On Thu, Dec 04, 2025 at 08:45:35AM +0100, Stefano Brivio wrote: > A fixed 10 ms ACK_TIMEOUT timer value served us relatively well > until Nit: it's called "ACK_INTERVAL" in the code. > the previous change, because we would generally cause retransmissions > for non-local outbound transfers with relatively high (> 100 Mbps) > bandwidth and non-local but low (< 5 ms) RTT. > > Now that retransmissions are less frequent, we don't have a proper > trigger to check for acknowledged bytes on the socket, and will > generally block the sender for a significant amount of time while > we could acknowledge more data, instead. > > Store the RTT reported by the kernel using an approximation (exponent), > to keep flow storage size within two (typical) cachelines. Check for > socket updates when half of this time elapses: it should be a good > indication of the one-way delay we're interested in (peer to us). Reasoning based on a one-way delay doesn't quite make sense to me. We can't know when anything happens at the peer, and - obviously - we can only set a timer starting at an event that occurs on our side. So, I think only RTT can matter to us, not one-way delay. That said, using half the RTT estimate still makes sense to me: we only have an approximation, and halving it gives us a pretty safe lower bound. > Representable values are between 100 us and 12.8 ms, and any value Nit: I think Unicode is long enough supported you can use µs > outside this range is clamped to these bounds. This choice appears > to be a good trade-off between additional overhead and throughput. > > This mechanism partially overlaps with the "low RTT" destinations, > which we use to infer that a socket is connected to an endpoint to > the same machine (while possibly in a different namespace) if the > RTT is reported as 10 us or less. > > This change doesn't, however, conflict with it: we are reading > TCP_INFO parameters for local connections anyway, so we can always > store the RTT approximation opportunistically. > > Then, if the RTT is "low", we don't really need a timer to > acknowledge data as we'll always acknowledge everything to the > sender right away. However, we have limited space in the array where > we store addresses of local destination, so the low RTT property of a > connection might toggle frequently. Because of this, it's actually > helpful to always have the RTT approximation stored. > > This could probably benefit from a future rework, though, introducing > a more integrated approach between these two mechanisms. Right, it feels like it should be possible to combine these mechanisms, but figuring out exactly how isn't trivial. Problem for another day. > > Signed-off-by: Stefano Brivio > --- > tcp.c | 29 ++++++++++++++++++++++------- > tcp_conn.h | 9 +++++++++ > util.c | 14 ++++++++++++++ > util.h | 1 + > 4 files changed, 46 insertions(+), 7 deletions(-) > > diff --git a/tcp.c b/tcp.c > index 863ccdb..b00b874 100644 > --- a/tcp.c > +++ b/tcp.c > @@ -202,9 +202,13 @@ > * - ACT_TIMEOUT, in the presence of any event: if no activity is detected on > * either side, the connection is reset > * > - * - ACK_INTERVAL elapsed after data segment received from tap without having > + * - RTT / 2 elapsed after data segment received from tap without having > * sent an ACK segment, or zero-sized window advertised to tap/guest (flag > - * ACK_TO_TAP_DUE): forcibly check if an ACK segment can be sent > + * ACK_TO_TAP_DUE): forcibly check if an ACK segment can be sent. > + * > + * RTT, here, is an approximation of the RTT value reported by the kernel via > + * TCP_INFO, with a representable range from RTT_STORE_MIN (100 us) to > + * RTT_STORE_MAX (12.8 ms). The timeout value is clamped accordingly. > * > * > * Summary of data flows (with ESTABLISHED event) > @@ -341,7 +345,6 @@ enum { > #define MSS_DEFAULT 536 > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > > -#define ACK_INTERVAL 10 /* ms */ > #define RTO_INIT 1 /* s, RFC 6298 */ > #define RTO_INIT_AFTER_SYN_RETRIES 3 /* s, RFC 6298 */ > #define FIN_TIMEOUT 60 > @@ -594,7 +597,9 @@ 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 = (long)ACK_INTERVAL * 1000 * 1000; > + it.it_value.tv_nsec = (long)RTT_GET(conn) * 1000 / 2; > + static_assert(RTT_STORE_MAX * 1000 / 2 < 1000 * 1000 * 1000, > + ".tv_nsec is greater than 1000 * 1000 * 1000"); > } else if (conn->flags & ACK_FROM_TAP_DUE) { > int exp = conn->retries, timeout = RTO_INIT; > if (!(conn->events & ESTABLISHED)) > @@ -609,9 +614,15 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn) > it.it_value.tv_sec = ACT_TIMEOUT; > } > > - flow_dbg(conn, "timer expires in %llu.%03llus", > - (unsigned long long)it.it_value.tv_sec, > - (unsigned long long)it.it_value.tv_nsec / 1000 / 1000); > + if (conn->flags & ACK_TO_TAP_DUE) { > + flow_trace(conn, "timer expires in %lu.%01llums", > + (unsigned long)it.it_value.tv_nsec / 1000 / 1000, > + (unsigned long long)it.it_value.tv_nsec / 1000); > + } else { > + flow_dbg(conn, "timer expires in %llu.%03llus", > + (unsigned long long)it.it_value.tv_sec, > + (unsigned long long)it.it_value.tv_nsec / 1000 / 1000); > + } One branch is flow_trace(), one is flow_dbg() which doesn't seem correct. Also, basing the range indirectly on the flags, rather than on the actual numbers in it.it_value seems fragile. But... this seems overly complex for a trace message anyway. Maybe just use the seconds formatting, but increase the resolution to µs. > > if (timerfd_settime(conn->timer, 0, &it, NULL)) > flow_perror(conn, "failed to set timer"); > @@ -1149,6 +1160,10 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, > conn_flag(c, conn, ACK_TO_TAP_DUE); > > out: > + /* Opportunistically store RTT approximation on valid TCP_INFO data */ > + if (tinfo) > + RTT_SET(conn, tinfo->tcpi_rtt); > + > return new_wnd_to_tap != prev_wnd_to_tap || > conn->seq_ack_to_tap != prev_ack_to_tap; > } > diff --git a/tcp_conn.h b/tcp_conn.h > index e36910c..76034f6 100644 > --- a/tcp_conn.h > +++ b/tcp_conn.h > @@ -49,6 +49,15 @@ struct tcp_tap_conn { > #define MSS_SET(conn, mss) (conn->tap_mss = (mss >> (16 - TCP_MSS_BITS))) > #define MSS_GET(conn) (conn->tap_mss << (16 - TCP_MSS_BITS)) > > +#define RTT_EXP_BITS 3 > + unsigned int rtt_exp :RTT_EXP_BITS; > +#define RTT_EXP_MAX MAX_FROM_BITS(RTT_EXP_BITS) > +#define RTT_STORE_MIN 100 /* us, minimum representable */ > +#define RTT_STORE_MAX (RTT_STORE_MIN << RTT_EXP_MAX) > +#define RTT_SET(conn, rtt) \ > + (conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN)))) > +#define RTT_GET(conn) (RTT_STORE_MIN << conn->rtt_exp) > + > int sock :FD_REF_BITS; > > uint8_t events; > diff --git a/util.c b/util.c > index 4beb7c2..590373d 100644 > --- a/util.c > +++ b/util.c > @@ -611,6 +611,9 @@ int __daemon(int pidfile_fd, int devnull_fd) > * fls() - Find last (most significant) bit set in word > * @x: Word > * > + * Note: unlike ffs() and other implementations of fls(), notably the one from > + * the Linux kernel, the starting position is 0 and not 1, that is, fls(1) = 0. > + * > * Return: position of most significant bit set, starting from 0, -1 if none > */ > int fls(unsigned long x) > @@ -626,6 +629,17 @@ int fls(unsigned long x) > return y; > } > > +/** > + * ilog2() - Integral part (floor) of binary logarithm (logarithm to the base 2) > + * @x: Argument > + * > + * Return: integral part of binary logarithm of @x, -1 if undefined (if @x is 0) > + */ > +int ilog2(unsigned long x) > +{ > + return fls(x); > +} > + > /** > * write_file() - Replace contents of file with a string > * @path: File to write > diff --git a/util.h b/util.h > index 7bf0701..40de694 100644 > --- a/util.h > +++ b/util.h > @@ -230,6 +230,7 @@ int output_file_open(const char *path, int flags); > void pidfile_write(int fd, pid_t pid); > int __daemon(int pidfile_fd, int devnull_fd); > int fls(unsigned long x); > +int ilog2(unsigned long x); > int write_file(const char *path, const char *buf); > intmax_t read_file_integer(const char *path, intmax_t fallback); > int write_all_buf(int fd, const void *buf, size_t len); > -- > 2.43.0 > -- 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