Re: [PATCH net-next] tcp: introduce skb_rbtree_walk_safe() and use it in tcp_clean_rtx_queue()

[Yafang Shao]

On Wed, Nov 28, 2018 at 10:44 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>
>
>
> On 11/28/2018 04:16 AM, Yafang Shao wrote:
> > When walk RB tree, we'd better use skb_rbtree_walk* helpers, that can make
> > the code more clear.
> > As skb_rbtree_walk() can't be used in tcp_clean_rtx_queue(), so the new
> > helper skb_rbtree_walk_safe() is introduced.
>
> This makes things slower. Let me explain inline.
>
> >
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> >  include/linux/skbuff.h | 5 +++++
> >  net/ipv4/tcp_input.c   | 5 ++---
> >  2 files changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 73902ac..37ff792 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -3256,6 +3256,11 @@ static inline int __skb_grow_rcsum(struct sk_buff *skb, unsigned int len)
> >               for (skb = skb_rb_first(root); skb != NULL;                     \
> >                    skb = skb_rb_next(skb))
> >
> > +#define skb_rbtree_walk_safe(skb, root, tmp)                                 \
> > +             for (skb = skb_rb_first(root);                                  \
> > +                  tmp = skb ? skb_rb_next(skb) : NULL, skb != NULL;          \
> > +                  skb = tmp)
> > +
> >  #define skb_rbtree_walk_from(skb)                                            \
> >               for (; skb != NULL;                                             \
> >                    skb = skb_rb_next(skb))
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index f323978..ab6add2 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -3043,7 +3043,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
> >       struct tcp_sock *tp = tcp_sk(sk);
> >       u32 prior_sacked = tp->sacked_out;
> >       u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
> > -     struct sk_buff *skb, *next;
> > +     struct sk_buff *skb, *tmp;
> >       bool fully_acked = true;
> >       long sack_rtt_us = -1L;
> >       long seq_rtt_us = -1L;
> > @@ -3055,7 +3055,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
> >
> >       first_ackt = 0;
> >
> > -     for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
> > +     skb_rbtree_walk_safe(skb, &sk->tcp_rtx_queue, tmp) {
> >               struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
> >               const u32 start_seq = scb->seq;
> >               u8 sacked = scb->sacked;
> > @@ -3126,7 +3126,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
> >               if (!fully_acked)
> >                       break;
> >
> > -             next = skb_rb_next(skb);
>
> We call skb_rb_next() here only, not at the beginning of the loop.
>
> Why ?
>
> Because we can break of the loop if the current skb is not fully acked.
>
> So your patch would add unnecessary overhead, since the extra sk_rb_next()
> could add more extra cache line misses.
>

I thought this extra sk_rb_next() doesn't add much overhead before,
since it won't take long time to execute.
Seems I made a mistake.

Thanks
Yafang