netdev - Re: [PATCH bpf-next 09/13] bpf: Add BPF_FUNC

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Date:   Sat, 14 Dec 2019 14:25:14 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <kernel-team@...com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 09/13] bpf: Add BPF_FUNC_jiffies

On Fri, Dec 13, 2019 at 9:00 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 12/13/19 4:47 PM, Martin KaFai Lau wrote:
> > This patch adds a helper to handle jiffies.  Some of the
> > tcp_sock's timing is stored in jiffies.  Although things
> > could be deduced by CONFIG_HZ, having an easy way to get
> > jiffies will make the later bpf-tcp-cc implementation easier.
> >
>
> ...
>
> > +
> > +BPF_CALL_2(bpf_jiffies, u64, in, u64, flags)
> > +{
> > +     if (!flags)
> > +             return get_jiffies_64();
> > +
> > +     if (flags & BPF_F_NS_TO_JIFFIES) {
> > +             return nsecs_to_jiffies(in);
> > +     } else if (flags & BPF_F_JIFFIES_TO_NS) {
> > +             if (!in)
> > +                     in = get_jiffies_64();
> > +             return jiffies_to_nsecs(in);
> > +     }
> > +
> > +     return 0;
> > +}
>
> This looks a bit convoluted :)
>
> Note that we could possibly change net/ipv4/tcp_cubic.c to no longer use jiffies at all.
>
> We have in tp->tcp_mstamp an accurate timestamp (in usec) that can be converted to ms.

If the jiffies functionality stays, how about 3 simple functions that
correspond to the underlying C functions, perhaps something like:

  bpf_nsecs_to_jiffies(nsecs)
  bpf_jiffies_to_nsecs(jiffies)
  bpf_get_jiffies_64()

Separate functions might be easier to read/maintain (and may even be
faster, given the corresponding reduction in branches).

neal