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

[Jamal Hadi Salim <jhs@mojatatu.com>]

On 2021-05-25 3:57 p.m., Alexei Starovoitov wrote:
> On Tue, May 25, 2021 at 12:35 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:

[..]
> The outcome of the last bpf office hours was a general agreement
> that we need new hooks in map update/delete operations
> (including auto-delete by LRU) that will trigger a bpf subprog.

This is certainly a useful feature (for other reasons as well).
Does this include create/update/delete issued from user space?

> It might look very similar to the timer callback that is part of this patch,
> but instead of being called by the timer the LRU logic will call it.
> This way the subprog can transfer the data stored in the
> about-to-be-deleted map element into some other map or pass
> to user space via ringbuf or do any other logic.
> 

The challenge we have in this case is LRU makes the decision
which entry to victimize. We do have some entries we want to
keep longer - even if they are not seeing a lot of activity.
You could just notify user space to re-add the entry but then
you have sync challenges.
The timers do provide us a way to implement custom GC.

So a question (which may have already been discussed),
assuming the following setup:
- 2 programs a) Ingress b) egress
- sharing a conntrack map which and said map pinned.
- a timer prog (with a map with just timers;
    even a single timer would be enough in some cases).

ingress and egress do std stuff like create/update
timer prog does the deletes. For simplicity sake assume
we just have one timer that does a foreach and iterates
all entries.

What happens when both ingress and egress are ejected?

cheers,
jamal