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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, May 24, 2021 at 10:22 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> On Mon, May 24, 2021 at 12:13 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > I second the use of BPF_PROG_TEST_RUN (a.k.a. BPF_PROG_RUN now) to
> > "mirror" such APIs to user-space. We have so much BPF-side
>
> Except the expiration time is stored in user-space too if you just
> use user-space timers to trigger BPF_PROG_TEST_RUN.
> Modifying expiration based on its current value in timer callbacks
> is very common. For example in conntrack use case, we want the
> GC timer to run sooner in the next run if we get certain amount of
> expired items in current run.

I'm not entirely sure what all this means, sorry. My general point is
that instead of doing bpf() syscall with a new custom command (e.g.,
BPF_TIMER_UPDATE), you can just fire your custom BPF program with
BPF_TEST_RUN. You can pass custom timeouts or any other
user-space-provided settings either through global variables, custom
maps, or directly as a context. So you have full control over what
should be set when and why, we just avoid adding tons of custom bpf()
syscall commands for every single feature.

>
>
> > functionality and APIs that reflecting all of that with special
> > user-space-facing BPF commands is becoming quite impractical. E.g., a
> > long time ago there was a proposal to add commands to push data to BPF
> > ringbuf from user-space for all kinds of testing scenarios. We never
> > did that because no one bothered enough, but now I'd advocate that a
> > small custom BPF program that is single-shot through BPF_PROG_RUN is a
> > better way to do this. Similarly for timers and whatever other
> > functionality. By doing everything from BPF program we also side-step
> > potential subtle differences in semantics between BPF-side and
> > user-space-side.
>
> I am confused about what you are saying, because we can already
> trigger BPF_PROG_RUN with a user-space timer for a single shot,
> with the current kernel, without any modification. So this sounds like
> you are against adding any timer on the eBPF side, but on the other
> hand, you are secoding to Alexei's patch... I am completely lost.

I'm arguing against adding more custom commands to bpf() syscall. And
I was talking about triggering BPF program directly from user-space
with BPF_PROG_TEST_RUN/BPF_PROG_RUN command, not through some timers.

>
> Very clearly, whatever you described as "single shot" is not what we
> want from any perspective.

I'm not sure we are even talking about the same things, so I doubt
"clearly" in this case.

>
> Thanks.