Re: [PATCH bpf-next 0/3] Add bpf_for_each helper

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

Joanne Koong <joannekoong@fb.com> writes:

> This patchset add a new helper, bpf_for_each.
>
> One of the complexities of using for loops in bpf programs is that the verifier
> needs to ensure that in every possibility of the loop logic, the loop will always
> terminate. As such, there is a limit on how many iterations the loop can do.
>
> The bpf_for_each helper moves the loop logic into the kernel and can thereby
> guarantee that the loop will always terminate. The bpf_for_each helper simplifies
> a lot of the complexity the verifier needs to check, as well as removes the
> constraint on the number of loops able to be run.
>
> From the test results, we see that using bpf_for_each in place
> of the traditional for loop led to a decrease in verification time
> and number of bpf instructions by 100%. The benchmark results show
> that as the number of iterations increases, the overhead per iteration
> decreases.

Small nit with the "by 100%" formulation: when giving such relative
quantities 100% has a particular meaning, namely "eliminates entirely".
Which this doesn't, obviously, it *almost* eliminates the verification
overhead. So I'd change this to 99.5% instead (which is the actual value
from your numbers in patch 2).