Re: [PATCH] mm, slab: Extend slab/shrink to shrink all the memcg caches

David Rientjes
2019-07-02T19:09:32Z

On Tue, 2 Jul 2019, Waiman Long wrote:

> diff --git a/Documentation/ABI/testing/sysfs-kernel-slab b/Documentation/ABI/testing/sysfs-kernel-slab
> index 29601d93a1c2..2a3d0fc4b4ac 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-slab
> +++ b/Documentation/ABI/testing/sysfs-kernel-slab
> @@ -429,10 +429,12 @@ KernelVersion: 2.6.22
> Contact: Pekka Enberg <penberg@cs.helsinki.fi>,
> Christoph Lameter <cl@linux-foundation.org>
> Description:
> - The shrink file is written when memory should be reclaimed from
> - a cache. Empty partial slabs are freed and the partial list is
> - sorted so the slabs with the fewest available objects are used
> - first.
> + A value of '1' is written to the shrink file when memory should
> + be reclaimed from a cache. Empty partial slabs are freed and
> + the partial list is sorted so the slabs with the fewest
> + available objects are used first. When a value of '2' is
> + written, all the corresponding child memory cgroup caches
> + should be shrunk as well. All other values are invalid.
>

This should likely call out that '2' also does '1', that might not be
clear enough.

> What: /sys/kernel/slab/cache/slab_size
> Date: May 2007
> diff --git a/mm/slab.h b/mm/slab.h
> index 3b22931bb557..a16b2c7ff4dd 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -174,6 +174,7 @@ int __kmem_cache_shrink(struct kmem_cache *);
> void __kmemcg_cache_deactivate(struct kmem_cache *s);
> void __kmemcg_cache_deactivate_after_rcu(struct kmem_cache *s);
> void slab_kmem_cache_release(struct kmem_cache *);
> +int kmem_cache_shrink_all(struct kmem_cache *s);
>
> struct seq_file;
> struct file;
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 464faaa9fd81..493697ba1da5 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -981,6 +981,49 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
> }
> EXPORT_SYMBOL(kmem_cache_shrink);
>
> +/**
> + * kmem_cache_shrink_all - shrink a cache and all its memcg children
> + * @s: The root cache to shrink.
> + *
> + * Return: 0 if successful, -EINVAL if not a root cache
> + */
> +int kmem_cache_shrink_all(struct kmem_cache *s)
> +{
> + struct kmem_cache *c;
> +
> + if (!IS_ENABLED(CONFIG_MEMCG_KMEM)) {
> + kmem_cache_shrink(s);
> + return 0;
> + }
> + if (!is_root_cache(s))
> + return -EINVAL;
> +
> + /*
> + * The caller should have a reference to the root cache and so
> + * we don't need to take the slab_mutex. We have to take the
> + * slab_mutex, however, to iterate the memcg caches.
> + */
> + get_online_cpus();
> + get_online_mems();
> + kasan_cache_shrink(s);
> + __kmem_cache_shrink(s);
> +
> + mutex_lock(&slab_mutex);
> + for_each_memcg_cache(c, s) {
> + /*
> + * Don't need to shrink deactivated memcg caches.
> + */
> + if (s->flags & SLAB_DEACTIVATED)
> + continue;
> + kasan_cache_shrink(c);
> + __kmem_cache_shrink(c);
> + }
> + mutex_unlock(&slab_mutex);
> + put_online_mems();
> + put_online_cpus();
> + return 0;
> +}
> +
> bool slab_is_available(void)
> {
> return slab_state >= UP;

I'm wondering how long this could take, i.e. how long we hold slab_mutex
while we traverse each cache and shrink it.

Acked-by: David Rientjes <rientjes@google.com>