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

[Waiman Long <longman@redhat.com>]

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