Re: [PATCH v3 bpf-next 1/8] bpf: Introduce bpf timers.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Jun 28, 2021 at 07:24:28PM -0700, Yonghong Song wrote:
> 
> > 
> > While talking to Martin about the api he pointed out that
> > callback_fn in timer_start() doesn't achieve the full use case
> > of replacing a prog. So in the next spin I'll split it into
> > bpf_timer_set_callback(timer, callback_fn);
> > bpf_timer_start(timer, nsec);
> > This way callback and prog can be replaced without resetting
> > timer expiry which could be useful.
> 
> I took a brief look for patch 4-6 and it looks okay. But since
> you will change helper signatures I will hold and check next
> revision instead.

Thanks. The verifier patches won't change though.

> 
> BTW, does this mean the following scenario will be supported?
>   prog1: bpf_timer_set_callback(time, callback_fn)
>   prog2: bpf_timer_start(timer, nsec)
> so here prog2 can start the timer which call prog1's callback_fn?

right.

> > 
> > Also Daniel and Andrii reminded that cpu pinning would be next
> > feature request. The api extensibility allows to add it in the future.
> > I'm going to delay implementing it until bpf_smp_call_single()
> > implications are understood.
> 
> Do we need to any a 'flags' parameter for bpf_timer_start() helper
> so we can encode target cpu in 'flags'?

I thought that bpf_timer_init will handle the cpu selection,
but you're right that having cpu in bpf_timer_start is more flexible.
So I'll add a flag.