On Tue, 2016-09-27 at 09:06 -0700, Lawrence Brakmo wrote:
> The current code changes txhash (flowlables) on every retransmitted
> SYN/ACK, but only after the 2nd retransmitted SYN and only after
> tcp_retries1 RTO retransmits.
>
> With this patch:
> 1) txhash is changed with every SYN retransmist
> 2) adds the option for the txhash to be changed before tcp_retries1
> RTO retransmits. A new sysctl tcp_rto_txhash_prob represents the
> probability that txhash will be changed. The default value is 0
> which maintains previous behavior and a value of 100 will always
> change it.
...
> @@ -213,6 +215,11 @@ static int tcp_write_timeout(struct sock *sk)
> tcp_mtu_probing(icsk, sk);
>
> dst_negative_advice(sk);
> + } else if (sk->sk_txhash && sysctl_tcp_rto_txhash_prob > 0) {
> + u32 v = prandom_u32() % 100;
> +
> + if (v < sysctl_tcp_rto_txhash_prob)
> + sk_set_txhash(sk);
>
Write timeouts are rare events and sk_txhash should already be set.
Also prandom_u32_max() is a bit faster (no divide, and even no multiply
for 100 which is 5*5*4, compiler can emit 2 lea instructions )
I would suggest not using all these tests and instead be very simple :
} else if (prandom_u32_max(100) < sysctl_tcp_rto_txhash_prob) {
sk_set_txhash(sk);
}
Finally, adding a new sysctl requires a Documentation update
( Documentation/networking/ip-sysctl.txt )
Thanks.
