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

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 18 Dec 2019 10:03:25 +0100

On Tue, Dec 17, 2019 at 07:22 PM CET, Martin Lau wrote:
> 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 <[email-masked]> 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.

Thank yor for the pointer, and Andrii for implementing it.
Selftest [0] from extern-var-support series demonstrates it nicely.

[0] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=330a73a7b6ca93a415de1b7da68d7a0698fe4937