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

[Yuchung Cheng]

On Wed, May 26, 2021 at 5:11 AM Neal Cardwell <ncardwell@xxxxxxxxxx> wrote:
>
> On Wed, May 26, 2021 at 6:38 AM Leonard Crestez <cdleonard@xxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Leonard Crestez <cdleonard@xxxxxxxxx>
> > ---
> >  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.
+1 to not have another sysctl

>
> >  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
>
> thanks,
> neal