On Wed, 2015-05-27 at 11:36 +0000, David Laight wrote: > 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. >
Have you followed previous discussions ? I guess we can have a helper macro, but for the moment only one spot was found. Its kind of depressing having to deal with HZ=100 issues, with modern NO_HZ configurations. TCP rtts have now usec resolution, so HZ=100 is pushing TCP to very imprecise behavior. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html