Re: [PATCH net-next v2] tcp: fix condition for increasing pingpong count

[LemmyHuang]

At 2022-07-21 02:49:35, "Neal Cardwell" <ncardwell@xxxxxxxxxx> wrote:
> On Wed, Jul 20, 2022 at 3:25 AM LemmyHuang <hlm3280@xxxxxxx> wrote:
>>
>> When CONFIG_HZ defaults to 1000Hz and the network transmission time is
>> less than 1ms, lsndtime and lrcvtime are likely to be equal, which will
>> lead to hundreds of interactions before entering pingpong mode.
>>
>> Fixes: 4a41f453bedf ("tcp: change pingpong threshold to 3")
>> Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>
>> Signed-off-by: LemmyHuang <hlm3280@xxxxxxx>
>> ---
>> v2:
>>   * Use !after() wrapping the values. (Jakub Kicinski)
>>
>> v1: https://lore.kernel.org/netdev/20220719130136.11907-1-hlm3280@xxxxxxx/
>> ---
>>  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 858a15cc2..c1c95dc40 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -172,7 +172,7 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
>>          * and it is a reply for ato after last received packet,
>>          * increase pingpong count.
>>          */
>> -       if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
>> +       if (!after(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
>>             (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
>>                 inet_csk_inc_pingpong_cnt(sk);
>>
>> --
>
> Thanks for pointing out this problem!
> 
> AFAICT this patch would result in incorrect behavior.
> 
> With this patch, we could have cases where tp->lsndtime ==
> icsk->icsk_ack.lrcvtime and (u32)(now - icsk->icsk_ack.lrcvtime) <
> icsk->icsk_ack.ato and yet we do not really have a ping-pong exchange.
> 
> For example, with this patch we could have:
> 
> T1: jiffies=J1; host B receives RPC request from host A
> T2: jiffies=J1; host B sends first RPC response data packet to host A;
>       -> calls inet_csk_inc_pingpong_cnt()
> T3: jiffies=J1; host B sends second RPC response data packet to host A;
>       -> calls inet_csk_inc_pingpong_cnt()
> 
> In this scenario there is only one ping-pong exchange but the code
> calls inet_csk_inc_pingpong_cnt() twice.
> 
> So I'm hoping we can come up with a better fix.
> 
> A simpler approach might be to simplify the model and go back to
> having a single ping-pong interaction cause delayed ACKs to be enabled
> on a connection endpoint. Our team has been seeing good results for a
> while with the simpler approach. What do folks think?
> 
> 
> neal

It seems better to go back.

Look at this revert patch:
https://lore.kernel.org/netdev/20220720233156.295074-1-weiwan@xxxxxxxxxx/