On Tue, 12 Jul 2022 21:52:25 +0100 Pavel Begunkov wrote:
> Even when zerocopy transmission is requested and possible,
> __ip_append_data() will still copy a small chunk of data just because it
> allocated some extra linear space (e.g. 148 bytes). It wastes CPU cycles
> on copy and iter manipulations and also misalignes potentially aligned
> data. Avoid such coies. And as a bonus we can allocate smaller skb.
s/coies/copies/ can fix when applying
>
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> ---
> net/ipv4/ip_output.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 00b4bf26fd93..581d1e233260 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -969,7 +969,6 @@ static int __ip_append_data(struct sock *sk,
> struct inet_sock *inet = inet_sk(sk);
> struct ubuf_info *uarg = NULL;
> struct sk_buff *skb;
> -
> struct ip_options *opt = cork->opt;
> int hh_len;
> int exthdrlen;
> @@ -977,6 +976,7 @@ static int __ip_append_data(struct sock *sk,
> int copy;
> int err;
> int offset = 0;
> + bool zc = false;
> unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
> int csummode = CHECKSUM_NONE;
> struct rtable *rt = (struct rtable *)cork->dst;
> @@ -1025,6 +1025,7 @@ static int __ip_append_data(struct sock *sk,
> if (rt->dst.dev->features & NETIF_F_SG &&
> csummode == CHECKSUM_PARTIAL) {
> paged = true;
> + zc = true;
> } else {
> uarg->zerocopy = 0;
> skb_zcopy_set(skb, uarg, &extra_uref);
> @@ -1091,9 +1092,12 @@ static int __ip_append_data(struct sock *sk,
> (fraglen + alloc_extra < SKB_MAX_ALLOC ||
> !(rt->dst.dev->features & NETIF_F_SG)))
> alloclen = fraglen;
> - else {
> + else if (!zc) {
> alloclen = min_t(int, fraglen, MAX_HEADER);
Willem, I think this came in with your GSO work, is there a reason we
use MAX_HEADER here? I thought MAX_HEADER is for headers (i.e. more or
less to be reserved) not for the min amount of data to be included.
I wanna make sure we're not missing something about GSO here.
Otherwise I don't think we need the extra branch but that can
be a follow up.
> pagedlen = fraglen - alloclen;
> + } else {
> + alloclen = fragheaderlen + transhdrlen;
> + pagedlen = datalen - transhdrlen;
> }
>
> alloclen += alloc_extra;
