Messages in this thread | | | Date | Wed, 27 May 2015 10:40:29 -0400 | From | Ido Yariv <> | Subject | Re: [PATCH] net: tcp: Fix a PTO timing granularity issue |
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Hi Eric,
On Wed, May 27, 2015 at 06:41:17AM -0700, Eric Dumazet wrote: > 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.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 > > > > 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.
HZ=100 is used on some embedded platforms, so it's still something we have to deal with unfortunately..
Since the '2' here is a lower bound, and msecs_to_jiffies(10) will return values greater than 2 for HZ>100 anyway, always ensuring the 2 jiffies lower bound shouldn't impact the behavior when HZ=1000.
However, as far as I can tell, comparing msecs_to_jiffies(10) to 2, or comparing the whole timeout to 2 doesn't make much difference, since msecs_to_jiffies isn't inlined.
In other words, keeping the #if shouldn't make much difference in behavior, but will save the small comparison.
Cheers, Ido.
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