Stock kernel scales worse than FreeBSD when doing a 20-way stat(2) on
the same tmpfs-backed file.
According to perf top:
38.09% lockref_put_return
26.08% lockref_get_not_dead
25.60% __d_lookup_rcu
0.89% clear_bhb_loop
__d_lookup_rcu is participating in cacheline ping pong due to the
embedded name sharing a cacheline with lockref.
Moving it out resolves the problem:
41.50% lockref_put_return
41.03% lockref_get_not_dead
1.54% clear_bhb_loop
benchmark (will-it-scale, Sapphire Rapids, tmpfs, ops/s):
FreeBSD:7219334
before: 5038006
after: 7842883 (+55%)
One minor remark: the 'after' result is unstable, fluctuating in the
range ~7.8 mln to ~9 mln during different runs.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240613001215.648829-3-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
18a5daf0e4