> > +     fd = open(path, O_RDONLY);
> > +     if (fd < 0) {
> > +             log_err("Open %s", path);
> > +             return -errno;
> > +     }
> > +     len = read(fd, buf, size);
> > +     if (len < 0)
> > +             log_err("Read %s", path);
> > +     else
> > +             buf[len] = 0;
> > +     close(fd);
> > +     return len < 0 ? -errno : 0;
> > +}
> > +
> > +static int setup_bpffs(void)
> > +{
> > +     int err;
> > +
> > +     /* Mount bpffs */
> > +     err = mount("bpf", BPFFS_ROOT, "bpf", 0, NULL);
> > +     mounted_bpffs = !err;
> > +     if (CHECK(err && errno != EBUSY, "mount bpffs",
>
> Please use ASSERT_* macros instead of CHECK.
> There are similar instances below as well.

CHECK is more flexible in providing a parameterized failure message,
but I guess we ideally shouldn't see those a lot anyway. Will change
them to ASSERTs in the next version.

>
> > +           "failed to mount bpffs at %s (%s)\n", BPFFS_ROOT,
> > +           strerror(errno)))
> > +             return err;
> > +
> > +     /* Create a directory to contain stat files in bpffs */
> > +     err = mkdir(BPFFS_VMSCAN, 0755);
> > +     CHECK(err, "mkdir bpffs", "failed to mkdir %s (%s)\n",
> > +           BPFFS_VMSCAN, strerror(errno));
> > +     return err;
> > +}
> > +
> > +static void cleanup_bpffs(void)
> > +{
> > +     /* Remove created directory in bpffs */
> > +     CHECK(rmdir(BPFFS_VMSCAN), "rmdir", "failed to rmdir %s (%s)\n",
> > +           BPFFS_VMSCAN, strerror(errno));
> > +
> > +     /* Unmount bpffs, if it wasn't already mounted when we started */
> > +     if (mounted_bpffs)
> > +             return;
> > +     CHECK(umount(BPFFS_ROOT), "umount", "failed to unmount bpffs (%s)\n",
> > +           strerror(errno));
> > +}
> > +
> > +static int setup_cgroups(void)
> > +{
> > +     int i, err;
> > +
> > +     err = setup_cgroup_environment();
> > +     if (CHECK(err, "setup_cgroup_environment", "failed: %d\n", err))
> > +             return err;
> > +
> > +     for (i = 0; i < N_CGROUPS; i++) {
> > +             int fd;
>
> You can put this to the top declaration 'int i, err'.

Will do in the next version. I thought declaring variables in the
innermost block that uses them is preferable.

>
> > +
> > +             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);
> > +             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)
> > +{
> > +     for (int i = 0; i < N_CGROUPS; i++)
> > +             close(cgroups[i].fd);
> > +     cleanup_cgroup_environment();
> > +}
> > +
> > +
> > +static int setup_hierarchy(void)
> > +{
> > +     return setup_bpffs() || setup_cgroups();
> > +}
> > +
> > +static void destroy_hierarchy(void)
> > +{
> > +     cleanup_cgroups();
> > +     cleanup_bpffs();
> > +}
> > +
> [...]
> > +
> > +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 = cgroup_id(cgrp);
> > +
> > +     /* 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_printk("error finding stats for cgroup %llu\n", cg_id);
> > +             BPF_SEQ_PRINTF(seq, "cg_id: -1, total_vmscan_delay: -1\n");
> > +             return 0;
> > +     }
> > +     BPF_SEQ_PRINTF(seq, "cg_id: %llu, total_vmscan_delay: %llu\n",
> > +                    cg_id, total_stat->state);
> > +     return 0;
> > +}
> > +
>
> Empty line here.

Will remove this in the next version.
Thanks for taking a look at this!

>

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