[Rpm] [Bloat] Relentless congestion control for testing purposes
Bob McMahon
bob.mcmahon at broadcom.com
Fri Oct 1 12:32:32 EDT 2021
hmm, this looks interesting to a test & measurement guy. Can it be done
with a setsockopt? I might want to add this as an iperf2 option,
particularly if it's broadly available,
Thanks,
Bob
On Fri, Oct 1, 2021 at 9:22 AM Luigi Rizzo <rizzo at iet.unipi.it> wrote:
> On Wed, Sep 29, 2021 at 1:17 AM Dave Taht <dave.taht at gmail.com> wrote:
> >
> > In today's rpm meeting I didn't quite manage to make a complicated
> > point. This long-ago proposal
> > of matt mathis's has often intrigued (inspired? frightened?) me:
> >
> >
> https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-relentless-tcp-00
> >
> > where he proposed that a tcp variant have no response at all to loss
> > or markings, merely
> > replacing lost segments as they are requested, continually ramping up
> > until the network
> > basically explodes.
>
>
> For a similar purpose, I use the following patch that ignores holes,
> thus mostly defeating congestion control.
>
> The nice thing is that you can just count sent and received bytes
> in the application to estimate losses
>
> commit 9c429c9644f2fd22d5fe2b6f2d4df6fb2a8962b2
> Author: Luigi Rizzo <lrizzo at google.com>
> Date: Fri Oct 1 09:10:44 2021 -0700
>
> test: module parameter to ignore holes in TCP
>
> Sometimes, for testing, it is useful to let the TCP receiver ignore
> drops (and defeat congestion control) and accept all packets as if they
> were in sequence. This will show whether a connection is sender or
> receiver throttled.
>
> This patch implements the above with 3
> /sys/module/tcp_input/parameters :
> - lossy_local_port, lossy_remote_port
> if non zero, indicate that sockets matching one of these ports
> will be set to ignore drops. (This socker flag could be set with a
> setsockopt(), but would require changes in the caller);
> - drop_freq
> if non zero, one every drop_freq packets will be artificially
> dropped on the receive side.
>
> Example
> echo 2345 > /sys/module/tcp_input/parameters/lossy_local_port
> echo 10 > /sys/module/tcp_input/parameters/drop_freq # drop one in 10
>
> ifconfig lo mtu 600
> nc -l 2345 > /tmp/a &
> MSG="this is a test this is a test this is a test count is"
> (for ((i=0;i<10000;i++)); do echo "${MSG} ${i}; $i"; done) | nc
> 127.0.0.1 2345
>
> The output file will have missing lines
>
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 48d8a363319e..3bb8888a56af 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -225,7 +225,8 @@ struct tcp_sock {
> u8 compressed_ack;
> u8 dup_ack_counter:2,
> tlp_retrans:1, /* TLP is a retransmission */
> - unused:5;
> + ignore_holes:1, /* ignore holes on rx. test only */
> + unused:4;
> u32 chrono_start; /* Start time in jiffies of a TCP chrono */
> u32 chrono_stat[3]; /* Time in jiffies for chrono_stat stats */
> u8 chrono_type:2, /* current chronograph type */
> @@ -250,6 +251,7 @@ struct tcp_sock {
> u32 tlp_high_seq; /* snd_nxt at the time of TLP */
>
> u32 tcp_tx_delay; /* delay (in usec) added to TX packets */
> + u32 tcp_test_drops; /* artificial packet drops. test only */
> u64 tcp_wstamp_ns; /* departure time for next sent data
> packet */
> u64 tcp_clock_cache; /* cache last tcp_clock_ns() (see
> tcp_mstamp_refresh()) */
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 414c179c28e0..fd770a176a68 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2344,6 +2344,9 @@ static int tcp_recvmsg_locked(struct sock *sk,
> struct msghdr *msg, size_t len,
> last = skb_peek_tail(&sk->sk_receive_queue);
> skb_queue_walk(&sk->sk_receive_queue, skb) {
> last = skb;
> + /* XXX if we allow holes, update copied_seq */
> + if (tp->ignore_holes && before(*seq,
> TCP_SKB_CB(skb)->seq))
> + *seq = TCP_SKB_CB(skb)->seq;
> /* Now that we have two receive queues this
> * shouldn't happen.
> */
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 246ab7b5e857..a5161ce78171 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -81,6 +81,14 @@
> #include <net/busy_poll.h>
> #include <net/mptcp.h>
>
> +#include <linux/module.h>
> +static int drop_freq; /* drop one pkt ever this many */
> +module_param(drop_freq, int, 0644);
> +static int lossy_local_port = 2345; /* drop pkts, ignore holes on this
> port */
> +module_param(lossy_local_port, int, 0644);
> +static int lossy_remote_port = 0; /* drop pkts, ignore holes on this port
> */
> +module_param(lossy_remote_port, int, 0644);
> +
> int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
>
> #define FLAG_DATA 0x01 /* Incoming frame contained data.
> */
> @@ -5806,6 +5814,16 @@ void tcp_rcv_established(struct sock *sk,
> struct sk_buff *skb)
>
> tp->rx_opt.saw_tstamp = 0;
>
> + if (tp->ignore_holes) {
> + const u32 seq = TCP_SKB_CB(skb)->seq;
> + if (drop_freq && tp->tcp_test_drops++ >= drop_freq) {
> + tp->tcp_test_drops = 0;
> + goto discard; /* artificial drop */
> + }
> + if (after(seq, tp->rcv_nxt)) /* Pretend this is in order */
> + tcp_rcv_nxt_update(tp, seq);
> + }
> +
> /* pred_flags is 0xS?10 << 16 + snd_wnd
> * if header_prediction is to be made
> * 'S' will always be tp->tcp_header_len >> 2
> @@ -5986,12 +6004,25 @@ void tcp_init_transfer(struct sock *sk, int
> bpf_op, struct sk_buff *skb)
> tcp_init_buffer_space(sk);
> }
>
> +/* Testing. Conditionally set tp->ignore_holes. Should be a setsockopt */
> +static void tcp_update_ignore_holes(struct sock *sk)
> +{
> + if (READ_ONCE(lossy_local_port) == ntohs(inet_sk(sk)->inet_sport)
> ||
> + READ_ONCE(lossy_remote_port) ==
> ntohs(inet_sk(sk)->inet_dport)) {
> + pr_info("XXX ignore holes for ports local %d remote %d\n",
> + ntohs(inet_sk(sk)->inet_sport),
> + ntohs(inet_sk(sk)->inet_dport));
> + tcp_sk(sk)->ignore_holes = 1;
> + }
> +}
> +
> void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
> {
> struct tcp_sock *tp = tcp_sk(sk);
> struct inet_connection_sock *icsk = inet_csk(sk);
>
> tcp_set_state(sk, TCP_ESTABLISHED);
> + tcp_update_ignore_holes(sk);
> icsk->icsk_ack.lrcvtime = tcp_jiffies32;
>
> if (skb) {
> @@ -6476,6 +6507,7 @@ int tcp_rcv_state_process(struct sock *sk,
> struct sk_buff *skb)
> }
> smp_mb();
> tcp_set_state(sk, TCP_ESTABLISHED);
> + tcp_update_ignore_holes(sk);
> sk->sk_state_change(sk);
>
> /* Note, that this wakeup is only for marginal crossed SYN
> case.
>
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