[PATCH v5 2/6] mm/memcg: Disable threshold event handlers on PREEMPT_RT

Sebastian Andrzej Siewior
2022-02-26T20:41:40Z

During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'. Moving the code around
and avoiding then would then lead to an additional local-irq-save
section within memcg_check_events(). While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature. Instead of
trying to get it to work under PREEMPT_RT just disable it. There should
be no users on PREEMPT_RT. From that perspective it makes even less
sense to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
Document that the two knobs are disabled on PREEMPT_RT.

Suggested-by: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Suggested-by: Michal Koutn√Ω <mkoutny-IBi9RG/b67k@public.gmane.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Acked-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>
Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Reviewed-by: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
---
Documentation/admin-guide/cgroup-v1/memory.rst | 2 ++
mm/memcontrol.c | 14 ++++++++++++--
2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index faac50149a222..2cc502a75ef64 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -64,6 +64,7 @@ Brief summary of control files.
threads
cgroup.procs show list of processes
cgroup.event_control an interface for event_fd()
+ This knob is not available on CONFIG_PREEMPT_RT systems.
memory.usage_in_bytes show current usage for memory
(See 5.5 for details)
memory.memsw.usage_in_bytes show current usage for memory+Swap
@@ -75,6 +76,7 @@ Brief summary of control files.
memory.max_usage_in_bytes show max memory usage recorded
memory.memsw.max_usage_in_bytes show max memory+Swap usage recorded
memory.soft_limit_in_bytes set/show soft limit of memory usage
+ This knob is not available on CONFIG_PREEMPT_RT systems.
memory.stat show various statistics
memory.use_hierarchy set/show hierarchical account enabled
This knob is deprecated and shouldn't be
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8ab2dc75e70ec..0b5117ed2ae08 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -859,6 +859,9 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
*/
static void memcg_check_events(struct mem_cgroup *memcg, int nid)
{
+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ return;
+
/* threshold event is triggered in finer grain than soft limit */
if (unlikely(mem_cgroup_event_ratelimit(memcg,
MEM_CGROUP_TARGET_THRESH))) {
@@ -3731,8 +3734,12 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
}
break;
case RES_SOFT_LIMIT:
- memcg->soft_limit = nr_pages;
- ret = 0;
+ if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+ ret = -EOPNOTSUPP;
+ } else {
+ memcg->soft_limit = nr_pages;
+ ret = 0;
+ }
break;
}
return ret ?: nbytes;
@@ -4708,6 +4715,9 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
char *endp;
int ret;

+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ return -EOPNOTSUPP;
+
buf = strstrip(buf);

efd = simple_strtoul(buf, &endp, 10);
--
2.35.1