Performance loss due to commit 37c3185 ([NET]: Added GSO toggle)

Performance loss due to commit 37c3185 ([NET]: Added GSO toggle)

[christophe leroy]

Hello Herbert,

In commit "[NET]: Added GSO toggle" 
37c3185a02d4b85fbe134bf5204535405dd2c957,
you force NETIF_F_HW_CSUM if GSO feature is selected.
By default, SW GSO is active as soon as a network board has NETIF_F_SG 
feature.
This means that function sk_setup_caps() forces NETIF_F_HW_CSUM for any 
board having NETIF_F_SG

For boards having no HW checksum capability, this results in performance 
loss due to data copy being done in skb_do_copy_data_nocache() with 
copy_from_user() then checksum being done later with csum_partial() 
instead of getting both done at the same time using 
csum_and_copy_from_user()

Is there a reason for forcing NETIF_F_HW_CSUM   ?

Christophe

---
L'absence de virus dans ce courrier électronique a été vérifiée par le logiciel antivirus Avast.
https://www.avast.com/antivirus

Re: Performance loss due to commit 37c3185 ([NET]: Added GSO toggle)

[Herbert Xu]

On Sat, Jun 20, 2015 at 11:55:03AM +0200, christophe leroy wrote:
> Hello Herbert,
> 
> In commit "[NET]: Added GSO toggle"
> 37c3185a02d4b85fbe134bf5204535405dd2c957,
> you force NETIF_F_HW_CSUM if GSO feature is selected.
> By default, SW GSO is active as soon as a network board has NETIF_F_SG
> feature.
> This means that function sk_setup_caps() forces NETIF_F_HW_CSUM for any
> board having NETIF_F_SG
> 
> For boards having no HW checksum capability, this results in performance
> loss due to data copy being done in skb_do_copy_data_nocache() with
> copy_from_user() then checksum being done later with csum_partial()
> instead of getting both done at the same time using
> csum_and_copy_from_user()
> 
> Is there a reason for forcing NETIF_F_HW_CSUM   ?

Well GSO requires hardware checksum for all the protocols that
it supports.  I guess we should also check that the hardware can
checksum both IPv4 and IPv6 before enabling it.

However, the benefit of GSO should cancel out the cost of copying
so I was hoping to just enable GSO unconditionally at some point.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt