block: enable per-cpu bio cache by default

Since after commit 12e4e8c7ab59 ("io_uring/rw: enable bio caches for
IRQ rw"), bio_put is safe for task and irq context, bio_alloc_bioset is
safe for task context and no one calls in irq context, so we can enable
per cpu bio cache by default.

Benchmarked with t/io_uring and ext4+nvme:
taskset -c 6 /root/fio/t/io_uring -p0 -d128 -b4096 -s1 -c1 -F1 -B1 -R1
-X1 -n1 -P1 /mnt/testfile
base IOPS is 562K, patch IOPS is 574K. The CPU usage of bio_alloc_bioset
decrease from 1.42% to 1.22%.

The worst case is allocate bio in CPU A but free in CPU B, still use
t/io_uring and ext4+nvme:
base IOPS is 648K, patch IOPS is 647K.

Also use fio test ext4/xfs with libaio/sync/io_uring on null_blk and
nvme, no obvious performance regression.

Signed-off-by: Fengnan Chang <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/bio.c

···
 	if (WARN_ON_ONCE(!mempool_initialized(&bs->bvec_pool) && nr_vecs > 0))
 		return NULL;
 
-	if (opf & REQ_ALLOC_CACHE) {
-		if (bs->cache && nr_vecs <= BIO_INLINE_VECS) {
-			bio = bio_alloc_percpu_cache(bdev, nr_vecs, opf,
-						     gfp_mask, bs);
-			if (bio)
-				return bio;
-			/*
-			 * No cached bio available, bio returned below marked with
-			 * REQ_ALLOC_CACHE to particpate in per-cpu alloc cache.
-			 */
-		} else {
-			opf &= ~REQ_ALLOC_CACHE;
-		}
-	}
+	if (bs->cache && nr_vecs <= BIO_INLINE_VECS) {
+		opf |= REQ_ALLOC_CACHE;
+		bio = bio_alloc_percpu_cache(bdev, nr_vecs, opf,
+					     gfp_mask, bs);
+		if (bio)
+			return bio;
+		/*
+		 * No cached bio available, bio returned below marked with
+		 * REQ_ALLOC_CACHE to participate in per-cpu alloc cache.
+		 */
+	} else
+		opf &= ~REQ_ALLOC_CACHE;
 
 	/*
 	 * submit_bio_noacct() converts recursion to iteration; this means if

block/fops.c

···
 	loff_t pos = iocb->ki_pos;
 	int ret = 0;
 
-	if (iocb->ki_flags & IOCB_ALLOC_CACHE)
-		opf |= REQ_ALLOC_CACHE;
 	bio = bio_alloc_bioset(bdev, nr_pages, opf, GFP_KERNEL,
 			       &blkdev_dio_pool);
 	dio = container_of(bio, struct blkdev_dio, bio);
···
 	loff_t pos = iocb->ki_pos;
 	int ret = 0;
 
-	if (iocb->ki_flags & IOCB_ALLOC_CACHE)
-		opf |= REQ_ALLOC_CACHE;
 	bio = bio_alloc_bioset(bdev, nr_pages, opf, GFP_KERNEL,
 			       &blkdev_dio_pool);
 	dio = container_of(bio, struct blkdev_dio, bio);

io_uring/rw.c

···
 	ret = kiocb_set_rw_flags(kiocb, rw->flags, rw_type);
 	if (unlikely(ret))
 		return ret;
-	kiocb->ki_flags |= IOCB_ALLOC_CACHE;
 
 	/*
 	 * If the file is marked O_NONBLOCK, still allow retry for it if it