netdev - Re: [PATCH net-next] sctp: define sctp_packet_gso

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Message-ID: <20180614113527.GA6733@hmswarspite.think-freely.org>
Date:   Thu, 14 Jun 2018 07:35:27 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        davem <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] sctp: define sctp_packet_gso_append to build
 GSO frames

On Thu, Jun 14, 2018 at 09:21:52AM +0800, Xin Long wrote:
> On Thu, Jun 14, 2018 at 8:46 AM, Neil Horman <nhorman@...driver.com> wrote:
> > On Thu, Jun 14, 2018 at 07:37:02AM +0800, Xin Long wrote:
> >> Now sctp GSO uses skb_gro_receive() to append the data into head
> >> skb frag_list. However it actually only needs very few code from
> >> skb_gro_receive(). Besides, NAPI_GRO_CB has to be set while most
> >> of its members are not needed here.
> >>
> >> This patch is to add sctp_packet_gso_append() to build GSO frames
> >> instead of skb_gro_receive(), and it would avoid many unnecessary
> >> checks and make the code clearer.
> >>
> >> Note that sctp will use page frags instead of frag_list to build
> >> GSO frames in another patch. But it may take time, as sctp's GSO
> >> frames may have different size. skb_segment() can only split it
> >> into the frags with the same size, which would break the border
> >> of sctp chunks.
> >>
> >> Signed-off-by: Xin Long <lucien.xin@...il.com>
> > Do you have any performance numbers to compare with and without this patch?
> > Adding a function like this implies that any fixes that go into skb_gro_receive
> > now need to be evaluated for this function too, which means theres an implied
> > overhead in maintaining it.  If we're signing up for that, I'd like to know that
> > theres a significant performance benefit.
> Hi Neil,
> 
> I don't think there's a noticeable performance benefit since it's
> just avoided some checks and variables settings.
> 
> The new function makes SCTP GSO code clearer and readable,
> as skb_gro_receive() should only be used in the GRO code paths,
> it's confusing in sctp tx path.
> 
> We're doing this, actually because skb_gro_receive() is being
> changed now, it would not be suitable for SCTP GSO, see:
> https://www.spinics.net/lists/netdev/msg507716.html
> 
Ok, I'm not on board if the only reason was to improve readability, as I didn't
want to maintain two separate code paths, but if skb_gro_receive isn't going to
be useable, then I'm ok with it

Acked-by: Neil Horman <nhorman@...driver.com>