tcp: pass maxseg around instead of locally calculating anew

[rscheff]

Summary

Improve slowpath processing (reordering, retransmissions)
slightly by passing maxseg, once calculated, around. This typically
saves one of two calls to tcp_maxseg() which is relatively complex.

[glebius]

I'm fine with the patch as is, but here is some idea you may consider.

Current code will call either tcp_sack_partialack() or tcp_do_prr_ack() and only once. It is:

if (foo) {
                if (bar)
                                tcp_do_prr_ack(tp, th, &to, sack_changed,
                                    (maxseg ? maxseg : tcp_maxseg(tp)));
                else
                                tcp_sack_partialack(tp, th,
                                    (maxseg ? maxseg : tcp_maxseg(tp)));
} else if (baz)
                        tcp_do_prr_ack(tp, th, &to, SACK_CHANGE,
                            (maxseg ? maxseg : tcp_maxseg(tp)));

However, we might have another function interested in maxseg or either of the existing functions called twice. If that happens we are doing two calls to tcp_maxseg() again. So alternative code would be a prototype that passes pointer to maxseg on the stack of caller and allows callee to update it if it is 0.

void
tcp_do_prr_ack(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to, sackstatus_t sack_changed, u_int *maxseg)
{
         if (*maxseg == 0)
                *maxseg = tcp_maxseg(tp)

[rscheff]

• update uninitialized maxseg for posterity

[rscheff]

• update uninitialized maxseg for posterity (no ptrs)

[rscheff]

• go back to passing a pointer but don't touch body of functions

[cc]

sys/netinet/tcp_input.c

1532

The "maxseg" won't be assigned a value after line#2793. So I am wondering why not initialize/zero it in the beginning. There may be techniques that make sure it is always zero, but at least zero it here removes uncertainty.

2792

The "maxseg", in my understanding, won't be assigned a value after line#2793.

[rscheff]

• properly initialize maxseg again

[rscheff]

sys/netinet/tcp_input.c

1532

Thanks. This got somehow dropped between revisions.