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

[Ido Yariv]

Hi Eric,

On Tue, May 26, 2015 at 09:23:55AM -0700, Eric Dumazet wrote:
> On Tue, 2015-05-26 at 10:25 -0400, Ido Yariv wrote:
> > 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 might expire
> > prematurely. This is especially problematic on systems in which HZ=100.
> > 
> > To work around this issue, increase the number of jiffies by one,
> > ensuring that the timeout won't expire in less than 10ms.
> > 
> > Signed-off-by: Ido Yariv <idox.yariv@xxxxxxxxx>
> > ---
> >  net/ipv4/tcp_output.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index 534e5fd..6f57d3d 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, msecs_to_jiffies(10) + 1);
> >  
> >  	/* If RTO is shorter, just schedule TLP in its place. */
> >  	tlp_time_stamp = tcp_time_stamp + timeout;
> 
> Have you really hit an issue, or did you send this patch after all these
> msecs_to_jiffies() discussions on lkml/netdev ?

This actually fixed a specific issue I ran into. This issue caused a
degradation in throughput in a benchmark which sent relatively small
chunks of data (100KB) in a loop. The impact was quite substantial -
with this patch, throughput increased by up to 50% on the platform this
was tested on.

> 
> Not sure this is the right fix.
> 
> TLP was really tested with an effective min delay of 10ms.
> 
> Adding 10% for the sake of crazy HZ=100 builds seems a high price.
> (All recent TCP changes were tested with HZ=1000 BTW ...)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 534e5fdb04c11152bae36f47a786e8b10b823cd3..5321df89af9b59c6727395c489e6f9b2770dcd5e 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;

This was actually the first incarnation of this patch. However, while
the impact of this issue when HZ=100 is the greatest, it can also impact
other settings as well. For instance, if HZ=250, the timer could expire
after a bit over 8ms instead of 10ms, and 9ms for HZ=1000.

By increasing the number of jiffies, we ensure that we'll wait at least
10ms but never less than that, so for HZ=1000, it'll be anywhere between
10ms and 11ms instead of 9ms and 10ms.

Thanks,
Ido.
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