[PATCH] net: tcp: Fix a PTO timing granularity issue

[Ido Yariv <ido@wizery.com>]

The Tail Loss Probe RFC specifies that the PTO value should be set to
max(2 * SRTT, 10ms), where SRTT is the smoothed round-trip time.

The PTO value is converted to jiffies, so the timer may expire
prematurely.

This is especially problematic on systems in which HZ <= 100, so work
around this by setting the timeout to at least 2 jiffies on such
systems.

The 10ms figure was originally selected based on tests performed with
the current implementation and HZ = 1000. Thus, leave the behavior on
systems with HZ > 100 unchanged.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
---
 net/ipv4/tcp_output.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 534e5fd..5321df8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2208,6 +2208,9 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 		timeout = max_t(u32, timeout,
 				(rtt + (rtt >> 1) + TCP_DELACK_MAX));
 	timeout = max_t(u32, timeout, msecs_to_jiffies(10));
+#if HZ <= 100
+	timeout = max_t(u32, timeout, 2);
+#endif
 
 	/* If RTO is shorter, just schedule TLP in its place. */
 	tlp_time_stamp = tcp_time_stamp + timeout;
-- 
2.1.0