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

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Date:   Tue, 17 Dec 2019 18:22:31 +0000
From:   Martin Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        "bpf@...r.kernel.org" <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 Tue, Dec 17, 2019 at 09:26:31AM +0100, Jakub Sitnicki wrote:
> On Sat, Dec 14, 2019 at 08:25 PM CET, Neal Cardwell wrote:
> > 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).
> 
> Having bpf_nsecs_to_jiffies() would be also handy for BPF sockops progs
> that configure SYN-RTO timeout (BPF_SOCK_OPS_TIMEOUT_INIT).
> 
> Right now user-space needs to go look for CONFIG_HZ in /proc/config.gz
Andrii's extern variable work (already landed) allows a bpf_prog
to read CONFIG_HZ as a global variable.  It is the path that I am
pursuing now for jiffies/nsecs conversion without relying on
a helper.