netdev - Re: [PATCH net] tcp: fix TLP timer not set when CA

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Date:   Sat, 23 Jan 2021 11:14:00 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Pengcheng Yang <yangpc@...gsu.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: fix TLP timer not set when CA_STATE changes from
 DISORDER to OPEN

On Sat, Jan 23, 2021 at 5:27 AM Pengcheng Yang <yangpc@...gsu.com> wrote:
>
> On Sat, Jan 23, 2021 at 5:02 AM "Yuchung Cheng" <ycheng@...gle.com> wrote:
> >
> > On Fri, Jan 22, 2021 at 6:37 AM Neal Cardwell <ncardwell@...gle.com> wrote:
> > >
> > > On Fri, Jan 22, 2021 at 5:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > On Fri, Jan 22, 2021 at 11:28 AM Pengcheng Yang <yangpc@...gsu.com> wrote:
> > > > >
> > > > > When CA_STATE is in DISORDER, the TLP timer is not set when receiving
> > > > > an ACK (a cumulative ACK covered out-of-order data) causes CA_STATE to
> > > > > change from DISORDER to OPEN. If the sender is app-limited, it can only
> >
> > Could you point which line of code causes the state to flip
> > incorrectly due to the TLP timer setting?
> >
>
> I mean TLP timer is not set due to receiving an ACK that changes CA_STATE
> from DISORDER to OPEN.
Acked-by: Yuchung Cheng <ycheng@...gle.com>


Thanks for the clarification and the fix. The initial cmsg reads like
the timer setting is causing state flips. I'd suggest this commit
message in your next rev.

"Upon receiving a cumulative ACK that changes the congestion state
from Disorder to Open, the TLP timer is not set. If the sender is
app-limited, it can only wait for the RTO timer to expire and
retransmit.

The reason for this ... "


>
> Receive an ACK covered out-of-order data in disorder state:
>
> tcp_ack()
> |-tcp_set_xmit_timer()  // RTO timer is set instead of TLP timer
> |  ...
> |-tcp_fastretrans_alert() // change from disorder to open
>
> > > > > wait for the RTO timer to expire and retransmit.
> > > > >
> > > > > The reason for this is that the TLP timer is set before CA_STATE changes
> > > > > in tcp_ack(), so we delay the time point of calling tcp_set_xmit_timer()
> > > > > until after tcp_fastretrans_alert() returns and remove the
> > > > > FLAG_SET_XMIT_TIMER from ack_flag when the RACK reorder timer is set.
> > > > >
> > > > > This commit has two additional benefits:
> > > > > 1) Make sure to reset RTO according to RFC6298 when receiving ACK, to
> > > > > avoid spurious RTO caused by RTO timer early expires.
> > > > > 2) Reduce the xmit timer reschedule once per ACK when the RACK reorder
> > > > > timer is set.
> > > > >
> > > > > Link: https://lore.kernel.org/netdev/1611139794-11254-1-git-send-email-yangpc@wangsu.com
> > > > > Signed-off-by: Pengcheng Yang <yangpc@...gsu.com>
> > > > > Cc: Neal Cardwell <ncardwell@...gle.com>
> > > > > ---
> > > >
> > > > This looks like a very nice patch, let me run packetdrill tests on it.
> > > >
> > > > By any chance, have you cooked a packetdrill test showing the issue
> > > > (failing on unpatched kernel) ?
> > >
> > > Thanks, Pengcheng. This patch looks good to me as well, assuming it
> > > passes our packetdrill tests. I agree with Eric that it would be good
> > > to have an explicit packetdrill test for this case.
> > >
> > > neal
>
> Here is a packetdrill test case:
>
> // Enable TLP
>     0 `sysctl -q net.ipv4.tcp_early_retrans=3`
>
> // Establish a connection
>    +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
>    +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
>    +0 bind(3, ..., ...) = 0
>    +0 listen(3, 1) = 0
>
> // RTT 100ms, RTO 300ms
>   +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
>    +0 > S. 0:0(0) ack 1 <...>
>   +.1 < . 1:1(0) ack 1 win 257
>    +0 accept(3, ..., ...) = 4
>
> // Send 4 data segments
>    +0 write(4, ..., 4000) = 4000
>    +0 > P. 1:4001(4000) ack 1
>
> // out-of-order: ca_state turns to disorder
>   +.1 < . 1:1(0) ack 1 win 257 <sack 1001:2001,nop,nop>
>
> // ACK covered out-of-order data: ca_state turns to open,
> // but RTO timer is set instead of TLP timer and the RTO
> // timer will expire at rtx_head_time+RTO (in 200ms).
>    +0 < . 1:1(0) ack 2001 win 257
>
> // Expect to send TLP packet in 2*rtt (200ms)
> +.2~+.25 > P. 3001:4001(1000) ack 1
>
>
> I ran this packetdrill test case on the kernel without
> the patch applied:
>
> tlp_timer_unset.pkt:31: error handling packet: live packet
> field tcp_seq: expected: 3001 (0xbb9) vs actual: 2001 (0x7d1)
> script packet:  0.644587 P. 3001:4001(1000) ack 1
> actual packet:  0.646197 . 2001:3001(1000) ack 1 win 502
>