netdev - Re: question about using UDP GSO in Linux kernel 4.19

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Message-ID: <CAEjGaqdo_6watKcGi1WUmrHiB9F=1+i+8LcxBXOMZvLneiEh7A@mail.gmail.com>
Date:   Sun, 26 Jul 2020 11:17:53 -0700
From:   Han <keepsimple@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>
Subject: Re: question about using UDP GSO in Linux kernel 4.19

On Sun, Jul 26, 2020 at 6:42 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Sat, Jul 25, 2020 at 7:08 PM Han <keepsimple@...il.com> wrote:
> >
> > My apologies if this is not the right place to ask this question.
> >
> > I'm trying to use UDP GSO to improve the throughput. My testing shows
> > that UDP GSO works with the local server (i.e. loopback interface) but
> > fails with a remote server (in WLAN, via wlan0 interface).
> >
> > My question is: do I need to explicitly enable UDP GSO for wlan0
> > interface? If yes, how do I do it? I searched online but could not
> > find a good answer.  I looked at "ethtool" but not clear which option
> > to use:
> >
> > $ ethtool  --show-offload wlan0 | grep -i generic-segment
> > generic-segmentation-offload: off [requested on]
>
> Which wireless driver does your device use. Does it have tx checksum offload?

It seems to be "brcmfmac" :

$ readlink /sys/class/net/wlan0/device/driver
../../../../../../../../bus/sdio/drivers/brcmfmac

I think tx checksum offload is off, and I couldn't turn it on. Does
"[fixed]" mean it cannot be changed?

$ ethtool  --show-offload wlan0 | grep -i sum
rx-checksumming: off [fixed]
tx-checksumming: off
        tx-checksum-ipv4: off [fixed]
        tx-checksum-ip-generic: off [fixed]
        tx-checksum-ipv6: off [fixed]
        tx-checksum-fcoe-crc: off [fixed]
        tx-checksum-sctp: off [fixed]
tx-gre-csum-segmentation: off [fixed]
tx-udp_tnl-csum-segmentation: off [fixed]
esp-tx-csum-hw-offload: off [fixed]

Tried this but didn't work:

$ sudo ethtool --offload wlan0 tx on
Cannot change tx-checksumming
Could not change any device features

$ sudo ethtool -K wlan0 tx-checksum-ipv4 on
Could not change any device features

> That is a hard requirement. In udp_send_skb:
>
>                 if (skb->ip_summed != CHECKSUM_PARTIAL || is_udplite ||
>                     dst_xfrm(skb_dst(skb))) {
>                         kfree_skb(skb);
>                         return -EIO;
>                 }
>
> > $ ethtool  --show-offload wlan0 | grep -i udp-segment
> > tx-udp-segmentation: off [fixed]
>
> This is hardware segmentation offload. It is not required.