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Tue, 28 Oct 2025 19:31:13 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEGUiySP7343ihJ35ABoQCuWt4qwQ4Awnsvwo0TDAdyBOFj/stzmIy95J5qKNM9TskpPYHDwFAV7cQ7UbnBOrM= X-Received: by 2002:a17:906:f586:b0:b6d:6a44:5fd9 with SMTP id a640c23a62f3a-b703d55e473mr110767866b.48.1761705073403; Tue, 28 Oct 2025 19:31:13 -0700 (PDT) MIME-Version: 1.0 References: <20251017062838.21041-1-yuhuang@redhat.com> <20251017062838.21041-4-yuhuang@redhat.com> <20251024010431.4329a843@elisabeth> <20251028124421.5e49bfc8@elisabeth> In-Reply-To: <20251028124421.5e49bfc8@elisabeth> From: Yumei Huang Date: Wed, 29 Oct 2025 10:31:02 +0800 X-Gm-Features: AWmQ_bkD-Nn45MDL8rGF4L-9NIeabRBaMzP6fWhj6SSkbZZ4PluRAfB_fXYFfk4 Message-ID: Subject: Re: [PATCH v6 3/4] tcp: Resend SYN for inbound connections To: Stefano Brivio X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: FQIDcZBqMIetsOLUT7ItJ7eDwPww0as8VFx7avl2ITM_1761705074 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Message-ID-Hash: QNOSM7PSPHAJ4GQL67W6DYKSHVOSYTBG X-Message-ID-Hash: QNOSM7PSPHAJ4GQL67W6DYKSHVOSYTBG X-MailFrom: yuhuang@redhat.com 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, david@gibson.dropbear.id.au 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: On Tue, Oct 28, 2025 at 7:44=E2=80=AFPM Stefano Brivio = wrote: > > On Tue, 28 Oct 2025 15:43:25 +0800 > Yumei Huang wrote: > > > On Fri, Oct 24, 2025 at 7:04=E2=80=AFAM Stefano Brivio wrote: > > > > > > On Fri, 17 Oct 2025 14:28:37 +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. > > > > > > Linux. > > > > > > > > > > > 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..9385132 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 hand= shake (flag > > > > - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time,= reset the > > > > - * connection > > > > + * - SYN_TIMEOUT_INIT: if no 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. If th= is persists > > > > > > "If this persists" makes sense for the existing ACK_TIMEOUT > > > description but not here, because it looks like it refers to "startin= g > > > timeout". > > > > > > Coupled with the next patch, it becomes increasingly difficult to > > > understand what "this" persisting thing is. > > > > > > Maybe directly say "Retry for ..., then reset the connection"? It's > > > shorter and clearer. > > > > > > > + * 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, a= fter sending > > > > * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send = data from the > > > > @@ -340,7 +342,7 @@ enum { > > > > #define WINDOW_DEFAULT 14600 /* RF= C 6928 */ > > > > > > > > #define ACK_INTERVAL 10 /* ms */ > > > > -#define SYN_TIMEOUT 10 /* s */ > > > > +#define SYN_TIMEOUT_INIT 1 /* s */ > > > > > > Maybe mention RFC 6928 as done above? That's where this value comes > > > from. > > > > > > I just noticed you do that in 4/4, so it's slightly nicer if you do > > > that right away here for ease of future reference, but not really > > > needed. > > > > > > > #define ACK_TIMEOUT 2 > > > > #define FIN_TIMEOUT 60 > > > > #define ACT_TIMEOUT 7200 > > > > @@ -365,6 +367,9 @@ uint8_t tcp_migrate_rcv_queue [TCP_= MIGRATE_RCV_QUEUE_MAX]; > > > > > > > > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller w= hen above this */ > > > > > > > > +#define TCP_SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_r= etries" > > > > +#define TCP_SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_l= inear_timeouts" \ > > > > > > This uses 121 columns. I'm not sure where all those tabs and \ come > > > from. > > > > My bad. I renamed the macro and it fits in 80 columns but I forgot to > > remove the '\' and the spaces before that. > > > > > > > + > > > > /* "Extended" data (not stored in the flow table) for TCP flow mig= ration */ > > > > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > > > > > > > > @@ -581,8 +586,13 @@ static void tcp_timer_ctl(const struct ctx *c,= struct tcp_tap_conn *conn) > > > > if (conn->flags & ACK_TO_TAP_DUE) { > > > > it.it_value.tv_nsec =3D (long)ACK_INTERVAL * 1000 * 1= 000; > > > > } 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_timeout= s) > > > > + it.it_value.tv_sec =3D SYN_TIMEOUT_IN= IT; > > > > + else > > > > + it.it_value.tv_sec =3D SYN_TIMEOUT_IN= IT << > > > > + (conn->retries - c->tcp.syn_l= inear_timeouts); > > > > > > Probably more readable, but I haven't tried: always start from > > > SYN_TIMEOUT_INIT, then multiply/shift if > > > conn->retries >=3D c->tcp.syn_linear_timeouts. > > > > I guess you meant: > > > > if (!(conn->events & ESTABLISHED)) { > > it.it_value.tv_sec =3D SYN_TIMEOUT_INIT; > > if (conn->retries >=3D c->tcp.syn_linear_timeouts) > > it.it_value.tv_sec =3D SYN_TIMEOUT_INIT << > > (conn->retries - c->tcp.syn_linear_timeouts); > > Well, yes, this, but without repeating the assignment, so > <<=3D (conn->retries - c->tcp.syn_linear_timeouts). > > > other than something like: > > > > it.it_value.tv_sec <<=3D 1 > > Hmm, no, why 1? I thought you meant: when retries >=3D syn_linear_timeouts, we don't use SYN_TIMEOUT_INIT anymore, but to multiply it.it_value.tv_sec itself, like if it's 2 this time, next time it's 4 (2<<=3D1). Now I get your point, and it makes sense to me. > > > in above second if block, since the latter would cause > > it.it_value.tv_sec=3D2 always when retries>=3Dsyn_linear_timeouts, as > > it.it_value.tv_sec is set to SYN_TIMEOUT_INIT before the if condition. > > > > If I'm correct, I'm not sure if it's more readable to change to that. > > I feel the if/else clause quite matches the way the kernel handles it. > > What do you think? > > Sorry, I took this for granted, but I rather meant: > > it.it_value.tv_sec =3D SYN_TIMEOUT_INIT; > if (conn->retries >=3D c->tcp.syn_linear_timeouts= ) { > it.it_value.tv_sec <<=3D > (conn->retries - > c->tcp.syn_linear_timeou= ts); > } > > because in both cases we're starting from SYN_TIMEOUT_INIT. > > And now that I wrote that, I'll pretend I actually meant this: > > if (!(conn->events & ESTABLISHED)) { > int factor =3D conn->retries - c->tcp.syn_linear_= timeouts; > > it.it_value.tv_sec =3D SYN_TIMEOUT_INIT << MAX(fa= ctor, 0); > } else { > ... > } > > ...where we make the conditional implicit (but I think intuitive) by > adding 0 as a lower bound. Am I missing something...? No, it was me misunderstanding your point. I will update accordingly. > > > > > + } > > > > 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, = 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); > > > > + if (conn->retries >=3D TCP_MAX_RETRIES || > > > > + conn->retries >=3D (c->tcp.tcp_syn_retrie= s + > > > > + c->tcp.syn_linear_timeo= uts)) { > > > > + flow_dbg(conn, "handshake timeout"); > > > > + tcp_rst(c, conn); > > > > + } else { > > > > + flow_trace(conn, "SYN timeout, retry"= ); > > > > + tcp_send_flag(c, conn, SYN); > > > > + conn->retries++; > > > > > > I think I already raised this point on a previous revision: this need= s > > > to be zeroed as the connection is established, but I don't see that i= n > > > the current version. > > > > > > > + tcp_timer_ctl(c, conn); > > > > + } > > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACK= ED)) { > > > > 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 parameters for inbound = connection > > > > > > They're not initial, they'll be used for all the connections if I > > > understand correctly. > > > > > > Maybe "Get SYN retries sysctl values"? > > > I think the _init() in the > > > function name is also somewhat misleading. > > > > Do you have a better idea for the function name? I named it just like > > other functions called in tcp_init(), such as tcp_sock_iov_init(), > > tcp_splice_init(). > > Given that David was suggesting "Get host kernel RTO parameters" as a > comment to this function, maybe tcp_get_rto_params()? > > The only similar function we have is tcp_get_sndbuf(), so I would make > it consistent with that. The most peculiar task of this function is > that it gets parameters ("get"), I think, rather than assigning them to > variables ("init"). Make sense. > > > > > + * @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); > > > > > > Why 8? Perhaps a #define would help? > > > > The 8 actually comes from your suggestion in v1: > > " > > (optional, and might be in another series, but keeping it together > > with the rest might be more convenient): read tcp_syn_retries, limit > > to 8, and also read tcp_syn_linear_timeouts > > " > > That was a *limit* of 8, coming from the fact that we have 3 bits to > store the retry count. This is a default instead. > > > I guess it can be replaced with 6 as it's the default value according t= o > > Documentation/networking/ip-sysctl.rst. > > I'd rather check the code because sometimes people forget to update > that documentation but yes, it matches: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree= /include/net/tcp.h?id=3Da7556779745c047efb7b0ce8732889b0cdc80936#n125 > > > We already have TCP_SYN_RETRIES for the sysctl file, maybe have > > another TCP_SYN_RETRIES_DEFAULT for the number? > > It sounds reasonable to me. > > > > > + syn_linear_timeouts =3D read_file_integer(TCP_SYN_LINEAR_TIME= OUTS, 1); > > > > The default value is 4 as Documentation/networking/ip-sysctl.rst. I > > don't quite remember where the 1 is from. Should we update it as well > > and add the similar macro as above? > > I would, yes. By the way, it's hardcoded here: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree= /net/ipv4/tcp_ipv4.c?id=3De97269257fe437910cddc7c642a636ca3cf9fb1d#n3669 > > but defined here for thin streams (connections where observed > throughput is low: https://docs.kernel.org/networking/tcp-thin.html): > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree= /include/net/tcp.h?id=3Da7556779745c047efb7b0ce8732889b0cdc80936#n266 > > I'd suggest to define a constant with value '4' and use it. Got it. > > > > > + c->tcp.tcp_syn_retries =3D MIN(tcp_syn_retries, UINT8_MAX); > > > > + c->tcp.syn_linear_timeouts =3D MIN(syn_linear_timeouts, UINT8= _MAX); > > > > + > > > > + debug("TCP SYN parameters: retries=3D%"PRIu8", linear_timeout= s=3D%"PRIu8, > > > > > > Similar to the comment above: these are not parameters of SYN segment= s > > > (which would seem to imply TCP options, such as the MSS). > > > > > > We typically don't print C assignments, rather human-readable message= s, > > > so that could be "Read sysctl values tcp_syn_retries: ..., > > > syn_linear_timeouts: ...". > > > > Got it. > > > > > > > > > > + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); > > > > +} > > > > + > > > > /** > > > > * tcp_init() - Get initial sequence, hash secret, initialise per-= socket 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_pool= 4)); > > > > diff --git a/tcp.h b/tcp.h > > > > index 234a803..4369b52 100644 > > > > --- a/tcp.h > > > > +++ b/tcp.h > > > > @@ -59,12 +59,17 @@ union tcp_listen_epoll_ref { > > > > * @fwd_out: Port forwarding configuration for outbound pa= ckets > > > > * @timer_run: Timestamp of most recent timer run > > > > * @pipe_size: Size of pipes for spliced connections > > > > + * @tcp_syn_retries: Number of SYN retries during handshake > > > > + * @syn_linear_timeouts: Number of SYN retries using linear backof= f timeout > > > > + * before switching to exponential backoff tim= eout > > > > > > Maybe more compact: > > > > > > * @syn_linear_timeouts: SYN retries before using exponential = timeout > > > > So we remove the '"Number of" as well? I guess we should make it > > consistent for both tcp_syn_retries and syn_linear_timeouts. > > Ah, yes, we should. > > I mean, I guess it's clear that we're talking about the count / number > of those and not their colour or taste... True. > > > > > > > > */ > > > > 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; > > > > }; > > > > > > -- > Stefano > --=20 Thanks, Yumei Huang