tcp: fix excessive TLP and RACK timeouts from HZ rounding

JIRA: https://issues.redhat.com/browse/RHEL-83546

commit 1c2709c
Author: Neal Cardwell <[email protected]>
Date:   Sun Oct 15 13:47:00 2023 -0400

    tcp: fix excessive TLP and RACK timeouts from HZ rounding

    We discovered from packet traces of slow loss recovery on kernels with
    the default HZ=250 setting (and min_rtt < 1ms) that after reordering,
    when receiving a SACKed sequence range, the RACK reordering timer was
    firing after about 16ms rather than the desired value of roughly
    min_rtt/4 + 2ms. The problem is largely due to the RACK reorder timer
    calculation adding in TCP_TIMEOUT_MIN, which is 2 jiffies. On kernels
    with HZ=250, this is 2*4ms = 8ms. The TLP timer calculation has the
    exact same issue.

    This commit fixes the TLP transmit timer and RACK reordering timer
    floor calculation to more closely match the intended 2ms floor even on
    kernels with HZ=250. It does this by adding in a new
    TCP_TIMEOUT_MIN_US floor of 2000 us and then converting to jiffies,
    instead of the current approach of converting to jiffies and then
    adding th TCP_TIMEOUT_MIN value of 2 jiffies.

    Our testing has verified that on kernels with HZ=1000, as expected,
    this does not produce significant changes in behavior, but on kernels
    with the default HZ=250 the latency improvement can be large. For
    example, our tests show that for HZ=250 kernels at low RTTs this fix
    roughly halves the latency for the RACK reorder timer: instead of
    mostly firing at 16ms it mostly fires at 8ms.

    Suggested-by: Eric Dumazet <[email protected]>
    Signed-off-by: Neal Cardwell <[email protected]>
    Signed-off-by: Yuchung Cheng <[email protected]>
    Fixes: bb4d991 ("tcp: adjust tail loss probe timeout")
    Reviewed-by: Eric Dumazet <[email protected]>
    Link: https://lore.kernel.org/r/[email protected]
    Signed-off-by: Jakub Kicinski <[email protected]>

Signed-off-by: CKI Backport Bot <[email protected]>

Author: CKI Backport Bot

include/net/tcp.h

@@ -143,6 +143,9 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCP_RTO_MAX	((unsigned)(120*HZ))
 #define TCP_RTO_MIN	((unsigned)(HZ/5))
 #define TCP_TIMEOUT_MIN	(2U) /* Min timeout for TCP timers in jiffies */
+
+#define TCP_TIMEOUT_MIN_US (2*USEC_PER_MSEC) /* Min TCP timeout in microsecs */
+
 #define TCP_TIMEOUT_INIT ((unsigned)(1*HZ))	/* RFC6298 2.1 initial RTO value	*/
 #define TCP_TIMEOUT_FALLBACK ((unsigned)(3*HZ))	/* RFC 1122 initial RTO value, now
 						 * used as a fallback RTO for the

net/ipv4/tcp_output.c

@@ -2746,7 +2746,7 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
-	u32 timeout, rto_delta_us;
+	u32 timeout, timeout_us, rto_delta_us;
 	int early_retrans;
 
 	/* Don't do any loss probe on a Fast Open connection before 3WHS
@@ -2770,11 +2770,12 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
 	 * sample is available then probe after TCP_TIMEOUT_INIT.
 	 */
 	if (tp->srtt_us) {
-		timeout = usecs_to_jiffies(tp->srtt_us >> 2);
+		timeout_us = tp->srtt_us >> 2;
 		if (tp->packets_out == 1)
-			timeout += TCP_RTO_MIN;
+			timeout_us += tcp_rto_min_us(sk);
 		else
-			timeout += TCP_TIMEOUT_MIN;
+			timeout_us += TCP_TIMEOUT_MIN_US;
+		timeout = usecs_to_jiffies(timeout_us);
 	} else {
 		timeout = TCP_TIMEOUT_INIT;
 	}

net/ipv4/tcp_recovery.c

@@ -109,7 +109,7 @@ bool tcp_rack_mark_lost(struct sock *sk)
 	tp->rack.advanced = 0;
 	tcp_rack_detect_loss(sk, &timeout);
 	if (timeout) {
-		timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
+		timeout = usecs_to_jiffies(timeout + TCP_TIMEOUT_MIN_US);
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 					  timeout, inet_csk(sk)->icsk_rto);
 	}