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


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