netdev - Re: [PATCH net v3 1/3] tcp_cubic: fix to run bictcp

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Message-ID: <db480e4e-2601-488b-a2a6-2ee9272e0f93@gmail.com>
Date: Fri, 16 Aug 2024 21:08:52 -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 v3 1/3] tcp_cubic: fix to run bictcp_update() at least
 once per RTT

On 8/16/24 13:30, Neal Cardwell wrote:
> On Thu, Aug 15, 2024 at 5:41 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.
>>
>> Fixes: df3271f3361b ("[TCP] BIC: CUBIC window growth (2.0)")
>> Fixes: ac35f562203a ("tcp: bic, cubic: use tcp_jiffies32 instead of tcp_time_stamp")
>> Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
>> Signed-off-by: Lisong Xu <xu@....edu>
>> ---
>> v2->v3: Corrent the "Fixes:" footer content
>> v1->v2: Separate patches
>>
>>  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.
>> --
> I'm getting a compiler error with our builds with clang:
>
> net/ipv4/tcp_cubic.c:230:46: error: comparison of distinct pointer
> types
> ('typeof (1000 / 32) *' (aka 'int *') and
> 'typeof (usecs_to_jiffies(ca->delay_min)) *' (aka 'unsigned long *'))
> [-Werror,-Wcompare-distinct-pointer-types]
>           (s32)(tcp_jiffies32 - ca->last_time) <= min(HZ / 32,
> usecs_to_jiffies(ca->delay_min)))
>
> Can you please try something like the following, which works for our build:
>
>            (s32)(tcp_jiffies32 - ca->last_time) <=
>            min_t(s32, HZ / 32, usecs_to_jiffies(ca->delay_min)))
>
> thanks,
> neal
Thank you, Neal,
We have tried your suggested changes, and they also work for our compile and experiment tests.
 
Thanks,
Mingrui