Re: [PATCH bpf-next v2 4/8] bpf: Introduce cgroup iter

Yosry Ahmed
2022-06-28T06:03:26Z

On Mon, Jun 27, 2022 at 9:14 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 6/10/22 12:44 PM, Yosry Ahmed wrote:
> > From: Hao Luo <haoluo@google.com>
> >
> > Cgroup_iter is a type of bpf_iter. It walks over cgroups in two modes:
> >
> > - walking a cgroup's descendants.
> > - walking a cgroup's ancestors.
> >
> > When attaching cgroup_iter, one can set a cgroup to the iter_link
> > created from attaching. This cgroup is passed as a file descriptor and
> > serves as the starting point of the walk. If no cgroup is specified,
> > the starting point will be the root cgroup.
> >
> > For walking descendants, one can specify the order: either pre-order or
> > post-order. For walking ancestors, the walk starts at the specified
> > cgroup and ends at the root.
> >
> > One can also terminate the walk early by returning 1 from the iter
> > program.
> >
> > Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
> > program is called with cgroup_mutex held.
> >
> > Signed-off-by: Hao Luo <haoluo@google.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> > include/linux/bpf.h | 8 ++
> > include/uapi/linux/bpf.h | 21 +++
> > kernel/bpf/Makefile | 2 +-
> > kernel/bpf/cgroup_iter.c | 235 +++++++++++++++++++++++++++++++++
> > tools/include/uapi/linux/bpf.h | 21 +++
> > 5 files changed, 286 insertions(+), 1 deletion(-)
> > create mode 100644 kernel/bpf/cgroup_iter.c
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 8e6092d0ea956..48d8e836b9748 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -44,6 +44,7 @@ struct kobject;
> > struct mem_cgroup;
> > struct module;
> > struct bpf_func_state;
> > +struct cgroup;
> >
> > extern struct idr btf_idr;
> > extern spinlock_t btf_idr_lock;
> > @@ -1590,7 +1591,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
> > int __init bpf_iter_ ## target(args) { return 0; }
> >
> > struct bpf_iter_aux_info {
> > + /* for map_elem iter */
> > struct bpf_map *map;
> > +
> > + /* for cgroup iter */
> > + struct {
> > + struct cgroup *start; /* starting cgroup */
> > + int order;
> > + } cgroup;
> > };
> >
> [...]
> > +
> > +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
> > +{
> > + struct cgroup_iter_priv *p = seq->private;
> > +
> > + mutex_lock(&cgroup_mutex);
> > +
> > + /* support only one session */
> > + if (*pos > 0)
> > + return NULL;
> > +
> > + ++*pos;
> > + p->terminate = false;
> > + if (p->order == BPF_ITER_CGROUP_PRE)
> > + return css_next_descendant_pre(NULL, p->start_css);
> > + else if (p->order == BPF_ITER_CGROUP_POST)
> > + return css_next_descendant_post(NULL, p->start_css);
> > + else /* BPF_ITER_CGROUP_PARENT_UP */
> > + return p->start_css;
> > +}
> > +
> > +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);
> > +}
> > +
> > +static void *cgroup_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > +{
> > + struct cgroup_subsys_state *curr = (struct cgroup_subsys_state *)v;
> > + struct cgroup_iter_priv *p = seq->private;
> > +
> > + ++*pos;
> > + if (p->terminate)
> > + return NULL;
> > +
> > + if (p->order == BPF_ITER_CGROUP_PRE)
> > + return css_next_descendant_pre(curr, p->start_css);
> > + else if (p->order == BPF_ITER_CGROUP_POST)
> > + return css_next_descendant_post(curr, p->start_css);
> > + else
> > + return curr->parent;
> > +}
> > +
> > +static int __cgroup_iter_seq_show(struct seq_file *seq,
> > + struct cgroup_subsys_state *css, int in_stop)
> > +{
> > + struct cgroup_iter_priv *p = seq->private;
> > + struct bpf_iter__cgroup ctx;
> > + struct bpf_iter_meta meta;
> > + struct bpf_prog *prog;
> > + int ret = 0;
> > +
> > + /* cgroup is dead, skip this element */
> > + if (css && cgroup_is_dead(css->cgroup))
> > + return 0;
> > +
> > + ctx.meta = &meta;
> > + ctx.cgroup = css ? css->cgroup : NULL;
> > + meta.seq = seq;
> > + prog = bpf_iter_get_info(&meta, in_stop);
> > + if (prog)
> > + ret = bpf_iter_run_prog(prog, &ctx);
>
> Do we need to do anything special to ensure bpf program gets
> up-to-date stat from ctx.cgroup?

Later patches in the series add cgroup_flush_rstat() kfunc which
flushes cgroup stats that use rstat (e.g. memcg stats). It can be
called directly from the bpf program if needed.

It would be better to leave this to the bpf program, it's an
unnecessary toll to flush the stats for any cgroup_iter program, that
could be not accessing stats, or stats that are not maintained using
rstat.

>
> > +
> > + /* if prog returns > 0, terminate after this element. */
> > + if (ret != 0)
> > + p->terminate = true;
> > +
> > + return 0;
> > +}
> > +
> …