netdev - Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

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Date:   Thu, 8 Sep 2022 05:20:03 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

On Thu, Sep 8, 2022 at 3:48 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Wed, 2022-09-07 at 13:40 -0700, Eric Dumazet wrote:
> > On 9/7/22 13:19, Paolo Abeni wrote:
> > > Hello,
> > >
> > > reviving an old thread...
> > > On Wed, 2021-01-13 at 08:18 -0800, Eric Dumazet wrote:
> > > > While using page fragments instead of a kmalloc backed skb->head might give
> > > > a small performance improvement in some cases, there is a huge risk of
> > > > under estimating memory usage.
> > > [...]
> > >
> > > > Note that we might in the future use the sk_buff napi cache,
> > > > instead of going through a more expensive __alloc_skb()
> > > >
> > > > Another idea would be to use separate page sizes depending
> > > > on the allocated length (to never have more than 4 frags per page)
> > > I'm investigating a couple of performance regressions pointing to this
> > > change and I'd like to have a try to the 2nd suggestion above.
> > >
> > > If I read correctly, it means:
> > > - extend the page_frag_cache alloc API to allow forcing max order==0
> > > - add a 2nd page_frag_cache into napi_alloc_cache (say page_order0 or
> > > page_small)
> > > - in __napi_alloc_skb(), when len <= SKB_WITH_OVERHEAD(1024), use the
> > > page_small cache with order 0 allocation.
> > > (all the above constrained to host with 4K pages)
> > >
> > > I'm not quite sure about the "never have more than 4 frags per page"
> > > part.
> > >
> > > What outlined above will allow for 10 min size frags in page_order0, as
> > > (SKB_DATA_ALIGN(0) + SKB_DATA_ALIGN(struct skb_shared_info) == 384. I'm
> > > not sure that anything will allocate such small frags.
> > > With a more reasonable GRO_MAX_HEAD, there will be 6 frags per page.
> >
> > Well, some arches have PAGE_SIZE=65536 :/
>
> Yes, the idea is to implement all the above only for arches with
> PAGE_SIZE==4K. Would that be reasonable?

Well, we also have changed MAX_SKB_FRAGS from 17 to 45 for BIG TCP.

And locally we have

#define GRO_MAX_HEAD 192

Reference:

commit fd9ea57f4e9514f9d0f0dec505eefd99a8faa148
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Wed Jun 8 09:04:38 2022 -0700

    net: add napi_get_frags_check() helper


>
> Thanks!
>
> Paolo
>