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Subject: Re: [PATCH bpf-next v2 4/8] bpf: Introduce cgroup iter
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To:     Yosry Ahmed <yosryahmed@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shuah Khan <shuah@kernel.org>, Michal Hocko <mhocko@kernel.org>
Cc:     Roman Gushchin <roman.gushchin@linux.dev>,
        David Rientjes <rientjes@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        Greg Thelen <gthelen@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        bpf@vger.kernel.org, cgroups@vger.kernel.org
References: <20220610194435.2268290-1-yosryahmed@google.com>
 <20220610194435.2268290-5-yosryahmed@google.com>
From:   Yonghong Song <yhs@fb.com>
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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?

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