netdev - Re: [RFC PATCH bpf-next] bpf: Introduce bpf

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Date:   Mon, 24 May 2021 10:33:56 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH bpf-next] bpf: Introduce bpf_timer

On Sun, May 23, 2021 at 8:58 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sun, May 23, 2021 at 4:48 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >
> > Still wrapping my head around this, but one thing immediately sprang to
> > mind:
> >
> > > + * long bpf_timer_mod(struct bpf_timer *timer, u64 msecs)
> > > + *   Description
> > > + *           Set the timer expiration N msecs from the current time.
> > > + *   Return
> > > + *           zero
> >
> > Could we make this use nanoseconds (and wire it up to hrtimers) instead?
> > I would like to eventually be able to use this for pacing out network
> > packets, and msec precision is way too coarse for that...
>
> msecs are used to avoid exposing jiffies to bpf prog, since msec_to_jiffies
> isn't trivial to do in the bpf prog unlike the kernel.
> hrtimer would be great to support as well.
> It could be implemented via flags (which are currently zero only)
> but probably not as a full replacement for jiffies based timers.
> Like array vs hash. bpf_timer can support both.

After reading the hrtimer code I might take the above statement back...
hrtimer looks strictly better than timerwheel and jiffies.
It scales well and there are no concerns with overload,
since sys_nanonsleep and tcp are heavy users.
So I'm thinking to drop jiffies approach and do hrtimer only.
wdyt?