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

[Eric Dumazet <edumazet@google.com>]

On Thu, Sep 8, 2022 at 7:26 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> On Thu, 2022-09-08 at 05:20 -0700, Eric Dumazet wrote:
> > On Thu, Sep 8, 2022 at 3:48 AM Paolo Abeni <pabeni@redhat.com> wrote:
> > > On Wed, 2022-09-07 at 13:40 -0700, Eric Dumazet wrote:
> > > > On 9/7/22 13:19, Paolo Abeni wrote:
> > > > > 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
>
> default allocation size for napi_get_frags() is ~960b in google kernel,
> right? It looks like it should fit the above quite nicely with 4 frags
> per page?!?
>

Yes, using order-0 pages on x86 would avoid problems.
But if this adds yet another tests in fast path, increasing icache
pressure, I am unsure.
So I will comment when I see actual code/implementation.

("Extending" page_frag_cache alloc API seems overkill to me. Just use
separate code maybe ?)

> Vanilla kernel may hit a larger number of fragments per page, even if
> very likely not as high as the theoretical maximum mentioned in my
> previous email (as noted by Alex).
>
> If in that case excessive truesize underestimation would still be
> problematic (with a order0 4k page) __napi_alloc_skb() could be patched
> to increase smaller sizes to some reasonable minimum.
>
> Likely there is some point in your reply I did not get. Luckily LPC is
> coming :)
>
> > Reference:
> >
> > commit fd9ea57f4e9514f9d0f0dec505eefd99a8faa148
> > Author: Eric Dumazet <edumazet@google.com>
> > Date:   Wed Jun 8 09:04:38 2022 -0700
> >
> >     net: add napi_get_frags_check() helper
>
> I guess such check should be revisited with all the above.
>
> Thanks,
>
> Paolo
>