netdev - Re: [PATCH RFC 0/2] veth, bridge, and GSO maximums

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Date:   Thu, 30 Nov 2017 12:50:03 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, sthemmin@...rosoft.com
Subject: Re: [PATCH RFC 0/2] veth, bridge, and GSO maximums

On Thu, Nov 30, 2017 at 9:11 AM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Thu, 30 Nov 2017 10:47:21 -0500 (EST)
> David Miller <davem@...emloft.net> wrote:
>
>> From: Stephen Hemminger <stephen@...workplumber.org>
>> Date: Sun, 26 Nov 2017 10:17:47 -0800
>>
>> > This pair of patchesimproves the performance when running
>> > containers in an environment where underlying device has lower
>> > GSO maximum (such as Azure).
>> >
>> > With containers a veth pair is created and one end is attached
>> > to the bridge device. The bridge device correctly reports
>> > computes GSO parameters that are the minimum of the lower devices.
>> >
>> > The problem is that the other end of the veth device (in container)
>> > reports the full GSO size. This patch propogates the upper
>> > (bridge device) parameters to the other end of the veth device.
>> >
>> > Please consider it as alternative to the sysfs GSO changes.
>>
>> I like this approach a lot, please resubmit this formally.
>
> Will do and add netif_needs_gso check as well.

Would it make sense to look at possibly moving something like this
over to being handled by something like GSO_PARTIAL? Essentially it is
the same type of issue where we are needing to split a TSO frame into
smaller TSO frames. We already did something like this for the GRO
with skb->fraglist case, I wonder if it wouldn't make sense to just
handle the oversized GSO the same way. It seems like you could just
split add a check against the size and tweak the mss at the start of
skb_segment and that should be about all you need to do to take care
of it.

If I am not mistaken we could probably just tweak the one line that
was computing "partial_segs" in skb_segment so that we did something
like "partial_segs = min(len, dev->gso_max_size) / mss".

This way we take care of the issue for things like GRO as well which
doesn't take the max size into account last I knew, and you would
still get most of the benefits of TSO.

- Alex