Re: [PATCH] tcp: do not update snd_una if it is same with ack_seq

[Yafang Shao]

On Sun, Nov 4, 2018 at 8:40 AM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>
>
>
> On 11/03/2018 09:54 AM, Yafang Shao wrote:
> > In the slow path, TCP_SKB_SB(skb)->ack_seq may be same with tp->snd_una,
> > and under this condition we don't need to update the snd_una.
> >
> > Furthermore, tcp_ack_update_window() is only called in slow path,
> > so introducing this check won't affect the fast path processing.
> >
> > By the way, '&' is a little faster than '-', so I replaced after() with
> > "flag & FLAG_SND_UNA_ADVANCED".
> >
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> >  net/ipv4/tcp_input.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 2868ef2..db5a6b7 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -3376,7 +3376,8 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
> >               }
> >       }
> >
> > -     tcp_snd_una_update(tp, ack);
> > +     if (after(ack, tp->snd_una))
> > +             tcp_snd_una_update(tp, ack);
> >
>
> Adding this after() here is confusing, how ack could be before snd_una ?
> That would be a serious bug.
>

ack can't be before snd_una, but it can be equal with snd_una.
Seems bellow change would be more specific,
    if (ack != tp->snd_una)
       tcp_snd_una_update(tp, ack);

> I do not see why another conditional has any gain here.
>
> You are trying to avoid very cheap operations :
>
>     u32 delta = ack - tp->snd_una;
>
>     tp->bytes_acked += delta;
>     tp->snd_una = ack;
>
> Maybe the real reason for this patch is not explained in the changelog ?

No additional reason. I just want to avoid these uneccessary operations.
Because I find that this uncessary operations always happen,
especially when head prediction fails and then the packet can't go to
fast path processing.

Thanks
Yafang