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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Fri, 3 Jun 2022 12:52:27 -0700
Message-ID: <CAJD7tkYwU5dW9Oof+pC81R9Bi-F=-EuiXpTn+HDeqbhTOTCcuw@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup
 hierarchical stats collection
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
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Thanks for taking a look at this!

On Fri, Jun 3, 2022 at 9:23 AM Michal Koutn=C3=BD <mkoutny@suse.com> wrote:
>
> On Fri, May 20, 2022 at 01:21:33AM +0000, Yosry Ahmed <yosryahmed@google.=
com> wrote:
> > +#define CGROUP_PATH(p, n) {.name =3D #n, .path =3D #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 *s=
c)
> > +{
> > [...]
> > +     struct cgroup *cgrp =3D 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