Re: [PATCH] mm/memcg: Do not check v1 event counter when not needed

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

On 2022-01-18 19:26:00 [+0100], Michal Koutný wrote:
> I think it would make sense inserting the patch into your series and
> subsequently reject enabling on PREEMPT_RT -- provided this patch makes sense
> to others too -- the justification is rather functionality splitting for
> this PREEMPT_RT effort.

Interesting. So while looking at this today I came up with the patch at
the bottom. The other things I had looked way uglier and then since
nobody probably will use it…
Let me know how you want it to be integrated.

------>8------

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Tue, 18 Jan 2022 17:28:07 +0100
Subject: [PATCH] mm/memcg: Disable threshold event handlers on PREEMPT_RT
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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-save block.

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
have not been any users on PREEMPT_RT. From that perspective makes it
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 memcg_check_events() empty on PREEMPT_RT
since it won't do anything. 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                                | 12 ++++++++++++
 2 files changed, 14 insertions(+)

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 2ed5f2a0879d3..3c4f7a0fd0039 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -821,6 +821,7 @@ static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
 	__this_cpu_add(memcg->vmstats_percpu->nr_page_events, nr_pages);
 }
 
+#ifndef CONFIG_PREEMPT_RT
 static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
 				       enum mem_cgroup_events_target target)
 {
@@ -864,6 +865,9 @@ static void memcg_check_events(struct mem_cgroup *memcg, int nid)
 			mem_cgroup_update_tree(memcg, nid);
 	}
 }
+#else
+static void memcg_check_events(struct mem_cgroup *memcg, int nid) { }
+#endif
 
 struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
 {
@@ -3751,8 +3755,12 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 		}
 		break;
 	case RES_SOFT_LIMIT:
+#ifndef CONFIG_PREEMPT_RT
 		memcg->soft_limit = nr_pages;
 		ret = 0;
+#else
+		ret = -EOPNOTSUPP;
+#endif
 		break;
 	}
 	return ret ?: nbytes;
@@ -4717,6 +4725,7 @@ static void memcg_event_ptable_queue_proc(struct file *file,
 static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 					 char *buf, size_t nbytes, loff_t off)
 {
+#ifndef CONFIG_PREEMPT_RT
 	struct cgroup_subsys_state *css = of_css(of);
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 	struct mem_cgroup_event *event;
@@ -4843,6 +4852,9 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 	kfree(event);
 
 	return ret;
+#else
+	return -EOPNOTSUPP;
+#endif
 }
 
 static struct cftype mem_cgroup_legacy_files[] = {
-- 
2.34.1