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Subject[PATCH bpf-next v1 0/5] bpf: rstat: cgroup hierarchical stats
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 and
propagate them through the cgroup hierarchy.

* Background on rstat (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

- 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.

Patches in this series are based off two patches in the mailing list:
- bpf/btf: also allow kfunc in tracing and syscall programs
- btf: Add a new kfunc set which allows to mark a function to be

Both by Benjamin Tissoires, from different versions of his HID patch
series (the second patch seems to have been dropped in the last

Patches in this series are organized as follows:
* The first patch adds a hook point, bpf_rstat_flush(), that is called
during rstat flushing. This allows bpf fentry programs to attach to it
to be called during rstat flushing (effectively registering themselves
as rstat flush callbacks).

* The second patch adds cgroup_rstat_updated() and cgorup_rstat_flush()
kfuncs, to allow bpf stat collectors and readers to communicate with rstat.

* The third patch is actually v2 of a previously submitted patch [1]
by Hao Luo. We agreed that it fits better as a part of this series. It
introduces cgroup_iter programs that can dump stats for cgroups to
v1 - > v2:
- Getting the cgroup's reference at the time at attaching, instead of
at the time when iterating. (Yonghong) (context [1])
- Remove .init_seq_private and .fini_seq_private callbacks for
cgroup_iter. They are not needed now. (Yonghong)

* The fourth patch extends bpf selftests cgroup helpers, as necessary
for the following patch.

* The fifth patch is a selftest that demonstrates the entire workflow.
It includes programs that collect, aggregate, and dump per-cgroup stats
by fully integrating with the rstat framework.


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

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
- Move rstat helper protos from bpf_base_func_proto() to
- rstat helpers argument (cgroup pointer) is now ARG_PTR_TO_BTF_ID, not
- 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.

Hao Luo (1):
bpf: Introduce cgroup iter

Yosry Ahmed (4):
cgroup: bpf: add a hook for bpf progs to attach to rstat flushing
cgroup: bpf: add cgroup_rstat_updated() and cgroup_rstat_flush()
selftests/bpf: extend cgroup helpers
bpf: add a selftest for cgroup hierarchical stats collection

include/linux/bpf.h | 2 +
include/uapi/linux/bpf.h | 6 +
kernel/bpf/Makefile | 3 +
kernel/bpf/cgroup_iter.c | 148 ++++++++
kernel/cgroup/rstat.c | 40 +++
tools/include/uapi/linux/bpf.h | 6 +
tools/testing/selftests/bpf/cgroup_helpers.c | 159 +++++---
tools/testing/selftests/bpf/cgroup_helpers.h | 14 +-
.../test_cgroup_hierarchical_stats.c | 339 ++++++++++++++++++
tools/testing/selftests/bpf/progs/bpf_iter.h | 7 +
.../selftests/bpf/progs/cgroup_vmscan.c | 221 ++++++++++++
11 files changed, 899 insertions(+), 46 deletions(-)
create mode 100644 kernel/bpf/cgroup_iter.c
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


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