Re: WARN_ON in TLP causing RT throttling

[Eric Dumazet <eric.dumazet@gmail.com>]

On 09/26/2018 04:46 PM, stranche@codeaurora.org wrote:
> Hi Eric,
> 
> Someone recently reported a crash to us on the 4.14.62 kernel where excessive
> WARNING prints were spamming the logs and causing watchdog bites. The kernel
> does have the following commit by Soheil:
> bffd168c3fc5 "tcp: clear tp->packets_out when purging write queue"
> 
> Before this bug we see over 1 second of continuous WARN_ON prints from
> tcp_send_loss_probe() like so:
> 
> 7795.530450:   <2>  tcp_send_loss_probe+0x194/0x1b8
> 7795.534833:   <2>  tcp_write_timer_handler+0xf8/0x1c4
> 7795.539492:   <2>  tcp_write_timer+0x4c/0x74
> 7795.543348:   <2>  call_timer_fn+0xc0/0x1b4
> 7795.547113:   <2>  run_timer_softirq+0x248/0x81c
> 
> Specifically, the prints come from the following check:
> 
>     /* Retransmit last segment. */
>     if (WARN_ON(!skb))
>         goto rearm_timer;
> 
> Since skb is always NULL, we know there's nothing on the write queue or the
> retransmit queue, so we just keep resetting the timer, waiting for more data
> to be queued. However, we were able to determine that the TCP socket is in the
> TCP_FIN_WAIT1 state, so we will no longer be sending any data and these queues
> remain empty.
> 
> Would it be appropriate to stop resetting the TLP timer if we detect that the
> connection is starting to close and we have no more data to send the probe with,
> or is there some way that this scenario should already be handled?
> 
> Unfortunately, we don't have a reproducer for this crash.
>

Something is fishy.

If there is no skb in the queues, then tp->packets_out should be 0,
therefore tcp_rearm_rto() should simply call inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);

I have never seen this report before.