linux-kernel - RE: [RFC] tcp_bbr2: use correct 64-bit division

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Date:   Tue, 24 May 2022 08:01:18 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Oleksandr Natalenko' <oleksandr@...alenko.name>,
        Neal Cardwell <ncardwell@...gle.com>
CC:     Yuchung Cheng <ycheng@...gle.com>,
        Yousuk Seung <ysseung@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Adithya Abraham Philip <abrahamphilip@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Konstantin Demin" <rockdrilla@...il.com>
Subject: RE: [RFC] tcp_bbr2: use correct 64-bit division

From: Oleksandr Natalenko
> Sent: 22 May 2022 23:30
> To: Neal Cardwell <ncardwell@...gle.com>
> 
> Hello Neal.
> 
> It was reported to me [1] by Konstantin (in Cc) that BBRv2 code suffers from integer division issue on
> 32 bit systems.

Do any of these divisions ever actually have 64bit operands?
Even on x86-64 64bit divide is significantly slower than 32bit divide.

It is quite clear that x * 8 / 1000 is the same as x / (1000 / 8).
So promoting to 64bit cannot be needed.

	David

> 
> Konstantin suggested a solution available in the same linked merge request and copy-pasted by me below
> for your convenience:
> 
> ```
> diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
> index 664c9e119787..fd3f89e3a8a6 100644
> --- a/net/ipv4/tcp_bbr.c
> +++ b/net/ipv4/tcp_bbr.c
> @@ -312,7 +312,7 @@ static u32 bbr_tso_segs_generic(struct sock *sk, unsigned int mss_now,
>  	bytes = sk->sk_pacing_rate >> sk->sk_pacing_shift;
> 
>  	bytes = min_t(u32, bytes, gso_max_size - 1 - MAX_TCP_HEADER);
> -	segs = max_t(u32, bytes / mss_now, bbr_min_tso_segs(sk));
> +	segs = max_t(u32, div_u64(bytes, mss_now), bbr_min_tso_segs(sk));
>  	return segs;
>  }
> 
> diff --git a/net/ipv4/tcp_bbr2.c b/net/ipv4/tcp_bbr2.c
> index fa49e17c47ca..488429f0f3d0 100644
> --- a/net/ipv4/tcp_bbr2.c
> +++ b/net/ipv4/tcp_bbr2.c
> @@ -588,7 +588,7 @@ static void bbr_debug(struct sock *sk, u32 acked,
>  		 bbr_rate_kbps(sk, bbr_max_bw(sk)), /* bw: max bw */
>  		 0ULL,				    /* lb: [obsolete] */
>  		 0ULL,				    /* ib: [obsolete] */
> -		 (u64)sk->sk_pacing_rate * 8 / 1000,
> +		 div_u64((u64)sk->sk_pacing_rate * 8, 1000),
>  		 acked,
>  		 tcp_packets_in_flight(tp),
>  		 rs->is_ack_delayed ? 'd' : '.',
> @@ -698,7 +698,7 @@ static u32 bbr_tso_segs_generic(struct sock *sk, unsigned int mss_now,
>  	}
> 
>  	bytes = min_t(u32, bytes, gso_max_size - 1 - MAX_TCP_HEADER);
> -	segs = max_t(u32, bytes / mss_now, bbr_min_tso_segs(sk));
> +	segs = max_t(u32, div_u64(bytes, mss_now), bbr_min_tso_segs(sk));
>  	return segs;
>  }
> ```
> 
> Could you please evaluate this report and check whether it is correct, and also check whether the
> suggested patch is acceptable?
> 
> Thanks.
> 
> [1] https://gitlab.com/post-factum/pf-kernel/-/merge_requests/6
> 
> --
> Oleksandr Natalenko (post-factum)
> 

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