On Thu, Jul 29, 2021 at 08:44:25PM +0200, Matthieu Baerts wrote:
> Commit 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny skbs")
> introduces a ~10% performance drop when using virtio-net drivers.
>
> This commit has been backported to v4.19 in commit 669c0b5782fb and this
> performance drop is also visible there.
> Here at Tessares, we can also notice this drop with the MPTCP fork [1]
> on top of the v4.19 kernel.
>
> Eric Dumazet already fixed this issue a few months ago, see
> commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head").
>
> Unfortunately, this patch has not been backported to < v5.4 because it
> caused issues [2]. Indeed, after having backported it, the kernel failed
> to compile because one commit was missing, see
> commit 503d539a6e41 ("virtio_net: Add XDP meta data support"). However,
> this missing commit has been added in 4.19.186 but probably because
> there were still some opened discussions [3] around
> commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head"),
> the latter has not been backported at all in v4.19.
>
> A cherry-pick of this patch without any modification is proposed here.
> It has been validated: it fixes the original issue on v4.19 as well.
>
> Please note that there is also a fix for the fix, see
> commit 38ec4944b593 ("gro: ensure frag0 meets IP header alignment").
>
> This second fix has also not been backported because it caused issues as
> well [4]. Here, it was due to a conflict but also a compilation error
> when the conflict has been resolved. Please refer to patch 2/2 for more
> details.
>
> One last note: these two patches have also been backported and validated
> on a v4.14 release. A second series is going to be sent.
> It looks like it could be interesting to backport these two patches to
> v4.9 and v4.4 as well but unfortunately, the backport of these two
> patches fails with conflicts and I don't have any setup to validate the
> performance drop and fix with v4.9 and v4.4 kernels.
Both sets of series now queued up, thanks!
greg k-h
