netdev - Performance regression with virtio

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Date:   Thu, 27 Jul 2017 12:09:42 -0500
From:   Seth Forshee <seth.forshee@...onical.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Performance regression with virtio_net

I'm seeing a performance regression with virtio_net that looks to have
started in 4.12-rc1. I only see it in one context though, downloading
snap packages from the Ubuntu snap store. For example:

 https://api.snapcraft.io/api/v1/snaps/download/b8X2psL1ryVrPt5WEmpYiqfr5emixTd7_1797.snap

which redirects to Internap's CDN. Normally this downloads in a few
seconds at ~10 MB/s, but with 4.12 and 4.13 it takes minutes with a rate
of ~150 KB/s. Everything else I've tried downloads as normal speeds.

I bisected this to 680557cf79f8 "virtio_net: rework mergeable buffer
handling". If I revert this on top of 4.13-rc2 (along with other changes
needed to successfully revert it) speeds return to normal.

Thanks,
Seth