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

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Message-ID: <CADVnQy=+Ad3cHHCxge37OKm=fHi6+aHyyy_0hp7p4VCSAGX4cw@mail.gmail.com>
Date: Fri, 16 Aug 2024 14:30:51 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Mingrui Zhang <mrzhang97@...il.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 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