Re: [PATCH net-next 0/5] skbuff: introduce skbuff_heads bulking and reusing

[Eric Dumazet]

On Wed, Jan 13, 2021 at 6:03 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 13 Jan 2021 05:46:05 +0100 Eric Dumazet wrote:
> > On Wed, Jan 13, 2021 at 2:02 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > >
> > > On Tue, 12 Jan 2021 13:23:16 +0100 Eric Dumazet wrote:
> > > > On Tue, Jan 12, 2021 at 12:08 PM Alexander Lobakin <alobakin@xxxxx> wrote:
> > > > >
> > > > > From: Edward Cree <ecree.xilinx@xxxxxxxxx>
> > > > > Date: Tue, 12 Jan 2021 09:54:04 +0000
> > > > >
> > > > > > Without wishing to weigh in on whether this caching is a good idea...
> > > > >
> > > > > Well, we already have a cache to bulk flush "consumed" skbs, although
> > > > > kmem_cache_free() is generally lighter than kmem_cache_alloc(), and
> > > > > a page frag cache to allocate skb->head that is also bulking the
> > > > > operations, since it contains a (compound) page with the size of
> > > > > min(SZ_32K, PAGE_SIZE).
> > > > > If they wouldn't give any visible boosts, I think they wouldn't hit
> > > > > mainline.
> > > > >
> > > > > > Wouldn't it be simpler, rather than having two separate "alloc" and "flush"
> > > > > >  caches, to have a single larger cache, such that whenever it becomes full
> > > > > >  we bulk flush the top half, and when it's empty we bulk alloc the bottom
> > > > > >  half?  That should mean fewer branches, fewer instructions etc. than
> > > > > >  having to decide which cache to act upon every time.
> > > > >
> > > > > I though about a unified cache, but couldn't decide whether to flush
> > > > > or to allocate heads and how much to process. Your suggestion answers
> > > > > these questions and generally seems great. I'll try that one, thanks!
> > > >
> > > > The thing is : kmalloc() is supposed to have batches already, and nice
> > > > per-cpu caches.
> > > >
> > > > This looks like an mm issue, are we sure we want to get over it ?
> > > >
> > > > I would like a full analysis of why SLAB/SLUB does not work well for
> > > > your test workload.
> > >
> > > +1, it does feel like we're getting into mm territory
> >
> > I read the existing code, and with Edward Cree idea of reusing the
> > existing cache (storage of pointers),
> > ths now all makes sense, since there will be not much added code (and
> > new storage of 64 pointers)
> >
> > The remaining issue is to make sure KASAN will still work, we need
> > this to detect old and new bugs.
>
> IDK much about MM, but we already have a kmem_cache for skbs and now
> we're building a cache on top of a cache.  Shouldn't MM take care of
> providing a per-CPU BH-only lockless cache?

I think part of the improvement comes from bulk operations, which are
provided by mm layer.

I also note Alexander made no provision for NUMA awareness.
Probably reusing skb located on a remote node will not be ideal.