netdev - Re: [PATCH net-next 0/5] skbuff: introduce skbuff

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Date:   Tue, 12 Jan 2021 17:02:42 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexander Lobakin <alobakin@...me>,
        Edward Cree <ecree.xilinx@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Edward Cree <ecree@...arflare.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Guillaume Nault <gnault@...hat.com>,
        Yadu Kishore <kyk.segfault@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 0/5] skbuff: introduce skbuff_heads bulking and
 reusing

On Tue, 12 Jan 2021 13:23:16 +0100 Eric Dumazet wrote:
> On Tue, Jan 12, 2021 at 12:08 PM Alexander Lobakin <alobakin@...me> wrote:
> >
> > From: Edward Cree <ecree.xilinx@...il.com>
> > 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

> More details, more numbers.... before we accept yet another
> 'networking optimization' adding more code to the 'fast' path.
> 
> More code means more latencies when all code needs to be brought up in
> cpu caches.