Re: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter

[Hao Luo]

On Wed, Jul 27, 2022 at 11:56 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 7/22/22 1:33 PM, Hao Luo wrote:
> > On Fri, Jul 22, 2022 at 11:36 AM Kumar Kartikeya Dwivedi
> > <memxor@xxxxxxxxx> wrote:
> >>
> >> On Fri, 22 Jul 2022 at 19:52, Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
<...>
> >>> +
> >>> +static int __cgroup_iter_seq_show(struct seq_file *seq,
> >>> +                                 struct cgroup_subsys_state *css, int in_stop);
> >>> +
> >>> +static void cgroup_iter_seq_stop(struct seq_file *seq, void *v)
> >>> +{
> >>> +       /* pass NULL to the prog for post-processing */
> >>> +       if (!v)
> >>> +               __cgroup_iter_seq_show(seq, NULL, true);
> >>> +       mutex_unlock(&cgroup_mutex);
> >>
> >> I'm just curious, but would it be a good optimization (maybe in a
> >> follow up) to move this mutex_unlock before the check on v? That
> >> allows you to store/buffer some info you want to print as a compressed
> >> struct in a map, then write the full text to the seq_file outside the
> >> cgroup_mutex lock in the post-processing invocation.
> >>
> >> It probably also allows you to walk the whole hierarchy, if one
> >> doesn't want to run into seq_file buffer limit (or it can decide what
> >> to print within the limit in the post processing invocation), or it
> >> can use some out of band way (ringbuf, hashmap, etc.) to send the data
> >> to userspace. But all of this can happen without holding cgroup_mutex
> >> lock.
> >
> > Thanks Kumar.
> >
> > It sounds like an idea, but the key thing is not about moving
> > cgroup_mutex unlock before the check IMHO. The user can achieve
> > compression using the current infra. Compression could actually be
> > done in the bpf program. user can define and output binary content and
> > implement a userspace library to parse/decompress when reading out the
> > data.
>
> Right mutex_unlock() can be moved to the beginning of the
> function since the cgroup is not used in
>    __cgroup_iter_seq_show(seq, NULL, true)

Ok. Will do.