Re: [PATCH bpf-next v5 0/8] bpf: rstat: cgroup hierarchical stats

[Yonghong Song <yhs@fb.com>]

On 7/22/22 10:48 AM, Yosry Ahmed wrote:
> This patch series allows for using bpf to collect hierarchical cgroup
> stats efficiently by integrating with the rstat framework. The rstat
> framework provides an efficient way to collect cgroup stats percpu and
> propagate them through the cgroup hierarchy.
> 
> The stats are exposed to userspace in textual form by reading files in
> bpffs, similar to cgroupfs stats by using a cgroup_iter program.
> cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:
> - walking a cgroup's descendants in pre-order.
> - walking a cgroup's descendants in post-order.
> - walking a cgroup's ancestors.
> 
> When attaching cgroup_iter, one needs to set a cgroup to the iter_link
> created from attaching. This cgroup is passed as a file descriptor and
> serves as the starting point of the walk.
> 
> One can also terminate the walk early by returning 1 from the iter
> program.
> 
> Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
> program is called with cgroup_mutex held.
> 
> ** Background on rstat for stats collection **
> (I am using a subscriber analogy that is not commonly used)
> 
> The rstat framework maintains a tree of cgroups that have updates and
> which cpus have updates. A subscriber to the rstat framework maintains
> their own stats. The framework is used to tell the subscriber when
> and what to flush, for the most efficient stats propagation. The
> workflow is as follows:
> 
> - When a subscriber updates a cgroup on a cpu, it informs the rstat
>    framework by calling cgroup_rstat_updated(cgrp, cpu).
> 
> - When a subscriber wants to read some stats for a cgroup, it asks
>    the rstat framework to initiate a stats flush (propagation) by calling
>    cgroup_rstat_flush(cgrp).
> 
> - When the rstat framework initiates a flush, it makes callbacks to
>    subscribers to aggregate stats on cpus that have updates, and
>    propagate updates to their parent.
> 
> Currently, the main subscribers to the rstat framework are cgroup
> subsystems (e.g. memory, block). This patch series allow bpf programs to
> become subscribers as well.
> 
> This patch series includes a resend of a patch from the mailing list by
> Benjamin Tissoires to support sleepable kfuncs [1], modified to use the
> new kfunc flags infrastructure.
> 
> Patches in this series are organized as follows:
> * Patch 1 is the updated sleepable kfuncs patch.
> * Patch 2 enables the use of cgroup_get_from_file() in cgroup1.
>    This is useful because it enables cgroup_iter to work with cgroup1, and
>    allows the entire stat collection workflow to be cgroup1-compatible.
> * Patches 3-5 introduce cgroup_iter prog, and a selftest.
> * Patches 6-8 allow bpf programs to integrate with rstat by adding the
>    necessary hook points and kfunc. A comprehensive selftest that
>    demonstrates the entire workflow for using bpf and rstat to
>    efficiently collect and output cgroup stats is added.
> 
> ---
> Changelog:
> 
> v4 -> v5:
> - Rebased on top of new kfunc flags infrastructure, updated patch 1 and
>    patch 6 accordingly.
> - Added docs for sleepable kfuncs.
> 
> v3 -> v4:
> - cgroup_iter:
>    * reorder fields in bpf_link_info to avoid break uapi (Yonghong)
>    * comment the behavior when cgroup_fd=0 (Yonghong)
>    * comment on the limit of number of cgroups supported by cgroup_iter.
>      (Yonghong)
> - cgroup_hierarchical_stats selftest:
>    * Do not return -1 if stats are not found (causes overflow in userspace).
>    * Check if child process failed to join cgroup.
>    * Make buf and path arrays in get_cgroup_vmscan_delay() static.
>    * Increase the test map sizes to accomodate cgroups that are not
>      created by the test.
> 
> v2 -> v3:
> - cgroup_iter:
>    * Added conditional compilation of cgroup_iter.c in kernel/bpf/Makefile
>      (kernel test) and dropped the !CONFIG_CGROUP patch.
>    * Added validation of traversal_order when attaching (Yonghong).
>    * Fixed previous wording "two modes" to "three modes" (Yonghong).
>    * Fixed the btf_dump selftest broken by this patch (Yonghong).
>    * Fixed ctx_arg_info[0] to use "PTR_TO_BTF_ID_OR_NULL" instead of
>      "PTR_TO_BTF_ID", because the "cgroup" pointer passed to iter prog can
>       be null.
> - Use __diag_push to eliminate __weak noinline warning in
>    bpf_rstat_flush().
> - cgroup_hierarchical_stats selftest:
>    * Added write_cgroup_file_parent() helper.
>    * Added error handling for failed map updates.
>    * Added null check for cgroup in vmscan_flush.
>    * Fixed the signature of vmscan_[start/end].
>    * Correctly return error code when attaching trace programs fail.
>    * Make sure all links are destroyed correctly and not leaking in
>      cgroup_hierarchical_stats selftest.
>    * Use memory.reclaim instead of memory.high as a more reliable way to
>      invoke reclaim.
>    * Eliminated sleeps, the test now runs faster.
> 
> v1 -> v2:
> - Redesign of cgroup_iter from v1, based on Alexei's idea [2]:
>    * supports walking cgroup subtree.
>    * supports walking ancestors of a cgroup. (Andrii)
>    * supports terminating the walk early.
>    * uses fd instead of cgroup_id as parameter for iter_link. Using fd is
>      a convention in bpf.
>    * gets cgroup's ref at attach time and deref at detach.
>    * brought back cgroup1 support for cgroup_iter.
> - Squashed the patches adding the rstat flush hook points and kfuncs
>    (Tejun).
> - Added a comment explaining why bpf_rstat_flush() needs to be weak
>    (Tejun).
> - Updated the final selftest with the new cgroup_iter design.
> - Changed CHECKs in the selftest with ASSERTs (Yonghong, Andrii).
> - Removed empty line at the end of the selftest (Yonghong).
> - Renamed test files to cgroup_hierarchical_stats.c.
> - Reordered CGROUP_PATH params order to match struct declaration
>    in the selftest (Michal).
> - Removed memory_subsys_enabled() and made sure memcg controller
>    enablement checks make sense and are documented (Michal).
> 
> RFC v2 -> v1:
> - Instead of introducing a new program type for rstat flushing, add an
>    empty hook point, bpf_rstat_flush(), and use fentry bpf programs to
>    attach to it and flush bpf stats.
> - Instead of using helpers, use kfuncs for rstat functions.
> - These changes simplify the patchset greatly, with minimal changes to
>    uapi.
> 
> RFC v1 -> RFC v2:
> - Instead of rstat flush programs attach to subsystems, they now attach
>    to rstat (global flushers, not per-subsystem), based on discussions
>    with Tejun. The first patch is entirely rewritten.
> - Pass cgroup pointers to rstat flushers instead of cgroup ids. This is
>    much more flexibility and less likely to need a uapi update later.
> - rstat helpers are now only defined if CGROUP_CONFIG.
> - Most of the code is now only defined if CGROUP_CONFIG and
>    CONFIG_BPF_SYSCALL.
> - Move rstat helper protos from bpf_base_func_proto() to
>    tracing_prog_func_proto().
> - rstat helpers argument (cgroup pointer) is now ARG_PTR_TO_BTF_ID, not
>    ARG_ANYTHING.
> - Rewrote the selftest to use the cgroup helpers.
> - Dropped bpf_map_lookup_percpu_elem (already added by Feng).
> - Dropped patch to support cgroup v1 for cgroup_iter.
> - Dropped patch to define some cgroup_put() when !CONFIG_CGROUP. The
>    code that calls it is no longer compiled when !CONFIG_CGROUP.
> 
> cgroup_iter was originally introduced in a different patch series[3].
> Hao and I agreed that it fits better as part of this series.
> RFC v1 of this patch series had the following changes from [3]:
> - Getting the cgroup's reference at the time at attaching, instead of
>    at the time when iterating. (Yonghong)
> - Remove .init_seq_private and .fini_seq_private callbacks for
>    cgroup_iter. They are not needed now. (Yonghong)
> 
> [1] https://lore.kernel.org/bpf/20220421140740.459558-5-benjamin.tissoires@redhat.com/
> [2] https://lore.kernel.org/bpf/20220520221919.jnqgv52k4ajlgzcl@MBP-98dd607d3435.dhcp.thefacebook.com/
> [3] https://lore.kernel.org/lkml/20220225234339.2386398-9-haoluo@google.com/
> ---
> 
> Benjamin Tissoires (1):
>    btf: Add a new kfunc flag which allows to mark a function to be
>      sleepable
> 
> Hao Luo (3):
>    bpf, iter: Fix the condition on p when calling stop.
>    bpf: Introduce cgroup iter
>    selftests/bpf: Test cgroup_iter.
> 
> Yosry Ahmed (4):
>    cgroup: enable cgroup_get_from_file() on cgroup1
>    cgroup: bpf: enable bpf programs to integrate with rstat
>    selftests/bpf: extend cgroup helpers
>    bpf: add a selftest for cgroup hierarchical stats collection

It would be great cgroup maintainers (Tejun?) can look at the above two 
cgroup related patches.

> 
>   Documentation/bpf/kfuncs.rst                  |   6 +
>   include/linux/bpf.h                           |   8 +
>   include/linux/btf.h                           |   1 +
>   include/uapi/linux/bpf.h                      |  30 ++
>   kernel/bpf/Makefile                           |   3 +
>   kernel/bpf/bpf_iter.c                         |   5 +
>   kernel/bpf/btf.c                              |   9 +
>   kernel/bpf/cgroup_iter.c                      | 252 ++++++++++++
>   kernel/cgroup/cgroup.c                        |   5 -
>   kernel/cgroup/rstat.c                         |  49 +++
>   tools/include/uapi/linux/bpf.h                |  30 ++
>   tools/testing/selftests/bpf/cgroup_helpers.c  | 201 ++++++++--
>   tools/testing/selftests/bpf/cgroup_helpers.h  |  19 +-
>   .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
>   .../prog_tests/cgroup_hierarchical_stats.c    | 364 ++++++++++++++++++
>   .../selftests/bpf/prog_tests/cgroup_iter.c    | 190 +++++++++
>   tools/testing/selftests/bpf/progs/bpf_iter.h  |   7 +
>   .../bpf/progs/cgroup_hierarchical_stats.c     | 239 ++++++++++++
>   .../testing/selftests/bpf/progs/cgroup_iter.c |  39 ++
>   19 files changed, 1407 insertions(+), 54 deletions(-)
>   create mode 100644 kernel/bpf/cgroup_iter.c
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_iter.c
>   create mode 100644 tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
>   create mode 100644 tools/testing/selftests/bpf/progs/cgroup_iter.c
>