Re: [PATCH v3 1/2] cgroup: Show # of subsystem CSSes in cgroup.stat

Waiman Long
2024-07-10T23:49:10Z

On 7/10/24 17:43, Roman Gushchin wrote:
> On Wed, Jul 10, 2024 at 02:23:52PM -0400, Waiman Long wrote:
>> Cgroup subsystem state (CSS) is an abstraction in the cgroup layer to
>> help manage different structures in various cgroup subsystems by being
>> an embedded element inside a larger structure like cpuset or mem_cgroup.
>>
>> The /proc/cgroups file shows the number of cgroups for each of the
>> subsystems. With cgroup v1, the number of CSSes is the same as the
>> number of cgroups. That is not the case anymore with cgroup v2. The
>> /proc/cgroups file cannot show the actual number of CSSes for the
>> subsystems that are bound to cgroup v2.
>>
>> So if a v2 cgroup subsystem is leaking cgroups (usually memory cgroup),
>> we can't tell by looking at /proc/cgroups which cgroup subsystems may
>> be responsible.
>>
>> As cgroup v2 had deprecated the use of /proc/cgroups, the hierarchical
>> cgroup.stat file is now being extended to show the number of live and
>> dying CSSes associated with all the non-inhibited cgroup subsystems
>> that have been bound to cgroup v2 as long as it is not zero. The number
>> includes CSSes in the current cgroup as well as in all the descendants
>> underneath it. This will help us pinpoint which subsystems are
>> responsible for the increasing number of dying (nr_dying_descendants)
>> cgroups.
>>
>> The cgroup-v2.rst file is updated to discuss this new behavior.
>>
>> With this patch applied, a sample output from root cgroup.stat file
>> was shown below.
>>
>> nr_descendants 54
>> nr_dying_descendants 44
>> nr_cpuset 1
>> nr_cpu 40
>> nr_io 40
>> nr_memory 54
>> nr_dying_memory 44
>> nr_perf_event 55
>> nr_hugetlb 1
>> nr_pids 54
>> nr_rdma 1
>> nr_misc 1
>>
>> Another sample output from system.slice/cgroup.stat was:
>>
>> nr_descendants 32
>> nr_dying_descendants 37
>> nr_cpu 30
>> nr_io 30
>> nr_memory 32
>> nr_dying_memory 37
>> nr_perf_event 33
>> nr_pids 32
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> I like it way more than the previous version, thank you for the update.
>
>> ---
>> Documentation/admin-guide/cgroup-v2.rst | 14 ++++++-
>> include/linux/cgroup-defs.h | 7 ++++
>> kernel/cgroup/cgroup.c | 50 ++++++++++++++++++++++++-
>> 3 files changed, 68 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>> index 52763d6b2919..9031419271cd 100644
>> --- a/Documentation/admin-guide/cgroup-v2.rst
>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>> @@ -981,6 +981,16 @@ All cgroup core files are prefixed with "cgroup."
>> A dying cgroup can consume system resources not exceeding
>> limits, which were active at the moment of cgroup deletion.
>>
>> + nr_<cgroup_subsys>
>> + Total number of live cgroups associated with that cgroup
>> + subsystem (e.g. memory) at and beneath the current
>> + cgroup. An entry will only be shown if it is not zero.
>> +
>> + nr_dying_<cgroup_subsys>
>> + Total number of dying cgroups associated with that cgroup
>> + subsystem (e.g. memory) beneath the current cgroup.
>> + An entry will only be shown if it is not zero.
>> +
>> cgroup.freeze
>> A read-write single value file which exists on non-root cgroups.
>> Allowed values are "0" and "1". The default is "0".
>> @@ -2930,8 +2940,8 @@ Deprecated v1 Core Features
>>
>> - "cgroup.clone_children" is removed.
>>
>> -- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file
>> - at the root instead.
>> +- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" or
>> + "cgroup.stat" files at the root instead.
>>
>>
>> Issues with v1 and Rationales for v2
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
>> index b36690ca0d3f..62de18874508 100644
>> --- a/include/linux/cgroup-defs.h
>> +++ b/include/linux/cgroup-defs.h
>> @@ -210,6 +210,13 @@ struct cgroup_subsys_state {
>> * fields of the containing structure.
>> */
>> struct cgroup_subsys_state *parent;
>> +
>> + /*
>> + * Keep track of total numbers of visible and dying descendant CSSes.
>> + * Protected by cgroup_mutex.
>> + */
>> + int nr_descendants;
>> + int nr_dying_descendants;
>> };
>>
>> /*
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index c8e4b62b436a..18c982a06446 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -3669,12 +3669,34 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
>> static int cgroup_stat_show(struct seq_file *seq, void *v)
>> {
>> struct cgroup *cgroup = seq_css(seq)->cgroup;
>> + struct cgroup_subsys_state *css;
>> + int ssid;
>>
>> + /* cgroup_mutex required for for_each_css() */
>> + cgroup_lock();
> I *guess* it can be done under a rcu_read_lock(), isn't it?
> That would eliminate a need for the second patch as well, which
> is questionable (e.g. one unprivileged user can block others?)

I am just following the instruction in the for_each_css() macro:

*
* Should be called under cgroup_mutex.
*/

I think taking rcu_read_lock() should also work in this case. Will try
it out and update the patch after some testing.

Thanks,
Longman