netdev - Re: [PATCH net v2 1/3] tcp_cubic: fix to run bictcp

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Message-ID: <73788a78-ada2-4e3c-bf33-e5e8f266920b@gmail.com>
Date: Thu, 15 Aug 2024 16:48:15 -0500
From: Mingrui Zhang <mrzhang97@...il.com>
To: Neal Cardwell <ncardwell@...gle.com>
Cc: edumazet@...gle.com, davem@...emloft.net, netdev@...r.kernel.org,
 Lisong Xu <xu@....edu>
Subject: Re: [PATCH net v2 1/3] tcp_cubic: fix to run bictcp_update() at least
 once per RTT

On 8/15/24 09:43, Neal Cardwell wrote:
> On Wed, Aug 14, 2024 at 8:19 PM Mingrui Zhang <mrzhang97@...il.com> wrote:
>> The original code bypasses bictcp_update() under certain conditions
>> to reduce the CPU overhead. Intuitively, when last_cwnd==cwnd,
>> bictcp_update() is executed 32 times per second. As a result,
>> it is possible that bictcp_update() is not executed for several
>> RTTs when RTT is short (specifically < 1/32 second = 31 ms and
>> last_cwnd==cwnd which may happen in small-BDP networks),
>> thus leading to low throughput in these RTTs.
>>
>> The patched code executes bictcp_update() 32 times per second
>> if RTT > 31 ms or every RTT if RTT < 31 ms, when last_cwnd==cwnd.
>>
>> Thanks
>> Mingrui, and Lisong
>>
>> Fixes: 91a4599c2ad8 ("tcp_cubic: fix to run bictcp_update() at least once per RTT")
> The Fixes: footer is not for restating the commit title in your new
> patch. Instead, it should list the SHA1 and first commit message line
> from the old commit (potentially years ago) that has buggy behavior
> that you are fixing. That way the Linux maintainers know which Linux
> releases have the bug, and they know how far back in release history
> the fix should be applied, when backporting fixes to stable releases.
>
> More information:
>  https://www.kernel.org/doc/html/v6.10/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes
>
> Please update the Fixes footers in the three commits and repost. :-)
>
> Thanks!
> neal
>
Thank you again, Neal.
I appreciate your detailed instructions on Fixes footer. I misunderstood its meaning.

Thanks
Mingrui
>> Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
>> Signed-off-by: Lisong Xu <xu@....edu>
>>
>> ---
>>  net/ipv4/tcp_cubic.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
>> index 5dbed91c6178..11bad5317a8f 100644
>> --- a/net/ipv4/tcp_cubic.c
>> +++ b/net/ipv4/tcp_cubic.c
>> @@ -218,8 +218,12 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
>>
>>         ca->ack_cnt += acked;   /* count the number of ACKed packets */
>>
>> +       /* Update 32 times per second if RTT > 1/32 second,
>> +        *        every RTT if RTT < 1/32 second
>> +        *        even when last_cwnd == cwnd
>> +        */
>>         if (ca->last_cwnd == cwnd &&
>> -           (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
>> +           (s32)(tcp_jiffies32 - ca->last_time) <= min(HZ / 32, usecs_to_jiffies(ca->delay_min)))
>>                 return;
>>
>>         /* The CUBIC function can update ca->cnt at most once per jiffy.
>> --
>> 2.34.1
>>