Re: [PATCH v3] mm, memcg: introduce memory.events.local

[Michal Hocko <mhocko@kernel.org>]

On Mon 27-05-19 10:46:43, Shakeel Butt wrote:
> The memory controller in cgroup v2 exposes memory.events file for each
> memcg which shows the number of times events like low, high, max, oom
> and oom_kill have happened for the whole tree rooted at that memcg.
> Users can also poll or register notification to monitor the changes in
> that file. Any event at any level of the tree rooted at memcg will
> notify all the listeners along the path till root_mem_cgroup. There are
> existing users which depend on this behavior.
> 
> However there are users which are only interested in the events
> happening at a specific level of the memcg tree and not in the events in
> the underlying tree rooted at that memcg. One such use-case is a
> centralized resource monitor which can dynamically adjust the limits of
> the jobs running on a system. The jobs can create their sub-hierarchy
> for their own sub-tasks. The centralized monitor is only interested in
> the events at the top level memcgs of the jobs as it can then act and
> adjust the limits of the jobs. Using the current memory.events for such
> centralized monitor is very inconvenient. The monitor will keep
> receiving events which it is not interested and to find if the received
> event is interesting, it has to read memory.event files of the next
> level and compare it with the top level one. So, let's introduce
> memory.events.local to the memcg which shows and notify for the events
> at the memcg level.
> 
> Now, does memory.stat and memory.pressure need their local versions.
> IMHO no due to the no internal process contraint of the cgroup v2. The
> memory.stat file of the top level memcg of a job shows the stats and
> vmevents of the whole tree. The local stats or vmevents of the top level
> memcg will only change if there is a process running in that memcg but
> v2 does not allow that. Similarly for memory.pressure there will not be
> any process in the internal nodes and thus no chance of local pressure.
> 
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Reviewed-by: Roman Gushchin <guro@fb.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

As there seems to be a larger agreement that the default behavior of
memory.events is going to be hierarchical then this addition makes a lot
of sense.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changelog since v2:
> - Added documentation.
> 
> Changelog since v1:
> - refactor memory_events_show to share between events and events.local
> 
>  Documentation/admin-guide/cgroup-v2.rst | 10 ++++++++
>  include/linux/memcontrol.h              |  7 ++++-
>  mm/memcontrol.c                         | 34 +++++++++++++++++--------
>  3 files changed, 40 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 19c4e78666ff..0e961fc90cd9 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1119,6 +1119,11 @@ PAGE_SIZE multiple when read back.
>  	otherwise, a value change in this file generates a file
>  	modified event.
>  
> +	Note that all fields in this file are hierarchical and the
> +	file modified event can be generated due to an event down the
> +	hierarchy. For for the local events at the cgroup level see
> +	memory.events.local.
> +
>  	  low
>  		The number of times the cgroup is reclaimed due to
>  		high memory pressure even though its usage is under
> @@ -1158,6 +1163,11 @@ PAGE_SIZE multiple when read back.
>  		The number of processes belonging to this cgroup
>  		killed by any kind of OOM killer.
>  
> +  memory.events.local
> +	Similar to memory.events but the fields in the file are local
> +	to the cgroup i.e. not hierarchical. The file modified event
> +	generated on this file reflects only the local events.
> +
>    memory.stat
>  	A read-only flat-keyed file which exists on non-root cgroups.
>  
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 36bdfe8e5965..de77405eec46 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -239,8 +239,9 @@ struct mem_cgroup {
>  	/* OOM-Killer disable */
>  	int		oom_kill_disable;
>  
> -	/* memory.events */
> +	/* memory.events and memory.events.local */
>  	struct cgroup_file events_file;
> +	struct cgroup_file events_local_file;
>  
>  	/* handle for "memory.swap.events" */
>  	struct cgroup_file swap_events_file;
> @@ -286,6 +287,7 @@ struct mem_cgroup {
>  	atomic_long_t		vmevents_local[NR_VM_EVENT_ITEMS];
>  
>  	atomic_long_t		memory_events[MEMCG_NR_MEMORY_EVENTS];
> +	atomic_long_t		memory_events_local[MEMCG_NR_MEMORY_EVENTS];
>  
>  	unsigned long		socket_pressure;
>  
> @@ -761,6 +763,9 @@ static inline void count_memcg_event_mm(struct mm_struct *mm,
>  static inline void memcg_memory_event(struct mem_cgroup *memcg,
>  				      enum memcg_memory_event event)
>  {
> +	atomic_long_inc(&memcg->memory_events_local[event]);
> +	cgroup_file_notify(&memcg->events_local_file);
> +
>  	do {
>  		atomic_long_inc(&memcg->memory_events[event]);
>  		cgroup_file_notify(&memcg->events_file);
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2713b45ec3f0..a57dfcc4c4a4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5630,21 +5630,29 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>  	return nbytes;
>  }
>  
> +static void __memory_events_show(struct seq_file *m, atomic_long_t *events)
> +{
> +	seq_printf(m, "low %lu\n", atomic_long_read(&events[MEMCG_LOW]));
> +	seq_printf(m, "high %lu\n", atomic_long_read(&events[MEMCG_HIGH]));
> +	seq_printf(m, "max %lu\n", atomic_long_read(&events[MEMCG_MAX]));
> +	seq_printf(m, "oom %lu\n", atomic_long_read(&events[MEMCG_OOM]));
> +	seq_printf(m, "oom_kill %lu\n",
> +		   atomic_long_read(&events[MEMCG_OOM_KILL]));
> +}
> +
>  static int memory_events_show(struct seq_file *m, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
>  
> -	seq_printf(m, "low %lu\n",
> -		   atomic_long_read(&memcg->memory_events[MEMCG_LOW]));
> -	seq_printf(m, "high %lu\n",
> -		   atomic_long_read(&memcg->memory_events[MEMCG_HIGH]));
> -	seq_printf(m, "max %lu\n",
> -		   atomic_long_read(&memcg->memory_events[MEMCG_MAX]));
> -	seq_printf(m, "oom %lu\n",
> -		   atomic_long_read(&memcg->memory_events[MEMCG_OOM]));
> -	seq_printf(m, "oom_kill %lu\n",
> -		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
> +	__memory_events_show(m, memcg->memory_events);
> +	return 0;
> +}
> +
> +static int memory_events_local_show(struct seq_file *m, void *v)
> +{
> +	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
>  
> +	__memory_events_show(m, memcg->memory_events_local);
>  	return 0;
>  }
>  
> @@ -5806,6 +5814,12 @@ static struct cftype memory_files[] = {
>  		.file_offset = offsetof(struct mem_cgroup, events_file),
>  		.seq_show = memory_events_show,
>  	},
> +	{
> +		.name = "events.local",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.file_offset = offsetof(struct mem_cgroup, events_local_file),
> +		.seq_show = memory_events_local_show,
> +	},
>  	{
>  		.name = "stat",
>  		.flags = CFTYPE_NOT_ON_ROOT,
> -- 
> 2.22.0.rc1.257.g3120a18244-goog

-- 
Michal Hocko
SUSE Labs