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@xxxxxxxxx> 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.
