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

Waiman Long
2019-07-02T19:15:57Z

On 7/2/19 3:09 PM, David Rientjes wrote:
> 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.

You are right. I will reword the text to make it clearer.

>> 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.

It will depends on how many memcg caches are there. Actually, I have
been thinking about using the show method to show the time spent in the
last shrink operation. I am just not sure if it is worth doing. What do
you think?

-Longman