Re: [PATCH RFC net-next 1/2] net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-02-14 17:08:44 [+0100], Toke Høiland-Jørgensen wrote:
> > During testing I forgot a spot in egress and the test module. You could
> > argue that the warning is enough since it should pop up in testing and
> > not production because the code is always missed and not by chance (go
> > boom, send a report). I *think* I covered all spots, at least the test
> > suite didn't point anything out to me.
> 
> Well, I would prefer if we could make sure we covered everything and not
> have this odd failure mode where redirect just mysteriously stops
> working. At the very least, if we keep the check we should have a
> WARN_ON in there to make it really obvious that something needs to be
> fixed.

Agree.

> This brings me to another thing I was going to point out separately, but
> may as well mention it here: It would be good if we could keep the
> difference between the RT and !RT versions as small as possible to avoid
> having subtle bugs that only appear in one configuration.

Yes. I do so, too.

> I agree with Jesper that the concept of a stack-allocated "run context"
> for the XDP program makes sense in general (and I have some vague ideas
> about other things that may be useful to stick in there). So I'm
> wondering if it makes sense to do that even in the !RT case? We can't
> stick the pointer to it into 'current' when running in softirq, but we
> could change the per-cpu variable to just be a pointer that gets
> populated by xdp_storage_set()?

I *think* that it could be added to current. The assignment currently
allows nesting so that is not a problem. Only the outer most set/clear
would do something. If you run in softirq, you would hijack a random
task_struct. If the pointer is already assigned then the list and so one
must be empty because access is only allowed in BH-disabled sections.

However, using per-CPU for the pointer (instead of task_struct) would
have the advantage that it is always CPU/node local memory while the
random task_struct could have been allocated on a different NUMA node.

> I'm not really sure if this would be performance neutral (it's just
> moving around a few bits of memory, but we do gain an extra pointer
> deref), but it should be simple enough to benchmark.

My guess is that we remain with one per-CPU dereference and an
additional "add offset". That is why I kept the !RT bits as they are
before getting yelled at.

I could prepare something and run a specific test if you have one.

> -Toke

Sebastian