On Wed, Aug 3, 2022 at 12:44 AM Yonghong Song [off-list ref] wrote:

On 8/1/22 10:54 AM, Hao Luo wrote:

quoted

Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:

  - walking a cgroup's descendants in pre-order.
  - walking a cgroup's descendants in post-order.
  - 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.

Currently only one session is supported, which means, depending on the
volume of data bpf program intends to send to user space, the number
of cgroups that can be walked is limited. For example, given the current
buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
cgroup, assuming PAGE_SIZE is 4kb, the total number of cgroups that can
be walked is 512. This is a limitation of cgroup_iter. If the output
data is larger than the buffer size, the second read() will signal
EOPNOTSUPP. In order to work around, the user may have to update their

'the second read() will signal EOPNOTSUPP' is not true. for bpf_iter,
we have user buffer from read() syscall and kernel buffer. The above
buffer size like 8 * PAGE_SIZE refers to the kernel buffer size.

If read() syscall buffer size is less than kernel buffer size,
the second read() will not signal EOPNOTSUPP. So to make it precise,
we can say
   If the output data is larger than the kernel buffer size, after
   all data in the kernel buffer is consumed by user space, the
   subsequent read() syscall will signal EOPNOTSUPP.

Thanks Yonghong. Will update.

quoted

program to reduce the volume of data sent to output. For example, skip
some uninteresting cgroups. In future, we may extend bpf_iter flags to
allow customizing buffer size.

Acked-by: Yonghong Song <redacted>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hao Luo <redacted>
---

[...]

quoted

+ *
+ * Currently only one session is supported, which means, depending on the
+ * volume of data bpf program intends to send to user space, the number
+ * of cgroups that can be walked is limited. For example, given the current
+ * buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
+ * cgroup, assuming PAGE_SIZE is 4kb, the total number of cgroups that can
+ * be walked is 512. This is a limitation of cgroup_iter. If the output data
+ * is larger than the buffer size, the second read() will signal EOPNOTSUPP.
+ * In order to work around, the user may have to update their program to

same here as above for better description.

SG. Will update.

quoted

+ * reduce the volume of data sent to output. For example, skip some
+ * uninteresting cgroups.
+ */
+
+struct bpf_iter__cgroup {
+     __bpf_md_ptr(struct bpf_iter_meta *, meta);
+     __bpf_md_ptr(struct cgroup *, cgroup);
+};
+
+struct cgroup_iter_priv {
+     struct cgroup_subsys_state *start_css;
+     bool visited_all;
+     bool terminate;
+     int order;
+};
+
+static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
+{
+     struct cgroup_iter_priv *p = seq->private;
+
+     mutex_lock(&cgroup_mutex);
+
+     /* cgroup_iter doesn't support read across multiple sessions. */
+     if (*pos > 0) {
+             if (p->visited_all)
+                     return NULL;

This looks good. thanks!

quoted

+
+             /* Haven't visited all, but because cgroup_mutex has dropped,
+              * return -EOPNOTSUPP to indicate incomplete iteration.
+              */
+             return ERR_PTR(-EOPNOTSUPP);
+     }
+
+     ++*pos;
+     p->terminate = false;
+     p->visited_all = 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;
+}
+

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