Re: [PATCH net-next] tcp: allow TLP in ECN CWR

[David Miller <davem@davemloft.net>]

From: Yuchung Cheng <ycheng@google.com>
Date: Mon, 11 Dec 2017 15:42:53 -0800

> From: Neal Cardwell <ncardwell@google.com>
> 
> This patch enables tail loss probe in cwnd reduction (CWR) state
> to detect potential losses. Prior to this patch, since the sender
> uses PRR to determine the cwnd in CWR state, the combination of
> CWR+PRR plus tcp_tso_should_defer() could cause unnecessary stalls
> upon losses: PRR makes cwnd so gentle that tcp_tso_should_defer()
> defers sending wait for more ACKs. The ACKs may not come due to
> packet losses.
> 
> Disallowing TLP when there is unused cwnd had the primary effect
> of disallowing TLP when there is TSO deferral, Nagle deferral,
> or we hit the rwin limit. Because basically every application
> write() or incoming ACK will cause us to run tcp_write_xmit()
> to see if we can send more, and then if we sent something we call
> tcp_schedule_loss_probe() to see if we should schedule a TLP. At
> that point, there are a few common reasons why some cwnd budget
> could still be unused:
> 
> (a) rwin limit
> (b) nagle check
> (c) TSO deferral
> (d) TSQ
> 
> For (d), after the next packet tx completion the TSQ mechanism
> will allow us to send more packets, so we don't really need a
> TLP (in practice it shouldn't matter whether we schedule one
> or not). But for (a), (b), (c) the sender won't send any more
> packets until it gets another ACK. But if the whole flight was
> lost, or all the ACKs were lost, then we won't get any more ACKs,
> and ideally we should schedule and send a TLP to get more feedback.
> In particular for a long time we have wanted some kind of timer for
> TSO deferral, and at least this would give us some kind of timer
> 
> Reported-by: Steve Ibanez <sibanez@stanford.edu>
> Signed-off-by: Neal Cardwell <ncardwell@google.com>
> Signed-off-by: Yuchung Cheng <ycheng@google.com>
> Reviewed-by: Nandita Dukkipati <nanditad@google.com>
> Reviewed-by: Eric Dumazet <edumazet@google.com>

Applied, thanks.