netdev - Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup hierarchical stats collection

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Date:   Fri, 3 Jun 2022 12:52:27 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Koutný <mkoutny@...e.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>, Yonghong Song <yhs@...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

Thanks for taking a look at this!

On Fri, Jun 3, 2022 at 9:23 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Fri, May 20, 2022 at 01:21:33AM +0000, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > +#define CGROUP_PATH(p, n) {.name = #n, .path = #p"/"#n}
> > +
> > +static struct {
> > +     const char *name, *path;
>
> Please unify the order of path and name with the macro (slightly
> confusing ;-).

Totally agree, will do.

>
> > +SEC("tp_btf/mm_vmscan_memcg_reclaim_end")
> > +int BPF_PROG(vmscan_end, struct lruvec *lruvec, struct scan_control *sc)
> > +{
> > [...]
> > +     struct cgroup *cgrp = task_memcg(current);
> > [...]
> > +     /* cgrp may not have memory controller enabled */
> > +     if (!cgrp)
> > +             return 0;
>
> Yes, the controller may not be enabled (for a cgroup).
> Just noting that the task_memcg() implementation will fall back to
> root_mem_cgroup in such a case (or nearest ancestor), you may want to
> use cgroup_ss_mask() for proper detection.

Good catch. I get confused between cgrp->subsys and
task->cgroups->subsys sometimes because of different fallback
behavior. IIUC cgrp->subsys should have NULL if the memory controller
is not enabled (no nearest ancestor fallback), and hence I can use
memory_subsys_enabled() that I defined just above task_memcg() to test
for this (I have no idea why I am not already using it here). Is my
understanding correct?

>
> Regards,
> Michal