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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. [email@example.com: kset_unregister(NULL) is legal] Link: http://lkml.kernel.org/r/20170204145203.GB26958@mtj.duckdns.org Signed-off-by: Tejun Heo <firstname.lastname@example.org> Cc: Christoph Lameter <email@example.com> Cc: Pekka Enberg <firstname.lastname@example.org> Cc: David Rientjes <email@example.com> Cc: Joonsoo Kim <firstname.lastname@example.org> Cc: Vladimir Davydov <email@example.com> Cc: Michal Hocko <firstname.lastname@example.org> Cc: Johannes Weiner <email@example.com> Signed-off-by: Andrew Morton <firstname.lastname@example.org> Signed-off-by: Linus Torvalds <email@example.com>
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