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@google.com>
Signed-off-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/r/20220824233117.1312810-6-haoluo@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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