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

[Martin Lau <kafai@fb.com>]

On Sat, Dec 14, 2019 at 02:25:14PM -0500, Neal Cardwell wrote:
> On Fri, Dec 13, 2019 at 9:00 PM Eric Dumazet <eric.dumazet@gmail.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).
Yes.  It could be different bpf helpers.

I will take another look on these.
I may not need the nsecs <=> jiffies with CONFIG_HZ and
Andrii's recent extern var support.  The first attempt I tried
end-up a lot of codes on the bpf_prog side.  I may not have done
it right.  I will give it another try on this side.

Thanks for the feedbacks!