mm/memcg: disable threshold event handlers on PREEMPT_RT

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.

Link: https://lkml.kernel.org/r/20220226204144.1008339-3-bigeasy@linutronix.de
Suggested-by: Michal Hocko <mhocko@kernel.org>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index faac501..2cc502a 100644 (file)
--- 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 7bf204b..6c61b4e 100644 (file)
--- 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))) {
@@ -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);