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

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 17 Dec 2019 09:26:31 +0100

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
or /boot/config-`uname -r`, or derive it from clock resolution [0]

        clock_getres(CLOCK_REALTIME_COARSE, &res);
        jiffy = res.tv_nsec / 1000000;

to pass timeout in jiffies to the BPF prog.

-jkbs

[0] https://www.mail-archive.com/[email-masked]/msg08850.html