Commit 2343e88d authored Mar 22, 2022 by Sebastian Andrzej Siewior Committed by Linus Torvalds Mar 22, 2022

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>

parent fead2b86

Documentation/admin-guide/cgroup-v1/memory.rst

+2 −0

Original line number	Diff line number	Diff line
		@@ -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

mm/memcontrol.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -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:
		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);