[PATCH bpf-next 0/4] Add internal-only BPF per-CPU instructions

[Andrii Nakryiko <andrii@kernel.org>]

Add two new BPF instructions for dealing with per-CPU memory.

One, BPF_LDX | BPF_ADDR_PERCPU | BPF_DW (where BPF_ADD_PERCPU is unused
0xe0 opcode), resolved provided per-CPU address (offset) to an absolute
address where per-CPU data resides for "this" CPU. This is the most universal,
and, strictly speaking, the only per-CPU BPF instruction necessary.

I also added BPF_LDX | BPF_MEM_PERCPU | BPF_{B,H,W,DW} (BPF_MEM_PERCPU using
another unused 0xc0 opcode), which can be considered an optimization
instruction, which allows to *read* per-CPU data up to 8 bytes in one
instruction, without having to first resolve the address and then
dereferencing the memory. This one is used in inlining of
bpf_get_smp_processor_id(), but it would be fine to implement the latter with
BPF_ADD_PERCPU, followed by normal BPF_LDX | BPF_MEM, so I'm fine dropping
this one, if requested.

This instructions are currently supported by x86-64 BPF JIT, but it would be
great if this was added for other arches ASAP, of course.

In either case, we also implement inlining for three cases:
  - bpf_get_smp_processor_id(), which allows to avoid unnecessary trivial
    function call, saving a bit of performance and also not polluting LBR
    records with unnecessary function call/return records;
  - PERCPU_ARRAY's bpf_map_lookup_elem() is completely inlined, bringing its
    performance to implementing per-CPU data structures using global variables
    in BPF (which is an awesome improvement, see benchmarks below);
  - PERCPU_HASH's bpf_map_lookup_elem() is partially inlined, just like the
    same for non-PERCPU HASH map; this still saves a bit of overhead.

To validate performance benefits, I hacked together a tiny benchmark doing
only bpf_map_lookup_elem() and incrementing the value by 1 for PERCPU_ARRAY
(arr-inc benchmark below) and PERCPU_HASH (hash-inc benchmark below) maps. To
establish a baseline, I also implemented logic similar to PERCPU_ARRAY based
on global variable array using bpf_get_smp_processor_id() to index array for
current CPU (glob-arr-inc benchmark below).

BEFORE
======
glob-arr-inc   :  163.685 ± 0.092M/s
arr-inc        :  138.096 ± 0.160M/s
hash-inc       :   66.855 ± 0.123M/s

AFTER
=====
glob-arr-inc   :  173.921 ± 0.039M/s (+6%)
arr-inc        :  170.729 ± 0.210M/s (+23.7%)
hash-inc       :   68.673 ± 0.070M/s (+2.7%)

As can be seen, PERCPU_HASH gets a modest +2.7% improvement, while global
array-based gets a nice +6% due to inlining of bpf_get_smp_processor_id().

But what's really important is that arr-inc benchmark basically catches up
with glob-arr-inc, resulting in +23.7% improvement. This means that in
practice it won't be necessary to avoid PERCPU_ARRAY anymore if performance is
critical (e.g., high-frequent stats collection, which is often a practical use
for PERCPU_ARRAY today).

Andrii Nakryiko (4):
  bpf: add internal-only per-CPU LDX instructions
  bpf: inline bpf_get_smp_processor_id() helper
  bpf: inline bpf_map_lookup_elem() for PERCPU_ARRAY maps
  bpf: inline bpf_map_lookup_elem() helper for PERCPU_HASH map

 arch/x86/net/bpf_jit_comp.c | 29 +++++++++++++++++++++++++++++
 include/linux/filter.h      | 27 +++++++++++++++++++++++++++
 kernel/bpf/arraymap.c       | 33 +++++++++++++++++++++++++++++++++
 kernel/bpf/core.c           |  5 +++++
 kernel/bpf/disasm.c         | 33 ++++++++++++++++++++++++++-------
 kernel/bpf/hashtab.c        | 21 +++++++++++++++++++++
 kernel/bpf/verifier.c       | 17 +++++++++++++++++
 7 files changed, 158 insertions(+), 7 deletions(-)

-- 
2.43.0