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

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Sep 6, 2024 at 10:29 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 9/6/24 19:19, Yosry Ahmed wrote:
> > [..]
> >> 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
> >
> > Aren't allocations done with kmalloc(__GFP_ACCOUNT) already accounted?
> > Why would we need to call kmem_cache_charge() for those?
>
> AFAIU current_obj_cgroup() returns NULL because we're in the interrupt
> context and no remote memcg context has been set. Thus the charging is
> skipped. The patch commit log describes such scenario for network receive.

Oh yeah I missed that part. I thought the networking allocations in
interrupt context are made without __GFP_ACCOUNT to begin with.

> But in case of kmalloc() the allocation must have been still attempted with
> __GFP_ACCOUNT so a kmalloc-cg cache is used even if the charging fails.

It is still possible that the initial allocation did not have
__GFP_ACCOUNT, but not from a KMALLOC_NORMAL cache (e.g. KMALLOC_DMA
or KMALLOC_RECLAIM). In this case kmem_cache_charge() should still
work, right?

>
> If there's another usage for kmem_cache_charge() where the memcg is
> available but we don't want to charge immediately on purpose (such as the
> Linus' idea for struct file), we might need to find another way to tell
> kmalloc() to use the kmalloc-cg cache but not charge immediately...

Can we just use a dedicated kmem_cache for this instead?