slub: make sysfs directories for memcg sub-caches optional

SLUB creates a per-cache directory under /sys/kernel/slab which hosts a
bunch of debug files.  Usually, there aren't that many caches on a
system and this doesn't really matter; however, if memcg is in use, each
cache can have per-cgroup sub-caches.  SLUB creates the same directories
for these sub-caches under /sys/kernel/slab/$CACHE/cgroup.

Unfortunately, because there can be a lot of cgroups, active or
draining, the product of the numbers of caches, cgroups and files in
each directory can reach a very high number - hundreds of thousands is
commonplace.  Millions and beyond aren't difficult to reach either.

What's under /sys/kernel/slab is primarily for debugging and the
information and control on the a root cache already cover its
sub-caches.  While having a separate directory for each sub-cache can be
helpful for development, it doesn't make much sense to pay this amount
of overhead by default.

This patch introduces a boot parameter slub_memcg_sysfs which determines
whether to create sysfs directories for per-memcg sub-caches.  It also
adds CONFIG_SLUB_MEMCG_SYSFS_ON which determines the boot parameter's
default value and defaults to 0.

[akpm@linux-foundation.org: kset_unregister(NULL) is legal]
Link: http://lkml.kernel.org/r/20170204145203.GB26958@mtj.duckdns.org
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 55bb6fb..22f437f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1779,6 +1779,20 @@ config SLUB_DEBUG
 	  SLUB sysfs support. /sys/slab will not exist and there will be
 	  no support for cache validation etc.
 
+config SLUB_MEMCG_SYSFS_ON
+	default n
+	bool "Enable memcg SLUB sysfs support by default" if EXPERT
+	depends on SLUB && SYSFS && MEMCG
+	help
+	  SLUB creates a directory under /sys/kernel/slab for each
+	  allocation cache to host info and debug files. If memory
+	  cgroup is enabled, each cache can have per memory cgroup
+	  caches. SLUB can create the same sysfs directories for these
+	  caches under /sys/kernel/slab/CACHE/cgroup but it can lead
+	  to a very high number of debug files being created. This is
+	  controlled by slub_memcg_sysfs boot parameter and this
+	  config option determines the parameter's default value.
+
 config COMPAT_BRK
 	bool "Disable heap randomization"
 	default y