Re: [PATCH] block: Optimize bio_init()

[Jens Axboe <axboe@kernel.dk>]

On 9/11/21 4:01 PM, Jens Axboe wrote:
> On 9/11/21 3:47 PM, Bart Van Assche wrote:
>> The following test:
>>
>> sudo taskset -c 0 t/io_uring -b512 -d128 -c32 -s32 -p1 -F1 -B1 /dev/nullb0
>>
>> reports 1366 K IOPS on my test setup without this patch and 1380 K IOPS
>> with this patch applied. In other words, this patch realizes a 1%
>> performance improvement. I think this is because this patch makes the
>> compiler generate better code. See also commit da521626ac62 ("bio:
>> optimize initialization of a bio").
>>
>> The assembler code generated by gcc without this patch is as follows:
>>
>>    0x0000000000000000 <+0>:     call   0x5 <bio_init+5>
>>    0x0000000000000005 <+5>:     xor    %eax,%eax
>>    0x0000000000000007 <+7>:     xor    %ecx,%ecx
>>    0x0000000000000009 <+9>:     movl   $0x1,0x1c(%rdi)
>>    0x0000000000000010 <+16>:    movq   $0x0,(%rdi)
>>    0x0000000000000017 <+23>:    movq   $0x0,0x8(%rdi)
>>    0x000000000000001f <+31>:    movq   $0x0,0x10(%rdi)
>>    0x0000000000000027 <+39>:    mov    %ax,0x18(%rdi)
>>    0x000000000000002b <+43>:    movb   $0x0,0x1a(%rdi)
>>    0x000000000000002f <+47>:    movq   $0x0,0x20(%rdi)
>>    0x0000000000000037 <+55>:    movq   $0x0,0x28(%rdi)
>>    0x000000000000003f <+63>:    movl   $0x0,0x30(%rdi)
>>    0x0000000000000046 <+70>:    movq   $0x0,0x38(%rdi)
>>    0x000000000000004e <+78>:    movq   $0x0,0x40(%rdi)
>>    0x0000000000000056 <+86>:    movq   $0x0,0x48(%rdi)
>>    0x000000000000005e <+94>:    movq   $0x0,0x50(%rdi)
>>    0x0000000000000066 <+102>:   movq   $0x0,0x58(%rdi)
>>    0x000000000000006e <+110>:   movq   $0x0,0x60(%rdi)
>>    0x0000000000000076 <+118>:   mov    %cx,0x68(%rdi)
>>    0x000000000000007a <+122>:   movl   $0x1,0x6c(%rdi)
>>    0x0000000000000081 <+129>:   mov    %dx,0x6a(%rdi)
>>    0x0000000000000085 <+133>:   mov    %rsi,0x70(%rdi)
>>    0x0000000000000089 <+137>:   movq   $0x0,0x78(%rdi)
>>    0x0000000000000091 <+145>:   ret
>>
>> With this patch bio_init() is compiled into the following assembly code:
>>
>>    0x0000000000000000 <+0>:     call   0x5 <bio_init+5>
>>    0x0000000000000005 <+5>:     mov    %rdi,%r8
>>    0x0000000000000008 <+8>:     mov    $0x10,%ecx
>>    0x000000000000000d <+13>:    xor    %eax,%eax
>>    0x000000000000000f <+15>:    rep stos %rax,%es:(%rdi)
>>    0x0000000000000012 <+18>:    movl   $0x1,0x1c(%r8)
>>    0x000000000000001a <+26>:    mov    %dx,0x6a(%r8)
>>    0x000000000000001f <+31>:    movl   $0x1,0x6c(%r8)
>>    0x0000000000000027 <+39>:    mov    %rsi,0x70(%r8)
>>    0x000000000000002b <+43>:    ret
>>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
>> ---
>>  block/bio.c | 45 ++++++++-------------------------------------
>>  1 file changed, 8 insertions(+), 37 deletions(-)
>>
>> diff --git a/block/bio.c b/block/bio.c
>> index 5df3dd282e40..775cd4274523 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -244,47 +244,18 @@ static void bio_free(struct bio *bio)
>>  }
>>  
>>  /*
>> - * Users of this function have their own bio allocation. Subsequently,
>> - * they must remember to pair any call to bio_init() with bio_uninit()
>> - * when IO has completed, or when the bio is released.
>> + * Users of this function must pair any call to bio_init() with a call to
>> + * bio_uninit() after IO has completed or when the bio is released.
>>   */
>>  void bio_init(struct bio *bio, struct bio_vec *table,
>>  	      unsigned short max_vecs)
>>  {
>> -	bio->bi_next = NULL;
>> -	bio->bi_bdev = NULL;
>> -	bio->bi_opf = 0;
>> -	bio->bi_flags = 0;
>> -	bio->bi_ioprio = 0;
>> -	bio->bi_write_hint = 0;
>> -	bio->bi_status = 0;
>> -	bio->bi_iter.bi_sector = 0;
>> -	bio->bi_iter.bi_size = 0;
>> -	bio->bi_iter.bi_idx = 0;
>> -	bio->bi_iter.bi_bvec_done = 0;
>> -	bio->bi_end_io = NULL;
>> -	bio->bi_private = NULL;
>> -#ifdef CONFIG_BLK_CGROUP
>> -	bio->bi_blkg = NULL;
>> -	bio->bi_issue.value = 0;
>> -#ifdef CONFIG_BLK_CGROUP_IOCOST
>> -	bio->bi_iocost_cost = 0;
>> -#endif
>> -#endif
>> -#ifdef CONFIG_BLK_INLINE_ENCRYPTION
>> -	bio->bi_crypt_context = NULL;
>> -#endif
>> -#ifdef CONFIG_BLK_DEV_INTEGRITY
>> -	bio->bi_integrity = NULL;
>> -#endif
>> -	bio->bi_vcnt = 0;
>> -
>> -	atomic_set(&bio->__bi_remaining, 1);
>> -	atomic_set(&bio->__bi_cnt, 1);
>> -
>> -	bio->bi_max_vecs = max_vecs;
>> -	bio->bi_io_vec = table;
>> -	bio->bi_pool = NULL;
>> +	*bio = (struct bio) {
>> +		.__bi_remaining	= ATOMIC_INIT(1),
>> +		.__bi_cnt	= ATOMIC_INIT(1),
>> +		.bi_max_vecs	= max_vecs,
>> +		.bi_io_vec	= table,
>> +	};
>>  }
> 
> I'll give this a whirl too, another upside is that it's less prone to
> errors if struct bio is changed.

Seems slower for me, by about 1-1.5%, which is consumed by
bio_alloc_kiocb() which is the only bio_init() caller in my test. Using
gcc 11.1 here, and my code generation seems to match your case too
(series of mov vs rep stos with the patch).

Probably a CPU thing. I'm running on an AMD 3970X for these tests.

-- 
Jens Axboe