Re: [PATCH 5/9] bpf: syscall: add percpu version of lookup/update elem

From: Alexei Starovoitov
Date: Mon Jan 11 2016 - 14:03:00 EST

On Mon, Jan 11, 2016 at 11:56:57PM +0800, Ming Lei wrote:
> Prepare for supporting percpu map in the following patch.
> Now userspace can lookup/update mapped value in one specific
> CPU in case of percpu map.
> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
> @@ -265,7 +272,10 @@ static int map_lookup_elem(union bpf_attr *attr)
> goto free_key;
> rcu_read_lock();
> - ptr = map->ops->map_lookup_elem(map, key);
> + if (!percpu)
> + ptr = map->ops->map_lookup_elem(map, key);
> + else
> + ptr = map->ops->map_lookup_elem_percpu(map, key, attr->cpu);

I think this approach is less potent than Martin's for several reasons:
- bpf program shouldn't be supplying bpf_smp_processor_id(), since
it's error prone and a bit slower than doing it explicitly as in:
although Martin's patch also needs to use this_cpu_ptr() instead
of per_cpu_ptr(.., smp_processor_id());

- two new bpf helpers are not necessary in Martin's approach.
regular map_lookup_elem() will work for both per-cpu maps.

- such map_lookup_elem_percpu() from syscall is not accurate.
Martin's approach via smp_call_function_single() returns precise value,
whereas here memcpy() will race with other cpus.

Overall I think both pre-cpu hash and per-cpu array maps are quite useful.
For this particular set I would suggest to rebase on top of Martin's
to reuse BPF_MAP_LOOKUP_PERCPU_ELEM command that should be applicable
to both per-cpu array and per-cpu hash maps.
and add BPF_MAP_UPDATE_PERCPU_ELEM via smp_call as another patch
that should work for both as well.