samples: introduce new samples subdir for cgroup

Patch series "samples: introduce cgroup events listeners", v3.

To begin with, this patch series relocates the cgroup example code to the
samples/cgroup directory, which is the appropriate location for such code
snippets.

Furthermore, a new memcg events listener is introduced.  This listener is
a simple yet effective tool for monitoring memory events and managing
counter changes during runtime.

Additionally, as per Andrew Morton's suggestion, a helpful reminder
comment is included in the memcontrol implementation.  This comment serves
to ensure that the samples code is updated whenever new events are added.


This patch (of 3):

Move the cgroup_event_listener for cgroup v1 to the samples directory. 
This suggestion was proposed by Andrew Morton during the discussion [1].

Link: https://lore.kernel.org/all/[email protected]/ [1]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dmitry Rokosov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

MAINTAINERS

@@ -5339,6 +5339,7 @@ L:	[email protected]
 S:	Maintained
 F:	mm/memcontrol.c
 F:	mm/swap_cgroup.c
+F:	samples/cgroup/*
 F:	tools/testing/selftests/cgroup/memcg_protection.m
 F:	tools/testing/selftests/cgroup/test_hugetlb_memcg.c
 F:	tools/testing/selftests/cgroup/test_kmem.c

samples/Kconfig

@@ -285,6 +285,12 @@ config SAMPLE_KMEMLEAK
           Build a sample program which have explicitly leaks memory to test
           kmemleak
 
+config SAMPLE_CGROUP
+	bool "Build cgroup sample code"
+	depends on CGROUPS && CC_CAN_LINK && HEADERS_INSTALL
+	help
+	  Build samples that demonstrate the usage of the cgroup API.
+
 source "samples/rust/Kconfig"
 
 endif # SAMPLES

samples/Makefile

@@ -3,6 +3,7 @@
 
 subdir-$(CONFIG_SAMPLE_AUXDISPLAY)	+= auxdisplay
 subdir-$(CONFIG_SAMPLE_ANDROID_BINDERFS) += binderfs
+subdir-$(CONFIG_SAMPLE_CGROUP) += cgroup
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
 obj-$(CONFIG_SAMPLE_FANOTIFY_ERROR)	+= fanotify/

samples/cgroup/Makefile

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0
+
+userprogs-always-y += cgroup_event_listener
+
+userccflags += -I usr/include