bpf: Optimize element count in non-preallocated hash map.

The atomic_inc/dec might cause extreme cache line bouncing when multiple cpus
access the same bpf map. Based on specified max_entries for the hash map
calculate when percpu_counter becomes faster than atomic_t and use it for such
maps. For example samples/bpf/map_perf_test is using hash map with max_entries
1000. On a system with 16 cpus the 'map_perf_test 4' shows 14k events per
second using atomic_t. On a system with 15 cpus it shows 100k events per second
using percpu. map_perf_test is an extreme case where all cpus colliding on
atomic_t which causes extreme cache bouncing. Note that the slow path of
percpu_counter is 5k events per secound vs 14k for atomic, so the heuristic is
necessary. See comment in the code why the heuristic is based on
num_online_cpus().

Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

0 files changed, 0 insertions, 0 deletions