On Sat, 4 Feb 2017 09:52:03 -0500 Tejun Heo <tj@xxxxxxxxxx> wrote:
> 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.
>
> ...
>
> #ifdef CONFIG_MEMCG
> - if (is_root_cache(s)) {
> + if (is_root_cache(s) && memcg_sysfs_enabled) {
This could be turned on and off after bootup but I guess the result
could be pretty confusing.
However there would be useful use cases? The user would normally have
this disabled but if he wants to do a bit of debugging then turn this
on, create a memcg, have a poke around then turn the feature off again.
> s->memcg_kset = kset_create_and_add("cgroup", NULL, &s->kobj);
> if (!s->memcg_kset) {
> err = -ENOMEM;
> @@ -5673,7 +5695,8 @@ static void sysfs_slab_remove(struct kme
> return;
>
> #ifdef CONFIG_MEMCG
> - kset_unregister(s->memcg_kset);
> + if (s->memcg_kset)
> + kset_unregister(s->memcg_kset);
kset_unregister(NULL) is legal
--- a/mm/slub.c~slub-make-sysfs-directories-for-memcg-sub-caches-optional-fix
+++ a/mm/slub.c
@@ -5699,8 +5699,7 @@ static void sysfs_slab_remove(struct kme
return;
#ifdef CONFIG_MEMCG
- if (s->memcg_kset)
- kset_unregister(s->memcg_kset);
+ kset_unregister(s->memcg_kset);
#endif
kobject_uevent(&s->kobj, KOBJ_REMOVE);
kobject_del(&s->kobj);
_
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