Re: [PATCH RFC bpf-next 4/4] i40e: remove rcu_read_lock() around XDP program invocation

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:

> On Fri, Apr 23, 2021 at 01:05:20PM +0200, Toke Høiland-Jørgensen wrote:
>> From: Toke Høiland-Jørgensen <toke@redhat.com>
>> 
>> The i40e driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
>> program invocations. However, the actual lifetime of the objects referred
>> by the XDP program invocation is longer, all the way through to the call to
>> xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
>> turns out to be harmless because it all happens in a single NAPI poll
>> cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
>> misleading.
>
> Okay, but what about the lifetime of the xdp_prog itself? Can xdp_prog
> change within a single NAPI poll? After reading previous discussions I
> would say it can't, right?

Well, bpf_prog objects are also RCU-protected so it's at least
guaranteed to stay alive until the end of the NAPI poll. But I don't
think there's anything preventing the program from being changed in the
middle of a NAPI poll.

> There are drivers that have a big RCU critical section in NAPI poll, but it
> seems that some read a xdp_prog a single time whereas others read it per
> processed frame.
>
> If we are sure that xdp_prog can't change on-the-fly then first low
> hanging fruit, at least for the Intel drivers, is to skip a test against
> NULL and read it only once at the beginning of NAPI poll. There might be
> also other micro-optimizations specific to each drivers that could be done
> based on that (that of course read the xdp_prog per each frame).

I think the main problem this could cause is that the dispatcher code
could have replaced the program in the dispatcher trampoline while the
driver was still using it, which would hurt performance. However,
ultimately this is under the control of the driver, since the program
install is a driver op. For instance, i40e_xdp_setup() does a
conditional synchronize_rcu() after removing a program; making this
unconditional (and maybe moving it after the writes to the rx_ring prog
pointers?) would ensure that the NAPI cycle had ended before the
bpf_op() call in dev_xdp_install(), which would delay the trampoline
replace.

I guess there could then be a window where the new program is being used
but has not been installed into the trampoline yet, then, so maybe
delaying that replace is not actually terribly important? Adding Björn,
maybe he has a better idea.

> Or am I nuts?

No I don't think so :)

I guess it remains to be seen whether there's a real performance
benefit, but at least I don't think there would be any safety or
correctness issues with attempting this.

-Toke