[PATCH] block: Optimize bio_init()

[Bart Van Assche <bvanassche@acm.org>]

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,
+	};
 }
 EXPORT_SYMBOL(bio_init);