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=AIDf1i+C; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id 089635A0BB9 for ; Mon, 10 Nov 2025 11:56:47 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1762772204; bh=pzeVz1EHTcJkt26XW+a3BUCLG5j8CpgQ03XeOA5V5x4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=AIDf1i+CQBGUMmyQJ1Rp4i5dQgJyr76DTuvzmIpCA0hQj+7S912Ua/b5evo2vz4Tz 3D5zUm7rVRNZuylpjJ+VKYcolW0s3TPCcPxDGwZK5BoNPFSEMsJpJ8xAcxDczPNF2h wIDTHrIOFi+Emlnx47K5NbiNfYVt7hlhxzR0accrjx6TzNRJl2p81X6dh6D4xIta8x Pu2lmp6ia0YBdYGTAxgIJwOvk157KQyh2RaqvO07P2tiK0YG9mXNT2nxMmTavfL8K6 RiEH2NuFH8fs0b5CrK5/skaHlcqMhKfT0Z5AUevMhNY8Bciqiry2mgUldRnHdKYJEW c2Mf6OMRwXLjg== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4d4mnS08kbz4w2R; Mon, 10 Nov 2025 21:56:44 +1100 (AEDT) Date: Mon, 10 Nov 2025 21:46:55 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v8 4/6] tcp: Resend SYN for inbound connections Message-ID: References: <20251110093137.87705-1-yuhuang@redhat.com> <20251110093137.87705-5-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="eUySxE5Ex0ZY1jFZ" Content-Disposition: inline In-Reply-To: <20251110093137.87705-5-yuhuang@redhat.com> Message-ID-Hash: P3DBEFQKEOWMLPD7ZTCGORMGSEWZEJKY X-Message-ID-Hash: P3DBEFQKEOWMLPD7ZTCGORMGSEWZEJKY 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: --eUySxE5Ex0ZY1jFZ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Nov 10, 2025 at 05:31:35PM +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 Though I have one small concern remaining > --- > tcp.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++--------- > tcp.h | 4 ++++ > 2 files changed, 57 insertions(+), 9 deletions(-) >=20 > diff --git a/tcp.c b/tcp.c > index 2f49327..da40a99 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) > + * times, reset the connection > * > * - 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 > @@ -585,10 +594,13 @@ static void tcp_timer_ctl(const struct ctx *c, stru= ct 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 =3D conn->retries - c->tcp.syn_linear_timeouts; As discussed in the thread on the last version, the subtraction will be done unsigned, so the final result depends on behaviour of casting a "negative" (that is, close to max value) unsigned value into a signed variable. I'm pretty sure that will do the right thing in practice, but I don't believe that behaviour is guaranteed by the C standard. This probably isn't worth a respin, though. > + 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 { > @@ -2425,8 +2437,18 @@ void tcp_timer_handler(const struct ctx *c, union = epoll_ref ref) > 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); > @@ -2782,6 +2804,26 @@ static socklen_t tcp_probe_tcp_info(void) > return sl; > } > =20 > +/** > + * tcp_get_rto_params() - Get host kernel RTO parameters > + * @c: Execution context > + */ > +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("Read sysctl values 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 +2834,8 @@ 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 > 2.51.0 >=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 --eUySxE5Ex0ZY1jFZ Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmkRwp4ACgkQzQJF27ox 2GcuFRAAgnHyifGspjIMcJlA5ZyFk+QocYsWxwjg5JRPIOMUPYd5q1+u2NRZxdwp nLYjt3uZ2Irwc9bO4zigyVcKcPX+7Er9QHpyS7Ki0Mc45W3J3rLLMBYRYxNNK4c4 YQF1nCtm+IvODsSiH9ICGUU0BUNuBz1b51tJjI6u98uKPduIqp3wG+6HpG48C23z Nn7AqqiuwuVgAGz83jsgqWref0vUPy+y0N8mKtefGjX8T52Jh5UjYOKx2LHw9A0t f2l7JOolXE2tYBAwhvIgDy8tCvi2j456RGT8sONrbkjgL+DqvuVkZVVjwOa4IrNc /R3kn2lFNNswrwlvyWOTqJ+jJlO+xjieWZI5T8BfwF7xiUU2RKYY4s7h7qLn9R5y IClE1JsXeLxBMT1qukDkScLToSGNUk0URa2wEUaKeSX1SBvBk8Fjt+nA0B+dgIGQ gDK8Zp4sD7+ddiSj/FoO3xEB2dfmkk/js0HGGQuRn6p0CUGAWeRRYwb4jJhuSfna 8AIqLnrGCBYxgXbVYxQb3kBDyuBmrfCWUbrPy1oRW4pzFG7LDYXjlC17/wMpGNU+ RF5nt4i5YUdiRztjjbom76ciRIzBKRa+LO8mgkUsVbNI6oTJDM0onipsHC6Hgjvv XBY8h/F7s7eps0Ktjf/PwgaymY6ZteARKrtCxFh1sRDf3n37Bdo= =4ZXc -----END PGP SIGNATURE----- --eUySxE5Ex0ZY1jFZ--