[PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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@kernel.org>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 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 4b1572ae990d8..c1caa662946dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -858,6 +858,9 @@ 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))) {
@@ -3724,8 +3727,12 @@ 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;
@@ -4701,6 +4708,9 @@ 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);
-- 
2.34.1