question about using UDP GSO in Linux kernel 4.19

question about using UDP GSO in Linux kernel 4.19

[Han]

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]

$ ethtool  --show-offload wlan0 | grep -i udp-segment
tx-udp-segmentation: off [fixed]

A quick try did not work:

$ sudo ethtool -K wlan0 gso on
Could not change any device features

My test hardware is Raspberry Pi 4, and the Linux kernel version is
4.19. My test program is in C.

Thanks.
Han

Re: question about using UDP GSO in Linux kernel 4.19

[Willem de Bruijn]

On Sat, Jul 25, 2020 at 7:08 PM Han <keepsimple@gmail.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?
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.

Re: question about using UDP GSO in Linux kernel 4.19

[Han]

On Sun, Jul 26, 2020 at 6:42 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Sat, Jul 25, 2020 at 7:08 PM Han <keepsimple@gmail.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.

Re: question about using UDP GSO in Linux kernel 4.19

[Willem de Bruijn]

On Sun, Jul 26, 2020 at 2:18 PM Han <keepsimple@gmail.com> wrote:
>
> On Sun, Jul 26, 2020 at 6:42 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > On Sat, Jul 25, 2020 at 7:08 PM Han <keepsimple@gmail.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?

Indeed. That explains it.