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

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Date:   Tue, 17 Dec 2019 09:26:31 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>, 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 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
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/kernelnewbies@nl.linux.org/msg08850.html