Re: [net,v2] tcp: correct handling of extreme memory squeeze

[Jon Maloy <jmaloy@redhat.com>]

On 2025-01-20 11:22, Eric Dumazet wrote:
> On Mon, Jan 20, 2025 at 5:10 PM Jon Maloy <jmaloy@redhat.com> wrote:
>>
>>
>>
>> On 2025-01-20 00:03, Jon Maloy wrote:
>>>
>>>

[...]

>>>> I agree with Eric that probably tp->pred_flags should be cleared, and
>>>> a packetdrill test for this would be super-helpful.
>>>
>>> I must admit I have never used packetdrill, but I can make an effort.
>>
>> I hear from other sources that you cannot force a memory exhaustion with
>> packetdrill anyway, so this sounds like a pointless exercise.
> 
> We certainly can and should add a feature like that to packetdrill.
> 
> Documentation/fault-injection/ has some relevant information.
> 
> Even without this, tcp_try_rmem_schedule() is reading sk->sk_rcvbuf
> that could be lowered by a packetdrill script I think.
> 
Neal, Eric,
How do you suggest we proceed with this?
I downloaded packetdrill and tried it a bit, but to understand it well 
enough to introduce a new feature would require more time than I am
able to spend on this. Maybe Neal, who I see is one of the contributors 
to packetdrill could help out?

I can certainly clear tp->pred_flags and post it again, maybe with
an improved and shortened log. Would that be acceptable?

I also made a run where I looked into why __tcp_select_window()
ignores all the space that has been freed up:


  tcp_recvmsg_locked(->)
    __tcp_cleanup_rbuf(->) (copied 131072)
      tp->rcv_wup: 1788299855, tp->rcv_wnd: 5812224,
      tp->rcv_nxt 1793800175
      __tcp_select_window(->)
        tcp_space(->)
        tcp_space(<-) returning 458163
        free_space = round_down(458163, 1 << 4096) = 454656
        (free_space > tp->rcv_ssthresh) -->
          free_space = tp->rcv_ssthresh = 261920
        window = ALIGN(261920, 4096) = 26144
      __tcp_select_window(<-) returning 262144
      [rcv_win_now 311904, 2 * rcv_win_now 623808, new_window 262144]
      (new_window >= (2 * rcv_win_now)) ? --> time_to_ack 0
      NOT calling tcp_send_ack()
    __tcp_cleanup_rbuf(<-)
    [tp->rcv_wup 1788299855, tp->rcv_wnd 5812224,
     tp->rcv_nxt 1793800175]
  tcp_recvmsg_locked(<-) returning 131072 bytes.
  [tp->rcv_nxt 1793800175, tp->rcv_wnd 5812224,
   tp->rcv_wup 1788299855, sk->last_ack 0, tcp_receive_win() 311904,
   copied_seq 1788299855->1788395953 (96098), unread 5404222,
   sk_rcv_qlen 83, ofo_qlen 0]


As we see tp->rcv_ssthresh is the limiting factor, causing
a consistent situation where (new_window < (rcv_win_now * 2)),
and even (new_window < rcv_win_now).

To me, it looks like tp->ssthresh should have a higher value
in this situation, or maybe we should alter this test.

The combination of these two issues, -not updating tp->wnd and
_tcp_select_window() returning a wrong value, is what is causing
this whole problem.

///jon