[PATCH 14/16] mm/percpu: optimize pcpu_alloc_area()

Yury Norov
2022-07-18T19:31:04Z

Don't call bitmap_clear() to clear 0 bits.

bitmap_clear() can handle 0-length requests properly, but it's not covered
with static optimizations, and falls to __bitmap_set(). So we are paying a
function call + prologue work cost just for nothing.

Caught with CONFIG_DEBUG_BITMAP:
[ 45.571799] <TASK>
[ 45.571801] pcpu_alloc_area+0x194/0x340
[ 45.571806] pcpu_alloc+0x2fb/0x8b0
[ 45.571811] ? kmem_cache_alloc_trace+0x177/0x2a0
[ 45.571815] __percpu_counter_init+0x22/0xa0
[ 45.571819] fprop_local_init_percpu+0x14/0x30
[ 45.571823] wb_get_create+0x15d/0x5f0
[ 45.571828] cleanup_offline_cgwb+0x73/0x210
[ 45.571831] cleanup_offline_cgwbs_workfn+0xcf/0x200
[ 45.571835] process_one_work+0x1e5/0x3b0
[ 45.571839] worker_thread+0x50/0x3a0
[ 45.571843] ? rescuer_thread+0x390/0x390
[ 45.571846] kthread+0xe8/0x110
[ 45.571849] ? kthread_complete_and_exit+0x20/0x20
[ 45.571853] ret_from_fork+0x22/0x30
[ 45.571858] </TASK>
[ 45.571859] ---[ end trace 0000000000000000 ]---
[ 45.571860] b1: ffffa8d5002e1000
[ 45.571861] b2: 0
[ 45.571861] b3: 0
[ 45.571862] nbits: 44638
[ 45.571863] start: 44638
[ 45.571864] off: 0
[ 45.571864] percpu: Bitmap: parameters check failed
[ 45.571865] percpu: include/linux/bitmap.h [538]: bitmap_clear

Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
mm/percpu.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 3633eeefaa0d..f720f7c36b91 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1239,7 +1239,8 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int alloc_bits,

/* update boundary map */
set_bit(bit_off, chunk->bound_map);
- bitmap_clear(chunk->bound_map, bit_off + 1, alloc_bits - 1);
+ if (alloc_bits > 1)
+ bitmap_clear(chunk->bound_map, bit_off + 1, alloc_bits - 1);
set_bit(bit_off + alloc_bits, chunk->bound_map);

chunk->free_bytes -= alloc_bits * PCPU_MIN_ALLOC_SIZE;
--
2.34.1