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

Dennis Zhou
2022-07-19T04:26:03Z

Hello,

On Mon, Jul 18, 2022 at 12:28:42PM -0700, Yury Norov wrote:
> 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
>

Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis