From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202510 header.b=odVJTEs/; dkim-atps=neutral Received: from mail.ozlabs.org (mail.ozlabs.org [IPv6:2404:9400:2221:ea00::3]) by passt.top (Postfix) with ESMTPS id 138C95A0271 for ; Wed, 26 Nov 2025 05:12:57 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1764130374; bh=fgmlA1kLVduRwiUXAw99nGsudGMDZODBkzoVPKk09rw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=odVJTEs/3/4r2yrowofLL2woPwIMeP6m4PvOlnuxbIBbh7wVK2YdjTz9cQvplZR7B EjOmDZineDqMSefzQ1igO5hjrAQv7+X+0Gag5CjfJjLymJwoDDcr0Vcgm2LYRCaOK3 8/45ogzB9EAuibmjYB4qNj3+wnMC3HzEZaxl7Wj/naxKSunv646PeIL6Sil/9Vr/Vh FD/zf7Fat3NWp1yYYcnnVHgbtxFiOlfDMxC7dxYRId+sEfbynRrvr+Gc62K2MD9mCd QzUjy6c92n3+Dc2r+y+aEW+cw0UfEnDJuGyWQb8yGagprlByqhtkGi8gh0df6vRt+W 4Uap6qI81x9aA== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4dGR465xfpz4w0Q; Wed, 26 Nov 2025 15:12:54 +1100 (AEDT) Date: Wed, 26 Nov 2025 15:12:44 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v9 3/5] tcp: Resend SYN for inbound connections Message-ID: References: <20251125072638.88896-1-yuhuang@redhat.com> <20251125072638.88896-4-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="ZO1nLCCv+yasFbo8" Content-Disposition: inline In-Reply-To: <20251125072638.88896-4-yuhuang@redhat.com> Message-ID-Hash: Z5UF5YPWS2SFJOTBM74DPZZMP4PU5J6B X-Message-ID-Hash: Z5UF5YPWS2SFJOTBM74DPZZMP4PU5J6B X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top, sbrivio@redhat.com X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --ZO1nLCCv+yasFbo8 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Nov 25, 2025 at 03:26:36PM +0800, Yumei Huang wrote: > If a client connects while guest is not connected or ready yet, > resend SYN instead of just resetting connection after 10 seconds. >=20 > Use the same backoff calculation for the timeout as Linux kernel. >=20 > Link: https://bugs.passt.top/show_bug.cgi?id=3D153 > Signed-off-by: Yumei Huang Reviewed-by: David Gibson With the exception of some tiny English usage notes below. > --- > tcp.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++------------- > tcp.h | 4 ++++ > 2 files changed, 65 insertions(+), 16 deletions(-) >=20 > diff --git a/tcp.c b/tcp.c > index 05f0d8b..2887f2c 100644 > --- a/tcp.c > +++ b/tcp.c > @@ -179,9 +179,11 @@ > * > * Timeouts are implemented by means of timerfd timers, set based on fla= gs: > * > - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake = (flag > - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset= the > - * connection > + * - SYN_TIMEOUT_INIT: if no SYN,ACK is received from tap/guest during > + * handshake (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within > + * this time, resend SYN. It's the starting timeout for the first SYN > + * retry. Retry for TCP_MAX_RETRIES or (syn_retries + syn_linear_timeo= uts) English usage nit: s/for // > + * times, reset the connection English usage nit: s/, reset/, then reset/ > * > * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after s= ending > * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data f= rom the > @@ -340,7 +342,7 @@ enum { > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > =20 > #define ACK_INTERVAL 10 /* ms */ > -#define SYN_TIMEOUT 10 /* s */ > +#define SYN_TIMEOUT_INIT 1 /* s, RFC 6928 */ > #define ACK_TIMEOUT 2 > #define FIN_TIMEOUT 60 > #define ACT_TIMEOUT 7200 > @@ -365,6 +367,13 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEU= E_MAX]; > =20 > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above thi= s */ > =20 > +#define SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_retries" > +#define SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" > + > +#define SYN_RETRIES_DEFAULT 6 > +#define SYN_LINEAR_TIMEOUTS_DEFAULT 4 > +#define MAX_SYNCNT 127 /* derived from kernel's limit */ > + > /* "Extended" data (not stored in the flow table) for TCP flow migration= */ > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > =20 > @@ -543,11 +552,12 @@ static int tcp_epoll_ctl(const struct ctx *c, struc= t tcp_tap_conn *conn) > =20 > /** > * tcp_timer_ctl() - Set timerfd based on flags/events, create timerfd i= f needed > + * @c: Execution context > * @conn: Connection pointer > * > * #syscalls timerfd_create timerfd_settime > */ > -static void tcp_timer_ctl(struct tcp_tap_conn *conn) > +static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) > { > struct itimerspec it =3D { { 0 }, { 0 } }; > =20 > @@ -584,10 +594,13 @@ static void tcp_timer_ctl(struct tcp_tap_conn *conn) > if (conn->flags & ACK_TO_TAP_DUE) { > it.it_value.tv_nsec =3D (long)ACK_INTERVAL * 1000 * 1000; > } else if (conn->flags & ACK_FROM_TAP_DUE) { > - if (!(conn->events & ESTABLISHED)) > - it.it_value.tv_sec =3D SYN_TIMEOUT; > - else > + if (!(conn->events & ESTABLISHED)) { > + int exp; > + exp =3D (int)conn->retries - c->tcp.syn_linear_timeouts; > + it.it_value.tv_sec =3D SYN_TIMEOUT_INIT << MAX(exp, 0); > + } else { > it.it_value.tv_sec =3D ACK_TIMEOUT; > + } > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > it.it_value.tv_sec =3D FIN_TIMEOUT; > } else { > @@ -631,7 +644,7 @@ void conn_flag_do(const struct ctx *c, struct tcp_tap= _conn *conn, > * flags and factor this into the logic below. > */ > if (flag =3D=3D ACK_FROM_TAP_DUE) > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > =20 > return; > } > @@ -647,7 +660,7 @@ void conn_flag_do(const struct ctx *c, struct tcp_tap= _conn *conn, > if (flag =3D=3D ACK_FROM_TAP_DUE || flag =3D=3D ACK_TO_TAP_DUE || > (flag =3D=3D ~ACK_FROM_TAP_DUE && (conn->flags & ACK_TO_TAP_DUE)) || > (flag =3D=3D ~ACK_TO_TAP_DUE && (conn->flags & ACK_FROM_TAP_DUE))) > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > } > =20 > /** > @@ -703,7 +716,7 @@ void conn_event_do(const struct ctx *c, struct tcp_ta= p_conn *conn, > } > =20 > if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > } > =20 > /** > @@ -1771,7 +1784,7 @@ static int tcp_data_from_tap(const struct ctx *c, s= truct tcp_tap_conn *conn, > seq, conn->seq_from_tap); > =20 > tcp_send_flag(c, conn, ACK); > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > =20 > if (p->count =3D=3D 1) { > tcp_tap_window_update(c, conn, > @@ -2422,11 +2435,21 @@ void tcp_timer_handler(const struct ctx *c, union= epoll_ref ref) > =20 > if (conn->flags & ACK_TO_TAP_DUE) { > tcp_send_flag(c, conn, ACK_IF_NEEDED); > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > } else if (conn->flags & ACK_FROM_TAP_DUE) { > if (!(conn->events & ESTABLISHED)) { > - flow_dbg(conn, "handshake timeout"); > - tcp_rst(c, conn); > + int max; > + max =3D c->tcp.syn_retries + c->tcp.syn_linear_timeouts; > + max =3D MIN(TCP_MAX_RETRIES, max); > + if (conn->retries >=3D max) { > + flow_dbg(conn, "handshake timeout"); > + tcp_rst(c, conn); > + } else { > + flow_trace(conn, "SYN timeout, retry"); > + tcp_send_flag(c, conn, SYN); > + conn->retries++; > + tcp_timer_ctl(c, conn); > + } > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > flow_dbg(conn, "FIN timeout"); > tcp_rst(c, conn); > @@ -2444,7 +2467,7 @@ void tcp_timer_handler(const struct ctx *c, union e= poll_ref ref) > return; > =20 > tcp_data_from_sock(c, conn); > - tcp_timer_ctl(conn); > + tcp_timer_ctl(c, conn); > } > } else { > struct itimerspec new =3D { { 0 }, { ACT_TIMEOUT, 0 } }; > @@ -2782,6 +2805,26 @@ static socklen_t tcp_probe_tcp_info(void) > return sl; > } > =20 > +/** > + * tcp_get_rto_params() - Get host kernel RTO parameters > + * @c: Execution context > + */ > +static void tcp_get_rto_params(struct ctx *c) > +{ > + intmax_t v; > + > + v =3D read_file_integer(SYN_RETRIES, SYN_RETRIES_DEFAULT); > + c->tcp.syn_retries =3D MIN(v, MAX_SYNCNT); > + > + v =3D read_file_integer(SYN_LINEAR_TIMEOUTS, SYN_LINEAR_TIMEOUTS_DEFAUL= T); > + c->tcp.syn_linear_timeouts =3D MIN(v, MAX_SYNCNT); > + > + debug("Using TCP RTO parameters, syn_retries: %"PRIu8 > + ", syn_linear_timeouts: %"PRIu8, > + c->tcp.syn_retries, > + c->tcp.syn_linear_timeouts); > +} > + > /** > * tcp_init() - Get initial sequence, hash secret, initialise per-socket= data > * @c: Execution context > @@ -2792,6 +2835,8 @@ int tcp_init(struct ctx *c) > { > ASSERT(!c->no_tcp); > =20 > + tcp_get_rto_params(c); > + > tcp_sock_iov_init(c); > =20 > memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); > diff --git a/tcp.h b/tcp.h > index 0082386..37d7758 100644 > --- a/tcp.h > +++ b/tcp.h > @@ -60,12 +60,16 @@ union tcp_listen_epoll_ref { > * @fwd_out: Port forwarding configuration for outbound packets > * @timer_run: Timestamp of most recent timer run > * @pipe_size: Size of pipes for spliced connections > + * @syn_retries: SYN retries using exponential backoff timeout > + * @syn_linear_timeouts: SYN retries before using exponential backoff ti= meout > */ > struct tcp_ctx { > struct fwd_ports fwd_in; > struct fwd_ports fwd_out; > struct timespec timer_run; > size_t pipe_size; > + uint8_t syn_retries; > + uint8_t syn_linear_timeouts; > }; > =20 > #endif /* TCP_H */ > --=20 > 2.51.1 >=20 --=20 David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson --ZO1nLCCv+yasFbo8 Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmkmfjsACgkQzQJF27ox 2GfCRg//bHJT79M0qWADdPbumBsf/MTxuwChVlQB3w2jvJKiquYu4DV+EXAhh8JG MUp6PbM2b/qjlTbV9/Y/Np7wa5kQvlKdY8500we1c47F+uhDExUYe25FcE0I8kFx FbS1vjZnvbFV8OBzEZzgW8ga1qgHG7vVPpTehrsuvIokc1aBx0y5hnD+o31JouQx f8XwqqOPa07KD57DCah1QW47f8qaSOv6r4Iw/V3Zw8tec8pbErXvUVpgnMrLoMk2 DP+EvRyFLQJwbnoxPtQhpQvGGv6IoDEwPyQBrzYJVd7o8shVoOXYA9kSQMElFpxA 8xSG4jrg/IjDKN8AuMjEItRlUevY4T3+m150ZQGl9RpLMGF5NbgA74m4KL5j4Gtg 6rujF+vwe1yL6BLczjkRL//MCqzLSFNxFLAnsunFGEtXIse2HY5qXteoTX2tp/1o 3uBSXXicXG1A/NPHFW0+300RmWQlemEJJ+/1om+cfwCECzob8rZwZt80NEtamnxV 4VDJASV6dc5igPfB99TsyOWwUUrq8yc+rHdGbAiglOJzA0d9ZQCdcnhCeIkojNA8 qD0A6ligIK2TMUMy5n3+ai2KoLJakTN+4jtoKK24/0xHo6x6jF4h+4sNR28EAJS3 hgFrr1ipQ9rU7mHBDbuNzjAedKDuGSy3ajup5g6IXJkTPxBRs+0= =tSeC -----END PGP SIGNATURE----- --ZO1nLCCv+yasFbo8--