Subject: Re: [RFC PATCH bpf-next] bpf: Introduce bpf_timer

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 24 May 2021 20:38:38 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Sun, May 23, 2021 at 4:48 AM Toke Høiland-Jørgensen <[email-masked]> 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.

Okay, so this is really:

long bpf_timer_mod(struct bpf_timer *timer, u64 interval)

where 'interval' will be expressed in either milliseconds or nanoseconds
depending on which flags are passed to bpf_timer_init()? That's fine by
me, then; I just wanted to make sure that that 'msecs' was not an
indication that this was the only granularity these timers would
support... :)

-Toke