Re: [RFCv2 1/3] tcp: Use smaller mtu probes if RACK is enabled

Leonard Crestez
2021-06-03T13:37:30Z

On 5/26/21 3:11 PM, Neal Cardwell wrote:
> On Wed, May 26, 2021 at 6:38 AM Leonard Crestez <cdleonard@gmail.com> wrote:
>>
>> RACK allows detecting a loss in rtt + min_rtt / 4 based on just one
>> extra packet. If enabled use this instead of relying of fast retransmit.
>
> IMHO it would be worth adding some more text to motivate the change,
> to justify the added complexity and risk from the change. The
> substance of the change seems to be decreasing the requirement for
> PMTU probing from needing roughly 5 packets worth of data to needing
> roughly 3 packets worth of data. It's not clear to me as a reader of
> this patch by itself that there are lots of applications that very
> often only have 3-4 packets worth of data to send and yet can benefit
> greatly from PMTU discovery.
>
>> Suggested-by: Neal Cardwell <ncardwell@google.com>
>> Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
>> ---
>> Documentation/networking/ip-sysctl.rst | 5 +++++
>> include/net/netns/ipv4.h | 1 +
>> net/ipv4/sysctl_net_ipv4.c | 7 +++++++
>> net/ipv4/tcp_ipv4.c | 1 +
>> net/ipv4/tcp_output.c | 26 +++++++++++++++++++++++++-
>> 5 files changed, 39 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
>> index a5c250044500..7ab52a105a5d 100644
>> --- a/Documentation/networking/ip-sysctl.rst
>> +++ b/Documentation/networking/ip-sysctl.rst
>> @@ -349,10 +349,15 @@ tcp_mtu_probe_floor - INTEGER
>> If MTU probing is enabled this caps the minimum MSS used for search_low
>> for the connection.
>>
>> Default : 48
>>
>> +tcp_mtu_probe_rack - BOOLEAN
>> + Try to use shorter probes if RACK is also enabled
>> +
>> + Default: 1
>
> I would vote to not have a sysctl for this. If we think it's a good
> idea to allow MTU probing with a smaller amount of data if RACK is
> enabled (which seems true to me), then this is a low-risk enough
> change that we should just change the behavior.
>
>> tcp_min_snd_mss - INTEGER
>> TCP SYN and SYNACK messages usually advertise an ADVMSS option,
>> as described in RFC 1122 and RFC 6691.
>>
>> If this ADVMSS option is smaller than tcp_min_snd_mss,
>> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
>> index 746c80cd4257..b4ff12f25a7f 100644
>> --- a/include/net/netns/ipv4.h
>> +++ b/include/net/netns/ipv4.h
>> @@ -112,10 +112,11 @@ struct netns_ipv4 {
>> #ifdef CONFIG_NET_L3_MASTER_DEV
>> u8 sysctl_tcp_l3mdev_accept;
>> #endif
>> u8 sysctl_tcp_mtu_probing;
>> int sysctl_tcp_mtu_probe_floor;
>> + int sysctl_tcp_mtu_probe_rack;
>> int sysctl_tcp_base_mss;
>> int sysctl_tcp_min_snd_mss;
>> int sysctl_tcp_probe_threshold;
>> u32 sysctl_tcp_probe_interval;
>>
>> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
>> index 4fa77f182dcb..275c91fb9cf8 100644
>> --- a/net/ipv4/sysctl_net_ipv4.c
>> +++ b/net/ipv4/sysctl_net_ipv4.c
>> @@ -847,10 +847,17 @@ static struct ctl_table ipv4_net_table[] = {
>> .mode = 0644,
>> .proc_handler = proc_dointvec_minmax,
>> .extra1 = &tcp_min_snd_mss_min,
>> .extra2 = &tcp_min_snd_mss_max,
>> },
>> + {
>> + .procname = "tcp_mtu_probe_rack",
>> + .data = &init_net.ipv4.sysctl_tcp_mtu_probe_rack,
>> + .maxlen = sizeof(int),
>> + .mode = 0644,
>> + .proc_handler = proc_dointvec,
>> + },
>> {
>> .procname = "tcp_probe_threshold",
>> .data = &init_net.ipv4.sysctl_tcp_probe_threshold,
>> .maxlen = sizeof(int),
>> .mode = 0644,
>> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
>> index 4f5b68a90be9..ed8af4a7325b 100644
>> --- a/net/ipv4/tcp_ipv4.c
>> +++ b/net/ipv4/tcp_ipv4.c
>> @@ -2892,10 +2892,11 @@ static int __net_init tcp_sk_init(struct net *net)
>> net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;
>> net->ipv4.sysctl_tcp_min_snd_mss = TCP_MIN_SND_MSS;
>> net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;
>> net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL;
>> net->ipv4.sysctl_tcp_mtu_probe_floor = TCP_MIN_SND_MSS;
>> + net->ipv4.sysctl_tcp_mtu_probe_rack = 1;
>>
>> net->ipv4.sysctl_tcp_keepalive_time = TCP_KEEPALIVE_TIME;
>> net->ipv4.sysctl_tcp_keepalive_probes = TCP_KEEPALIVE_PROBES;
>> net->ipv4.sysctl_tcp_keepalive_intvl = TCP_KEEPALIVE_INTVL;
>>
>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> index bde781f46b41..9691f435477b 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -2311,10 +2311,19 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
>> }
>>
>> return true;
>> }
>>
>> +/* Check if rack is supported for current connection */
>> +static int tcp_mtu_probe_is_rack(const struct sock *sk)
>> +{
>> + struct net *net = sock_net(sk);
>> +
>> + return (net->ipv4.sysctl_tcp_recovery & TCP_RACK_LOSS_DETECTION &&
>> + net->ipv4.sysctl_tcp_mtu_probe_rack);
>> +}
>
> You may want to use the existing helper, tcp_is_rack(), by moving it
> to include/net/tcp.h

OK, for this and other comments.

Initially I though that maybe a more elaborate check is required but it
seems to be only up to the sender to keep individual timeouts.

--
Regards,
Leonard