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=H/1nd6Ff; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id A84B75A026F for ; Fri, 17 Oct 2025 09:48:16 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1760687294; bh=U9EQ2R6PYNoo7ICC5/0LxONnWmwlwSePU5yPZWvdWu8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H/1nd6FfsifDC/+pr4MItlbE0488FtF6ZAdwpWiE2moQX6U2IeDDUYWH1/yQrKsoj ng7fEs+c8GHrTPIKTA9wIcD2lLGir1/ygRkziY4ROdlFiZPWqr1ORP9kYTXbaBeT5K QLYSI6i3aHKCci1+SO9qYcqZyS7zQGK8oGC1x2YhUc0cWXcdsDWs0pgAt1koUXFp8x oyiRK4FJFD7+zOv/F93HTID6k4VMGnLdRFKs9Wx3yJXEKrVxhqtAm/HHZxV4OMCaZE FwXadHDAezIZ1vsnwh6qUQVuoORXIC4eJNcamYJ8vh/QuVgZ1fPypIrSoE+tPz5nMe 34VAVyLAS6ewg== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4cnxl20785z4w9w; Fri, 17 Oct 2025 18:48:14 +1100 (AEDT) Date: Fri, 17 Oct 2025 18:27:29 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v5 3/4] tcp: Resend SYN for inbound connections Message-ID: References: <20251017042743.15519-1-yuhuang@redhat.com> <20251017042743.15519-4-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="CJoTDDlspE8STQLn" Content-Disposition: inline In-Reply-To: Message-ID-Hash: A7RK3GQE4F643OITGO4RL7Z3JDG7CORD X-Message-ID-Hash: A7RK3GQE4F643OITGO4RL7Z3JDG7CORD 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: --CJoTDDlspE8STQLn Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 17, 2025 at 01:46:15PM +0800, Yumei Huang wrote: > On Fri, Oct 17, 2025 at 12:44=E2=80=AFPM David Gibson > wrote: > > > > On Fri, Oct 17, 2025 at 12:27:42PM +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. > > > > > > Use the same backoff calculation for the timeout as linux kernel. > > > > > > Link: https://bugs.passt.top/show_bug.cgi?id=3D153 > > > Signed-off-by: Yumei Huang > > > --- > > > tcp.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++-------- > > > tcp.h | 5 +++++ > > > 2 files changed, 52 insertions(+), 8 deletions(-) > > > > > > diff --git a/tcp.c b/tcp.c > > > index 2ec4b0c..71e2da0 100644 > > > --- a/tcp.c > > > +++ b/tcp.c > > > @@ -179,9 +179,11 @@ > > > * > > > * Timeouts are implemented by means of timerfd timers, set based on= flags: > > > * > > > - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handsh= ake (flag > > > - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, r= eset the > > > - * connection > > > + * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during h= andshake > > > + * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this t= ime, resend > > > + * SYN. It's the starting timeout for the first SYN retry. If this= persists > > > + * for more than TCP_MAX_RETRIES or (tcp_syn_retries + > > > + * tcp_syn_linear_timeouts) times in a row, reset the connection > > > * > > > * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, aft= er sending > > > * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send da= ta from the > > > @@ -340,7 +342,7 @@ enum { > > > #define WINDOW_DEFAULT 14600 /* RFC = 6928 */ > > > > > > #define ACK_INTERVAL 10 /* ms */ > > > -#define SYN_TIMEOUT 10 /* s */ > > > +#define SYN_TIMEOUT_INIT 1 /* s */ > > > #define ACK_TIMEOUT 2 > > > #define FIN_TIMEOUT 60 > > > #define ACT_TIMEOUT 7200 > > > @@ -365,6 +367,9 @@ uint8_t tcp_migrate_rcv_queue [TCP_MI= GRATE_RCV_QUEUE_MAX]; > > > > > > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller whe= n above this */ > > > > > > +#define TCP_SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_ret= ries" > > > +#define TCP_SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_lin= ear_timeouts" \ > > > + > > > /* "Extended" data (not stored in the flow table) for TCP flow migra= tion */ > > > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > > > > > > @@ -581,8 +586,13 @@ static void tcp_timer_ctl(const struct ctx *c, s= truct tcp_tap_conn *conn) > > > if (conn->flags & ACK_TO_TAP_DUE) { > > > it.it_value.tv_nsec =3D (long)ACK_INTERVAL * 1000 * 100= 0; > > > } else if (conn->flags & ACK_FROM_TAP_DUE) { > > > - if (!(conn->events & ESTABLISHED)) > > > - it.it_value.tv_sec =3D SYN_TIMEOUT; > > > + if (!(conn->events & ESTABLISHED)) { > > > + if (conn->retries < c->tcp.syn_linear_timeouts) > > > + it.it_value.tv_sec =3D SYN_TIMEOUT_INIT; > > > + else > > > + it.it_value.tv_sec =3D SYN_TIMEOUT_INIT= << > > > + (conn->retries - c->tcp.syn_lin= ear_timeouts); > > > + } > > > else > > > it.it_value.tv_sec =3D ACK_TIMEOUT; > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > > > @@ -2409,8 +2419,17 @@ void tcp_timer_handler(const struct ctx *c, un= ion 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); > > > + if (conn->retries >=3D TCP_MAX_RETRIES || > > > + conn->retries >=3D (c->tcp.tcp_syn_retries + > > > + c->tcp.syn_linear_timeout= s)) { > > > + 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); > > > @@ -2766,6 +2785,24 @@ static socklen_t tcp_probe_tcp_info(void) > > > return sl; > > > } > > > > > > +/** > > > + * tcp_syn_params_init() - Get initial syn params for inbound connec= tion > > > > Stefano's comment wasn't 100% clear, but I believe he was suggesting > > s/syn params/SYN parameters/ > > in the comment here for clarity. >=20 > Oops, I missed this one. > > > > > + * @c: Execution context > > > +*/ > > > +void tcp_syn_params_init(struct ctx *c) > > > +{ > > > + intmax_t tcp_syn_retries, syn_linear_timeouts; > > > + > > > + tcp_syn_retries =3D read_file_integer(TCP_SYN_RETRIES, 8); > > > + syn_linear_timeouts =3D read_file_integer(TCP_SYN_LINEAR_TIMEOU= TS, 1); > > > + > > > + c->tcp.tcp_syn_retries =3D MIN(tcp_syn_retries, UINT8_MAX); > > > + c->tcp.syn_linear_timeouts =3D MIN(syn_linear_timeouts, UINT8_M= AX); > > > + > > > + debug("TCP SYN parameters: retries=3D%"PRIu8", linear_timeouts= =3D%"PRIu8, > > > + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); > > > +} > > > + > > > /** > > > * tcp_init() - Get initial sequence, hash secret, initialise per-so= cket data > > > * @c: Execution context > > > @@ -2776,6 +2813,8 @@ int tcp_init(struct ctx *c) > > > { > > > ASSERT(!c->no_tcp); > > > > > > + tcp_syn_params_init(c); > > > + > > > tcp_sock_iov_init(c); > > > > > > memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)= ); > > > diff --git a/tcp.h b/tcp.h > > > index 234a803..bb58324 100644 > > > --- a/tcp.h > > > +++ b/tcp.h > > > @@ -59,12 +59,17 @@ union tcp_listen_epoll_ref { > > > * @fwd_out: Port forwarding configuration for outbound pack= ets > > > * @timer_run: Timestamp of most recent timer run > > > * @pipe_size: Size of pipes for spliced connections > > > + * @tcp_syn_retries: Number of times initial SYNs during handshake > > > > Perhaps, > > > > "Number of SYN retries using exponential backoff" >=20 > It's not actually retries using exponential backoff. It's the total > number of retries, including linear backoff and exponential backoff. That's what ip-sysctl.rst says, but as we discussed the other day, the kernel code and behaviour seem to treat it as just the retries after exhausting linear_timeouts. > I got the "Number of times initial SYNs" from the kernel doc > https://www.kernel.org/doc/Documentation/networking/ip-sysctl.rst. Right, but by cutting off the "will be retransmitted", it's no longer grammatical (admittedly it's a pretty long confusing sentence). > Maybe "Number of SYN retries during handshake" ? What do you think? That would be good if the description in ip-sysctl.rst was accurate, but it seems not to be. > > > > > + * @syn_linear_timeouts: Number of SYN retries using linear backoff = timeout > > > + * before switching to exponential backoff timeo= ut > > > */ > > > struct tcp_ctx { > > > struct fwd_ports fwd_in; > > > struct fwd_ports fwd_out; > > > struct timespec timer_run; > > > size_t pipe_size; > > > + uint8_t tcp_syn_retries; > > > + uint8_t syn_linear_timeouts; > > > }; > > > > > > #endif /* TCP_H */ > > > -- > > > 2.47.0 > > > > > > > -- > > David Gibson (he or they) | I'll have my music baroque, and my co= de > > david AT gibson.dropbear.id.au | minimalist, thank you, not the other = way > > | around. > > http://www.ozlabs.org/~dgibson >=20 >=20 >=20 > --=20 > Thanks, >=20 > Yumei Huang >=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 --CJoTDDlspE8STQLn Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmjx79MACgkQzQJF27ox 2GfrvA//X77DMADKseD4OK0dhYIl8dmbgFfe9S4WX+wuPu6I+rJ5dS35zAdkueQU 5uC+IVZAIBpomdRcOj/1fZkt6nPCCddDfdOPvoYuPTMLmZ54X7Gsoc5mcE4Zk8yl dRP9kZdpT7JUgfyQF7Z4DtoOMokN4g/FD5sSirt3cGQ2OJpqqOyJaUR5pCkDhwgg zmC6fWTkB+CmyGDsPhF5Uny85Ai1cBpbj1clmRfra5jYFV7EMPQgrY47DQgBpVUp Qx2of6k8RfQ/zZSP9xmn98+RK20n68StFMBkv2v9kbE2zzRKqnmzHvpI1thRAXr3 XW7etwV+nu6cdX0Mlujn224cBFWwaBCu6IOF65fnyd5rcjR5zIzypt5HtMewnQ2d GYmeGuL1PxDqrRxnsekR/ZLRfLV8mga3Enh9tTAkrNYi5WMgiUhM0hfuRDRXXRZw mtvLQLX9VSqAj+U97o1ntyVWPflrormwvS1gDSJwgw8CzHuaTuXv1ZUY2KkhYYKm dHNi+pdJBjh1MIXhuc2X55gNzw3fvXIu8E63cF2c4Msd99MNiztFeG0DByBFO/OG InHmuc8hGqvB/Y/KS4R/iRYjxUlVywFHRGmc3Eo2bsPJi+zBnEBabECq4wMDFhPQ 6saWUwcLSv5pLAqswC9pz+cj0+br0BmcGqJu4kFZuapGc/sV7N8= =/LDh -----END PGP SIGNATURE----- --CJoTDDlspE8STQLn--