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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 24 May 2022 13:22:49 -0700
Message-ID: <CANn89iKsJ53GKJyKGLLvn_NhY_oe5TzHLGRtRttZpHaUOVmiBw@mail.gmail.com>
Subject: Re: [RFC] tcp_bbr2: use correct 64-bit division
To:     Neal Cardwell <ncardwell@google.com>
Cc:     David Laight <David.Laight@aculab.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Yuchung Cheng <ycheng@google.com>,
        Yousuk Seung <ysseung@google.com>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        Adithya Abraham Philip <abrahamphilip@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Konstantin Demin <rockdrilla@gmail.com>
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On Tue, May 24, 2022 at 1:06 PM Neal Cardwell <ncardwell@google.com> wrote:
>
> On Tue, May 24, 2022 at 4:01 AM David Laight <David.Laight@aculab.com> wrote:
> >
> > From: Oleksandr Natalenko
> > > Sent: 22 May 2022 23:30
> > > To: Neal Cardwell <ncardwell@google.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
>
> The sk->sk_pacing_rate can definitely be bigger than 32 bits if the
> network path can support more than 34 Gbit/sec  (a pacing rate of 2^32
> bytes per sec is roughly 34 Gibt/sec). This definitely happens.
>
> So  this one seems reasonable to me (and is only in debug code, so the
> performance is probably fine):
> -                (u64)sk->sk_pacing_rate * 8 / 1000,
> +                div_u64((u64)sk->sk_pacing_rate * 8, 1000),
>
> For the other two I agree we should rework them to avoid the 64-bit
> divide, since we don't need it.
>
> There is similar logic in mainline Linux in tcp_tso_autosize(), which
> is currently using "unsigned long" for bytes.
>

Not sure I follow.

sk_pacing_rate is also 'unsigned long'

So tcp_tso_autosize() is correct on 32bit and 64bit arches.
There is no forced 64bit operation there.


> Eric, what do you advise?
>
> thanks,
> neal