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SubjectRe: [PATCH net-next 0/5] skbuff: introduce skbuff_heads bulking and reusing
On Tue, Jan 12, 2021 at 7:26 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> From: Eric Dumazet <edumazet@google.com>
> Date: Tue, 12 Jan 2021 13:32:56 +0100
>
> > On Tue, Jan 12, 2021 at 11:56 AM Alexander Lobakin <alobakin@pm.me> wrote:
> >>
> >
> >>
> >> Ah, I should've mentioned that I use UDP GRO Fraglists, so these
> >> numbers are for GRO.
> >>
> >
> > Right, this suggests UDP GRO fraglist is a pathological case of GRO,
> > not saving memory.
> >
> > Real GRO (TCP in most cases) will consume one skb, and have page
> > fragments for each segment.
> >
> > Having skbs linked together is not cache friendly.
>
> OK, so I rebased test setup a bit to clarify the things out.
>
> I disabled fraglists and GRO/GSO fraglists support advertisement
> in driver to exclude any "pathological" cases and switched it
> from napi_get_frags() + napi_gro_frags() to napi_alloc_skb() +
> napi_gro_receive() to disable local skb reusing (napi_reuse_skb()).
> I also enabled GSO UDP L4 ("classic" one: one skbuff_head + frags)
> for forwarding, not only local traffic, and disabled NF flow offload
> to increase CPU loading and drop performance below link speed so I
> could see the changes.
>
> So, the traffic flows looked like:
> - TCP GRO (one head + frags) -> NAT -> hardware TSO;
> - UDP GRO (one head + frags) -> NAT -> driver-side GSO.
>
> Baseline 5.11-rc3:
> - 865 Mbps TCP, 866 Mbps UDP.
>
> This patch (both separate caches and Edward's unified cache):
> - 899 Mbps TCP, 893 Mbps UDP.
>
> So that's cleary *not* only "pathological" UDP GRO Fraglists
> "problem" as TCP also got ~35 Mbps from this, as well as
> non-fraglisted UDP.
>
> Regarding latencies: I remember there were talks about latencies when
> Edward introduced batched GRO (using linked lists to pass skbs from
> GRO layer to core stack instead of passing one by one), so I think
> it's a perennial question when it comes to batching/caching.
>
> Thanks for the feedback, will post v2 soon.
> The question about if this caching is reasonable isn't closed anyway,
> but I don't see significant "cons" for now.
>

Also it would be nice to have KASAN support.

We do not want to unconditionally to recycle stuff, since this might
hide use-after-free.

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