Re: [PATCH v4] memcg: add charging of already allocated slab objects

[Vlastimil Babka]

On 9/5/24 20:48, Shakeel Butt wrote:
>> > ---
>> > v3: https://lore.kernel.org/all/20240829175339.2424521-1-shakeel.butt@xxxxxxxxx/
>> > Changes since v3:
>> > - Add kernel doc for kmem_cache_charge.
>> >
>> > v2: https://lore.kernel.org/all/20240827235228.1591842-1-shakeel.butt@xxxxxxxxx/
>> > Change since v2:
>> > - Add handling of already charged large kmalloc objects.
>> > - Move the normal kmalloc cache check into a function.
>> >
>> > v1: https://lore.kernel.org/all/20240826232908.4076417-1-shakeel.butt@xxxxxxxxx/
>> > Changes since v1:
>> > - Correctly handle large allocations which bypass slab
>> > - Rearrange code to avoid compilation errors for !CONFIG_MEMCG builds
>> >
>> > RFC: https://lore.kernel.org/all/20240824010139.1293051-1-shakeel.butt@xxxxxxxxx/
>> > Changes since the RFC:
>> > - Added check for already charged slab objects.
>> > - Added performance results from neper's tcp_crr
>> >
>> >
>> >  include/linux/slab.h            | 20 ++++++++++++++
>> >  mm/slab.h                       |  7 +++++
>> >  mm/slub.c                       | 49 +++++++++++++++++++++++++++++++++
>> >  net/ipv4/inet_connection_sock.c |  5 ++--
>> >  4 files changed, 79 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/include/linux/slab.h b/include/linux/slab.h
>> > index eb2bf4629157..68789c79a530 100644
>> > --- a/include/linux/slab.h
>> > +++ b/include/linux/slab.h
>> > @@ -547,6 +547,26 @@ void *kmem_cache_alloc_lru_noprof(struct kmem_cache *s, struct list_lru *lru,
>> >                             gfp_t gfpflags) __assume_slab_alignment __malloc;
>> >  #define kmem_cache_alloc_lru(...)      alloc_hooks(kmem_cache_alloc_lru_noprof(__VA_ARGS__))
>> >
>> > +/**
>> > + * kmem_cache_charge - memcg charge an already allocated slab memory
>> > + * @objp: address of the slab object to memcg charge.
>> > + * @gfpflags: describe the allocation context
>> > + *
>> > + * kmem_cache_charge is the normal method to charge a slab object to the current

what is "normal method"? 

>> > + * memcg. The objp should be pointer returned by the slab allocator functions
>> > + * like kmalloc or kmem_cache_alloc. The memcg charge behavior can be controller
>> 
>> s/controller/controlled
> 
> Thanks. Vlastimil please fix this when you pick this up.

I felt it could be improved more, so ended up with this. Thoughts?

/**
 * kmem_cache_charge - memcg charge an already allocated slab memory
 * @objp: address of the slab object to memcg charge
 * @gfpflags: describe the allocation context
 *
 * kmem_cache_charge allows charging a slab object to the current memcg,
 * primarily in cases where charging at allocation time might not be possible
 * because the target memcg is not known (i.e. softirq context)
 *
 * The objp should be pointer returned by the slab allocator functions like
 * kmalloc (with __GFP_ACCOUNT in flags) or kmem_cache_alloc. The memcg charge
 * behavior can be controlled through gfpflags parameter, which affects how the
 * necessary internal metadata can be allocated. Including __GFP_NOFAIL denotes
 * that overcharging is requested instead of failure, but is not applied for the
 * internal metadata allocation.
 *
 * There are several cases where it will return true even if the charging was
 * not done:
 * More specifically:
 *
 * 1. For !CONFIG_MEMCG or cgroup_disable=memory systems.
 * 2. Already charged slab objects.
 * 3. For slab objects from KMALLOC_NORMAL caches - allocated by kmalloc()
 *    without __GFP_ACCOUNT
 * 4. Allocating internal metadata has failed
 *
 * Return: true if charge was successful otherwise false.
 */
 
>> > +
>> > +       /* Ignore KMALLOC_NORMAL cache to avoid circular dependency. */
>> 
>> Is it possible to point to the commit that has the explanation here?
>> The one you pointed me to before? Otherwise it's not really obvious
>> where the circular dependency comes from (at least to me).
>> 
> 
> Not sure about the commit reference. We can add more text here.
> Vlastimil, how much detail do you prefer?

What about:

        /*
         * Ignore KMALLOC_NORMAL cache to avoid possible circular dependency
         * of slab_obj_exts being allocated from the same slab and thus the slab
         * becoming effectively unfreeable.
         */

 
> thanks,
> Shakeel