linux-kernel - Re: [PATCH net-next] tcp: clamp window like before the cleanup

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Message-ID: <a3266974-d561-4e8f-a23a-9c0774ee2bbe@kernel.org>
Date: Thu, 6 Mar 2025 11:12:36 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Jason Xing <kerneljasonxing@...il.com>, mptcp@...ts.linux.dev,
 Neal Cardwell <ncardwell@...gle.com>, Kuniyuki Iwashima <kuniyu@...zon.com>,
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: clamp window like before the cleanup

On 06/03/2025 11:02, Eric Dumazet wrote:
> On Thu, Mar 6, 2025 at 10:55 AM Matthieu Baerts <matttbe@...nel.org> wrote:
>>
>> Hi Eric,
>>
>> On 06/03/2025 10:45, Eric Dumazet wrote:
>>> On Thu, Mar 6, 2025 at 6:22 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>>>>
>>>> On Wed, Mar 5, 2025 at 10:49 PM Matthieu Baerts (NGI0)
>>>> <matttbe@...nel.org> wrote:
>>>>>
>>>>> A recent cleanup changed the behaviour of tcp_set_window_clamp(). This
>>>>> looks unintentional, and affects MPTCP selftests, e.g. some tests
>>>>> re-establishing a connection after a disconnect are now unstable.
>>>>>
>>>>> Before the cleanup, this operation was done:
>>>>>
>>>>>   new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
>>>>>   tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);
>>>>>
>>>>> The cleanup used the 'clamp' macro which takes 3 arguments -- value,
>>>>> lowest, and highest -- and returns a value between the lowest and the
>>>>> highest allowable values. This then assumes ...
>>>>>
>>>>>   lowest (rcv_ssthresh) <= highest (rcv_wnd)
>>>>>
>>>>> ... which doesn't seem to be always the case here according to the MPTCP
>>>>> selftests, even when running them without MPTCP, but only TCP.
>>>>>
>>>>> For example, when we have ...
>>>>>
>>>>>   rcv_wnd < rcv_ssthresh < new_rcv_ssthresh
>>>>>
>>>>> ... before the cleanup, the rcv_ssthresh was not changed, while after
>>>>> the cleanup, it is lowered down to rcv_wnd (highest).
>>>>>
>>>>> During a simple test with TCP, here are the values I observed:
>>>>>
>>>>>   new_window_clamp (val)  rcv_ssthresh (lo)  rcv_wnd (hi)
>>>>>       117760   (out)         65495         <  65536
>>>>>       128512   (out)         109595        >  80256  => lo > hi
>>>>>       1184975  (out)         328987        <  329088
>>>>>
>>>>>       113664   (out)         65483         <  65536
>>>>>       117760   (out)         110968        <  110976
>>>>>       129024   (out)         116527        >  109696 => lo > hi
>>>>>
>>>>> Here, we can see that it is not that rare to have rcv_ssthresh (lo)
>>>>> higher than rcv_wnd (hi), so having a different behaviour when the
>>>>> clamp() macro is used, even without MPTCP.
>>>>>
>>>>> Note: new_window_clamp is always out of range (rcv_ssthresh < rcv_wnd)
>>>>> here, which seems to be generally the case in my tests with small
>>>>> connections.
>>>>>
>>>>> I then suggests reverting this part, not to change the behaviour.
>>>>>
>>>>> Fixes: 863a952eb79a ("tcp: tcp_set_window_clamp() cleanup")
>>>>> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/551
>>>>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
>>>>
>>>> Tested-by: Jason Xing <kerneljasonxing@...il.com>
>>>>
>>>> Thanks for catching this. I should have done more tests :(
>>>>
>>>> Now I use netperf with TCP_CRR to test loopback and easily see the
>>>> case where tp->rcv_ssthresh is larger than tp->rcv_wnd, which means
>>>> tp->rcv_wnd is not the upper bound as you said.
>>>>
>>>> Thanks,
>>>> Jason
>>>>
>>>
>>> Patch looks fine to me but all our tests are passing with the current kernel,
>>> and I was not able to trigger the condition.
>>
>> Thank you for having looked at this patch!
>>
>>
>>> Can you share what precise test you did ?
>>
>> To be able to get a situation where "rcv_ssthresh > rcv_wnd", I simply
>> executed MPTCP Connect selftest. You can also force creating TCP only
>> connections with '-tt', e.g.
>>
>>   ./mptcp_connect.sh -tt
> 
> I was asking Jason about TCP tests. He mentioned TCP_CRR

Oops, I'm sorry, I didn't look at the "To:" field.

> I made several of them, with temporary debug in the kernel that did
> not show the issue.
> 
> 
> I am wondering if this could hide an issue in MPTCP ?
Indeed, I was wondering the same thing. I didn't see anything obvious
when looking at this issue. The behaviours around the window clamping,
with MPTCP single flow, and "plain" TCP were quite similar I think.

Cheers,
Matt
-- 
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