From: Xin Long <lucien.xin@gmail.com>
To: Paolo Abeni <pabeni@redhat.com>
Cc: network dev <netdev@vger.kernel.org>,
quic@lists.linux.dev, davem@davemloft.net, kuba@kernel.org,
Eric Dumazet <edumazet@google.com>,
Simon Horman <horms@kernel.org>,
Stefan Metzmacher <metze@samba.org>,
Moritz Buhl <mbuhl@openbsd.org>,
Tyler Fanelli <tfanelli@redhat.com>,
Pengtao He <hepengtao@xiaomi.com>,
Thomas Dreibholz <dreibh@simula.no>,
linux-cifs@vger.kernel.org, Steve French <smfrench@gmail.com>,
Namjae Jeon <linkinjeon@kernel.org>,
Paulo Alcantara <pc@manguebit.com>, Tom Talpey <tom@talpey.com>,
kernel-tls-handshake@lists.linux.dev,
Chuck Lever <chuck.lever@oracle.com>,
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Alexander Aring <aahringo@redhat.com>,
David Howells <dhowells@redhat.com>,
Matthieu Baerts <matttbe@kernel.org>,
John Ericson <mail@johnericson.me>,
Cong Wang <xiyou.wangcong@gmail.com>,
"D . Wythe" <alibuda@linux.alibaba.com>,
Jason Baron <jbaron@akamai.com>,
illiliti <illiliti@protonmail.com>,
Sabrina Dubroca <sd@queasysnail.net>,
Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
Daniel Stenberg <daniel@haxx.se>,
Andy Gospodarek <andrew.gospodarek@broadcom.com>
Subject: Re: [PATCH net-next v4 09/15] quic: add congestion control
Date: Thu, 6 Nov 2025 15:24:30 -0500 [thread overview]
Message-ID: <CADvbK_d8WoKJkU7ACK6nzbv7hzxxkAYZ5--DPzVQHsSZbEJnuw@mail.gmail.com> (raw)
In-Reply-To: <43ea4062-75a8-4152-bf19-2eca561036bd@redhat.com>
On Tue, Nov 4, 2025 at 7:02 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> On 10/29/25 3:35 PM, Xin Long wrote:
> > +/* Compute and update the pacing rate based on congestion window and smoothed RTT. */
> > +static void quic_cong_pace_update(struct quic_cong *cong, u32 bytes, u32 max_rate)
> > +{
> > + u64 rate;
> > +
> > + /* rate = N * congestion_window / smoothed_rtt */
> > + rate = (u64)cong->window * USEC_PER_SEC * 2;
> > + if (likely(cong->smoothed_rtt))
> > + rate = div64_ul(rate, cong->smoothed_rtt);
> > +
> > + WRITE_ONCE(cong->pacing_rate, min_t(u64, rate, max_rate));
> > + pr_debug("%s: update pacing rate: %u, max rate: %u, srtt: %u\n",
> > + __func__, cong->pacing_rate, max_rate, cong->smoothed_rtt);
>
> I think you should skip entirely the pacing_rate update when
> `smoothed_rtt == 0`
>
will update it.
> [...]> +/* rfc9002#section-5: Estimating the Round-Trip Time */
> > +void quic_cong_rtt_update(struct quic_cong *cong, u32 time, u32 ack_delay)
> > +{
> > + u32 adjusted_rtt, rttvar_sample;
> > +
> > + /* Ignore RTT sample if ACK delay is suspiciously large. */
> > + if (ack_delay > cong->max_ack_delay * 2)
> > + return;
> > +
> > + /* rfc9002#section-5.1: latest_rtt = ack_time - send_time_of_largest_acked */
> > + cong->latest_rtt = cong->time - time;
> > +
> > + /* rfc9002#section-5.2: Estimating min_rtt */
> > + if (!cong->min_rtt_valid) {
> > + cong->min_rtt = cong->latest_rtt;
> > + cong->min_rtt_valid = 1;
> > + }
> > + if (cong->min_rtt > cong->latest_rtt)
> > + cong->min_rtt = cong->latest_rtt;
> > +
> > + if (!cong->is_rtt_set) {
> > + /* rfc9002#section-5.3:
> > + * smoothed_rtt = latest_rtt
> > + * rttvar = latest_rtt / 2
> > + */
> > + cong->smoothed_rtt = cong->latest_rtt;
> > + cong->rttvar = cong->smoothed_rtt / 2;
> > + quic_cong_pto_update(cong);
> > + cong->is_rtt_set = 1;
> > + return;
> > + }
> > +
> > + /* rfc9002#section-5.3:
> > + * adjusted_rtt = latest_rtt
> > + * if (latest_rtt >= min_rtt + ack_delay):
> > + * adjusted_rtt = latest_rtt - ack_delay
> > + * smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
> > + * rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
> > + * rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
> > + */
> > + adjusted_rtt = cong->latest_rtt;
> > + if (cong->latest_rtt >= cong->min_rtt + ack_delay)
> > + adjusted_rtt = cong->latest_rtt - ack_delay;
> > +
> > + cong->smoothed_rtt = (cong->smoothed_rtt * 7 + adjusted_rtt) / 8;
>
> Out of sheer curiosity, is the compiler smart enough to use a 'srl 3'
> for the above?
>
Yes.
266 cong->smoothed_rtt = (cong->smoothed_rtt * 7 + adjusted_rtt) / 8;
0x593d <+77>: mov (%rbx),%ecx ; ecx = cong->smoothed_rtt
0x593f <+79>: lea (%rax,%rcx,8),%edx ; edx = adjusted_rtt + (ecx * 8)
0x5942 <+82>: sub %ecx,%edx ; edx = adjusted_rtt +
(8*ecx) - ecx = ecx*7 + adjusted_rtt
0x5946 <+86>: shr $0x3,%edx ; edx >>= 3 → divide by 8
0x594d <+93>: mov %edx,(%rbx) ; store result back to
cong->smoothed_rtt
Thanks.
next prev parent reply other threads:[~2025-11-06 20:24 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-29 14:35 [PATCH net-next v4 00/15] net: introduce QUIC infrastructure and core subcomponents Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 01/15] net: define IPPROTO_QUIC and SOL_QUIC constants Xin Long
2025-11-04 9:20 ` Paolo Abeni
2025-10-29 14:35 ` [PATCH net-next v4 02/15] net: build socket infrastructure for QUIC protocol Xin Long
2025-10-29 16:22 ` Stefan Metzmacher
2025-10-29 19:57 ` Xin Long
2025-10-30 11:29 ` Stefan Metzmacher
2025-10-30 14:13 ` Xin Long
2025-10-30 14:17 ` Stefan Metzmacher
2025-10-30 14:28 ` Xin Long
2025-11-04 9:38 ` Paolo Abeni
2025-11-05 22:20 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 03/15] quic: provide common utilities and data structures Xin Long
2025-11-04 9:55 ` Paolo Abeni
2025-10-29 14:35 ` [PATCH net-next v4 04/15] quic: provide family ops for address and protocol Xin Long
2025-11-04 10:27 ` Paolo Abeni
2025-11-06 1:01 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 05/15] quic: provide quic.h header files for kernel and userspace Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 06/15] quic: add stream management Xin Long
2025-11-04 11:05 ` Paolo Abeni
2025-11-06 1:27 ` Xin Long
2025-11-06 8:51 ` Paolo Abeni
2025-11-06 16:22 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 07/15] quic: add connection id management Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 08/15] quic: add path management Xin Long
2025-11-04 11:50 ` Paolo Abeni
2025-11-06 1:28 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 09/15] quic: add congestion control Xin Long
2025-11-04 12:02 ` Paolo Abeni
2025-11-06 20:24 ` Xin Long [this message]
2025-10-29 14:35 ` [PATCH net-next v4 10/15] quic: add packet number space Xin Long
2025-11-04 12:17 ` Paolo Abeni
2025-11-06 16:40 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 11/15] quic: add crypto key derivation and installation Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 12/15] quic: add crypto packet encryption and decryption Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 13/15] quic: add timer management Xin Long
2025-11-04 12:33 ` Paolo Abeni
2025-11-06 16:49 ` Xin Long
2025-11-13 21:23 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 14/15] quic: add frame encoder and decoder base Xin Long
2025-11-04 12:47 ` Paolo Abeni
2025-11-06 17:22 ` Xin Long
2025-11-13 21:26 ` Xin Long
2025-10-29 14:35 ` [PATCH net-next v4 15/15] quic: add packet builder and parser base Xin Long
2025-11-04 14:44 ` Paolo Abeni
2025-11-06 19:24 ` Xin Long
2025-11-04 2:41 ` [PATCH net-next v4 00/15] net: introduce QUIC infrastructure and core subcomponents Jakub Kicinski
2025-11-05 22:19 ` Xin Long
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