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.ya...@intel.com>
---
include/net/tcp.h | 20 ++++++++++++++++++++
net/ipv4/tcp_output.c | 2 +-
2 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2bb2bad..2ff0181 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1751,4 +1751,24 @@ static inline void skb_set_tcp_pure_ack(struct sk_buff
*skb)
skb->truesize = 2;
}
+/* Convert msecs to jiffies, ensuring that the return value is at least 2
+ * jiffies.
+ * This can be used when setting tick-based timers to guarantee that they won't
+ * expire right away.
+ */
+static inline unsigned long tcp_msecs_to_jiffies_min_2(const unsigned int m)
+{
+ if (__builtin_constant_p(m)) {
+ /* The theoretical upper bound of m for 2 jiffies is 2 seconds,
+ * so compare m with that to avoid potential integer overflows.
+ */
+ if ((m > 2 * MSEC_PER_SEC) || (m * HZ > 2 * MSEC_PER_SEC))
+ return msecs_to_jiffies(m);
+
+ return 2;
+ }
+
+ return max_t(u32, 2, msecs_to_jiffies(m));
+}
+
#endif /* _TCP_H */
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 190538a..37694d2 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2207,7 +2207,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
if (tp->packets_out == 1)
timeout = max_t(u32, timeout,
(rtt + (rtt >> 1) + TCP_DELACK_MAX));
- timeout = max_t(u32, timeout, msecs_to_jiffies(10));
+ timeout = max_t(u32, timeout, tcp_msecs_to_jiffies_min_2(10));
/* If RTO is shorter, just schedule TLP in its place. */
tlp_time_stamp = tcp_time_stamp + timeout;
--
2.1.0
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