Re: [PATCH bpf-next v2 8/8] bpf: add a selftest for cgroup hierarchical stats collection

[Yonghong Song]

On 6/28/22 12:14 AM, Yosry Ahmed wrote:
> On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song <yhs@xxxxxx> wrote:
> > > On 6/10/22 12:44 PM, Yosry Ahmed wrote:
> > > > Add a selftest that tests the whole workflow for collecting,
> > > > aggregating (flushing), and displaying 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, 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.
> > > >
> > > > 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. 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).
> > > >
> > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > >
> > > There are a selftest failure with test:
> > >
> > > get_cgroup_vmscan_delay:PASS:output format 0 nsec
> > > get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
> > > get_cgroup_vmscan_delay:PASS:vmscan_reading 0 nsec
> > > get_cgroup_vmscan_delay:PASS:read cgroup_iter 0 nsec
> > > get_cgroup_vmscan_delay:PASS:output format 0 nsec
> > > get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
> > > get_cgroup_vmscan_delay:FAIL:vmscan_reading unexpected vmscan_reading:
> > > actual 0 <= expected 0
> > > check_vmscan_stats:FAIL:child1_vmscan unexpected child1_vmscan: actual
> > > 781874 != expected 382092
> > > check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan: actual
> > > -1 != expected -2
> > > check_vmscan_stats:FAIL:test_vmscan unexpected test_vmscan: actual
> > > 781874 != expected 781873
> > > check_vmscan_stats:FAIL:root_vmscan unexpected root_vmscan: actual 0 <
> > > expected 781874
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter pin 0 nsec
> > > destroy_progs:PASS:remove cgroup_iter root pin 0 nsec
> > > cleanup_bpffs:PASS:rmdir /sys/fs/bpf/vmscan/ 0 nsec
> > > #33      cgroup_hierarchical_stats:FAIL
> >
> > The test is passing on my setup. I am trying to figure out if there is
> > something outside the setup done by the test that can cause the test
> > to fail.
>
> I can't reproduce the failure on my machine. It seems like for some
> reason reclaim is not invoked in one of the test cgroups which results
> in the expected stats not being there. I have a few suspicions as to
> what might cause this but I am not sure.
>
> If you have the capacity, do you mind re-running the test with the
> attached diff1.patch? (and maybe diff2.patch if that fails, this will
> cause OOMs in the test cgroup, you might see some process killed
> warnings).

The patch doesn't help. Still failed.

get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
get_cgroup_vmscan_delay:FAIL:vmscan_reading unexpected vmscan_reading: 
actual 0 <= expected 0
check_vmscan_stats:FAIL:child1_vmscan unexpected child1_vmscan: actual 
676612 != expected 339142
check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan: actual 
-1 != expected -2
check_vmscan_stats:FAIL:test_vmscan unexpected test_vmscan: actual 
676612 != expected 676611
check_vmscan_stats:FAIL:root_vmscan unexpected root_vmscan: actual 0 < 
expected 676612
destroy_progs:PASS:remove cgroup_iter pin 0 nsec
destroy_progs:PASS:remove cgroup_iter pin 0 nsec


> Thanks!
>
>
> > > Also an existing test also failed.
[...]