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

[Shakeel Butt]

On Fri, Sep 06, 2024 at 10:52:04AM GMT, Vlastimil Babka wrote:
> 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"? 

This is just a copy-paste from kmalloc() documentation.

> 
> >> > + * 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.
>  */
>  

Yes, this is much better.

> >> > +
> >> > +       /* 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.
>          */
> 

Looks great to me.

thanks,
Shakeel