From: Of Ido Yariv
> Sent: 26 May 2015 21:17
> 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>
> ---
> 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
Why not:
timeout = max_t(u32, timeout, max_t(u32, 2, msecs_to_jiffies(10)));
I think the RH max_t() is a compile time constant.
You need 2 jiffies to guarantee a non-zero timeout.
Even if HZ=199 with a 'rounding down' msecs_to_jiffies() you get 1 jiffy
and a possible immediate timeout.
There are probably other places where msecs_to_jiffies() better not return 1.
David
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