Andrii Nakryiko wrote: > Add a new BPF instruction for resolving per-CPU memory addresses. > > New instruction is a special form of BPF_ALU64 | BPF_MOV | BPF_DW, with > insns->off set to BPF_ADDR_PERCPU (== -1). It resolves provided per-CPU offset > to an absolute address where per-CPU data resides for "this" CPU. > > This patch set implements support for it in x86-64 BPF JIT only. > > Using the new instruction, 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). Out of curiousity did we consider exposing this instruction outside internal inlining? It seems it would help compiler some to not believe its doing a function call. We could do some runtime rewrites to find the address for global vars for example. FWIW I don't think one should block this necessarily perhaps as follow up? Or at least worth considering if I didn't miss some reason its not plausible. > > v1->v2: > - use BPF_ALU64 | BPF_MOV instruction instead of LDX (Alexei); > - dropped the direct per-CPU memory read instruction, it can always be added > back, if necessary; > - guarded bpf_get_smp_processor_id() behind x86-64 check (Alexei); > - switched all per-cpu addr casts to (unsigned long) to avoid sparse > warnings. > > Andrii Nakryiko (4): > bpf: add special internal-only MOV instruction to resolve per-CPU > addrs > 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 | 16 ++++++++++++++++ > include/linux/filter.h | 20 ++++++++++++++++++++ > kernel/bpf/arraymap.c | 33 +++++++++++++++++++++++++++++++++ > kernel/bpf/core.c | 5 +++++ > kernel/bpf/disasm.c | 14 ++++++++++++++ > kernel/bpf/hashtab.c | 21 +++++++++++++++++++++ > kernel/bpf/verifier.c | 24 ++++++++++++++++++++++++ > 7 files changed, 133 insertions(+) > > -- > 2.43.0 > >
