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Date: Fri, 9 Jun 2023 12:53:38 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <haiyangz@...rosoft.com>
CC: <ncardwell@...gle.com>, <atenart@...nel.org>, <bagasdotme@...il.com>,
<corbet@....net>, <davem@...emloft.net>, <dsahern@...nel.org>,
<edumazet@...gle.com>, <kuba@...nel.org>, <kuniyu@...zon.com>,
<kys@...rosoft.com>, <linux-doc@...r.kernel.org>,
<linux-hyperv@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<liushixin2@...wei.com>, <maheshb@...gle.com>, <netdev@...r.kernel.org>,
<olaf@...fle.de>, <pabeni@...hat.com>, <simon.horman@...igine.com>,
<soheil@...gle.com>, <stephen@...workplumber.org>,
<tim.gardner@...onical.com>, <vkuznets@...hat.com>, <weiwan@...gle.com>,
<ycheng@...gle.com>, <ykaliuta@...hat.com>
Subject: Re: [PATCH net-next] tcp: Make pingpong threshold tunable
From: Neal Cardwell <ncardwell@...gle.com>
Date: Fri, 9 Jun 2023 15:16:00 -0400
> On Fri, Jun 9, 2023 at 12:26 PM Haiyang Zhang <haiyangz@...rosoft.com> wrote:
>
> Regarding the patch title:
> > [PATCH net-next] tcp: Make pingpong threshold tunable
>
> There are many ways to make something tunable these days, including
> BPF, setsockopt(), and sysctl. :-) This patch only uses sysctl. Please
> consider a more clear/specific title, like:
>
> [PATCH net-next] tcp: set pingpong threshold via sysctl
>
> > TCP pingpong threshold is 1 by default. But some applications, like SQL DB
> > may prefer a higher pingpong threshold to activate delayed acks in quick
> > ack mode for better performance.
> >
> > The pingpong threshold and related code were changed to 3 in the year
> > 2019, and reverted to 1 in the year 2022.
>
> Please include the specific commit, like:
>
> The pingpong threshold and related code were changed to 3 in the year
> 2019 in:
> commit 4a41f453bedf ("tcp: change pingpong threshold to 3")
> and reverted to 1 in the year 2022 in:
> commit 4d8f24eeedc5 ("Revert "tcp: change pingpong threshold to 3"")
>
> Then please make sure to use scripts/checkpatch.pl on your resulting
> patch to check the formatting of the commit references, among other
> things.
>
> > There is no single value that
> > fits all applications.
> >
> > Add net.core.tcp_pingpong_thresh sysctl tunable,
>
> For consistency, TCP sysctls should be in net.ipv4 rather than
> net.core. Yes, that is awkward, given IPv6 support. But consistency is
> very important here. :-)
>
> > so it can be tuned for
> > optimal performance based on the application needs.
> >
> > Signed-off-by: Haiyang Zhang <haiyangz@...rosoft.com>
> > ---
> > Documentation/admin-guide/sysctl/net.rst | 8 ++++++++
> > include/net/inet_connection_sock.h | 14 +++++++++++---
> > net/core/sysctl_net_core.c | 9 +++++++++
> > net/ipv4/tcp.c | 2 ++
> > net/ipv4/tcp_output.c | 17 +++++++++++++++--
> > 5 files changed, 45 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
> > index 4877563241f3..16f54be9461f 100644
> > --- a/Documentation/admin-guide/sysctl/net.rst
> > +++ b/Documentation/admin-guide/sysctl/net.rst
> > @@ -413,6 +413,14 @@ historical importance.
> >
> > Default: 0
> >
> > +tcp_pingpong_thresh
> > +-------------------
> > +
> > +TCP pingpong threshold is 1 by default, but some application may need a higher
> > +threshold for optimal performance.
> > +
> > +Default: 1, min: 1, max: 3
>
> If we want to make this tunable, it seems sad to make the max 3. I'd
> suggest making the max 255, since we have 8 bits of space anyway in
> the inet_csk(sk)->icsk_ack.pingpong field.
>
> > +
> > 2. /proc/sys/net/unix - Parameters for Unix domain sockets
> > ----------------------------------------------------------
> >
> > diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> > index c2b15f7e5516..e84e33ddae49 100644
> > --- a/include/net/inet_connection_sock.h
> > +++ b/include/net/inet_connection_sock.h
> > @@ -324,11 +324,11 @@ void inet_csk_update_fastreuse(struct inet_bind_bucket *tb,
> >
> > struct dst_entry *inet_csk_update_pmtu(struct sock *sk, u32 mtu);
> >
> > -#define TCP_PINGPONG_THRESH 1
> > +extern int tcp_pingpong_thresh;
>
> To match most TCP sysctls, this should be per-namespace, rather than global.
Also, please change int to u8.
>
> Please follow a recent example by Eric, perhaps:
> 65466904b015f6eeb9225b51aeb29b01a1d4b59c
> tcp: adjust TSO packet sizes based on min_rtt
>
>
> >
> > static inline void inet_csk_enter_pingpong_mode(struct sock *sk)
> > {
> > - inet_csk(sk)->icsk_ack.pingpong = TCP_PINGPONG_THRESH;
> > + inet_csk(sk)->icsk_ack.pingpong = tcp_pingpong_thresh;
> > }
>
> inet_csk(sk)->icsk_ack.pingpong = sock_net(sk)->sysctl_tcp_pingpong_thresh;
Let's use READ_ONCE(sock_net(sk)->sysctl_tcp_pingpong_thresh).
Same for other sysctl reads.
>
> > static inline void inet_csk_exit_pingpong_mode(struct sock *sk)
> > @@ -338,7 +338,15 @@ static inline void inet_csk_exit_pingpong_mode(struct sock *sk)
> >
> > static inline bool inet_csk_in_pingpong_mode(struct sock *sk)
> > {
> > - return inet_csk(sk)->icsk_ack.pingpong >= TCP_PINGPONG_THRESH;
> > + return inet_csk(sk)->icsk_ack.pingpong >= tcp_pingpong_thresh;
> > +}
>
> Again, sock_net(sk)->sysctl_tcp_pingpong_thresh rather than tcp_pingpong_thresh.
>
> > +static inline void inet_csk_inc_pingpong_cnt(struct sock *sk)
> > +{
> > + struct inet_connection_sock *icsk = inet_csk(sk);
> > +
> > + if (icsk->icsk_ack.pingpong < U8_MAX)
> > + icsk->icsk_ack.pingpong++;
> > }
> >
> > static inline bool inet_csk_has_ulp(struct sock *sk)
> > diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> > index 782273bb93c2..b5253567f2bd 100644
> > --- a/net/core/sysctl_net_core.c
> > +++ b/net/core/sysctl_net_core.c
> > @@ -653,6 +653,15 @@ static struct ctl_table net_core_table[] = {
>
> Again, in net.ipv4, not net.core.
>
> > .proc_handler = proc_dointvec_minmax,
> > .extra1 = SYSCTL_ZERO,
> > },
> > + {
> > + .procname = "tcp_pingpong_thresh",
> > + .data = &tcp_pingpong_thresh,
> > + .maxlen = sizeof(int),
> > + .mode = 0644,
> > + .proc_handler = proc_dointvec_minmax,
> > + .extra1 = SYSCTL_ONE,
> > + .extra2 = SYSCTL_THREE,
>
> Please make the max U8_MAX to allow more flexibility (since we have 8
> bits of space anyway in the inet_csk(sk)->icsk_ack.pingpong field).
Please use proc_dou8vec_minmax(), then you can drop .extra2.
.maxlen = sizeof(u8),
.mode = 0644,
.proc_handler = proc_dou8vec_minmax,
.extra1 = SYSCTL_ONE,
Thanks,
Kuniyuki
>
> > + },
> > { }
> > };
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 53b7751b68e1..dcd143193d41 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -308,6 +308,8 @@ EXPORT_SYMBOL(tcp_have_smc);
> > struct percpu_counter tcp_sockets_allocated ____cacheline_aligned_in_smp;
> > EXPORT_SYMBOL(tcp_sockets_allocated);
> >
> > +int tcp_pingpong_thresh __read_mostly = 1;
> > +
>
> Again, per-network-namespace. You will need to initialize the
> per-netns value in tcp_sk_init(). Again, see Eric's
> 65466904b015f6eeb9225b51aeb29b01a1d4b59c commit for an example.
>
> > * TCP splice context
> > */
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index cfe128b81a01..576d21621778 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -167,12 +167,25 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
> > if (tcp_packets_in_flight(tp) == 0)
> > tcp_ca_event(sk, CA_EVENT_TX_START);
> >
> > + /* If tcp_pingpong_thresh > 1, and
> > + * this is the first data packet sent in response to the
> > + * previous received data,
> > + * and it is a reply for ato after last received packet,
> > + * increase pingpong count.
> > + */
> > + if (tcp_pingpong_thresh > 1 &&
> > + before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
> > + (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
> > + inet_csk_inc_pingpong_cnt(sk);
> > +
>
> Introducing this new code re-introduces a bug fixed in 4d8f24eeedc5.
> As that commit description noted:
>
> This to-be-reverted commit was meant to apply a stricter rule for the
> stack to enter pingpong mode. However, the condition used to check for
> interactive session "before(tp->lsndtime, icsk->icsk_ack.lrcvtime)" is
> jiffy based and might be too coarse, which delays the stack entering
> pingpong mode.
> We revert this patch so that we no longer use the above condition to
> determine interactive session,
>
> > tp->lsndtime = now;
> >
> > - /* If it is a reply for ato after last received
> > + /* If tcp_pingpong_thresh == 1, and
>
> Please remove the "If tcp_pingpong_thresh == 1, and" part, since this
> is the correct code path no matter the value of the threshold.
>
> > + * it is a reply for ato after last received
> > * packet, enter pingpong mode.
> > */
> > - if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
> > + if (tcp_pingpong_thresh == 1 &&
>
> Please remove the "if (tcp_pingpong_thresh == 1 &&" part, since this
> is the correct code path no matter the value of the threshold.
>
> > + (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
> > inet_csk_enter_pingpong_mode(sk);
>
> Please make this call inet_csk_inc_pingpong_cnt(), since this is the
> correct code path no matter the value of the threshold.
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