Re: [PATCH v3 1/2] md: add support for REQ_NOWAIT

On 11/6/21 8:38 AM, Guoqing Jiang wrote:

On 11/4/21 12:51 PM, Vishal Verma wrote:

quoted

commit 021a24460dc2 ("block: add QUEUE_FLAG_NOWAIT") added support
for checking whether a given bdev supports handling of REQ_NOWAIT or not.
Since then commit 6abc49468eea ("dm: add support for REQ_NOWAIT and enable
it for linear target") added support for REQ_NOWAIT for dm. This uses
a similar approach to incorporate REQ_NOWAIT for md based bios.

This patch was tested using t/io_uring tool within FIO. A nvme drive
was partitioned into 2 partitions and a simple raid 0 configuration
/dev/md0 was created.

md0 : active raid0 nvme4n1p1[1] nvme4n1p2[0]
       937423872 blocks super 1.2 512k chunks

Before patch:

$ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100

Running top while the above runs:

$ ps -eL | grep $(pidof io_uring)

   38396   38396 pts/2    00:00:00 io_uring
   38396   38397 pts/2    00:00:15 io_uring
   38396   38398 pts/2    00:00:13 iou-wrk-38397

We can see iou-wrk-38397 io worker thread created which gets created
when io_uring sees that the underlying device (/dev/md0 in this case)
doesn't support nowait.

After patch:

$ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100

Running top while the above runs:

$ ps -eL | grep $(pidof io_uring)

   38341   38341 pts/2    00:10:22 io_uring
   38341   38342 pts/2    00:10:37 io_uring

After running this patch, we don't see any io worker thread
being created which indicated that io_uring saw that the
underlying device does support nowait. This is the exact behaviour
noticed on a dm device which also supports nowait.

For all the other raid personalities except raid0, we would need
to train pieces which involves make_request fn in order for them
to correctly handle REQ_NOWAIT.

Signed-off-by: Vishal Verma <redacted>
---
  drivers/md/md.c | 14 ++++++++++++++
  1 file changed, 14 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 5111ed966947..73089776475f 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -5792,6 +5792,7 @@ int md_run(struct mddev *mddev)

      int err;
      struct md_rdev *rdev;
      struct md_personality *pers;
+    bool nowait = true;
        if (list_empty(&mddev->disks))
          /* cannot run an array with no devices.. */

@@ -5862,8 +5863,13 @@ int md_run(struct mddev *mddev)

              }
          }
          sysfs_notify_dirent_safe(rdev->sysfs_state);
+        nowait = nowait && blk_queue_nowait(bdev_get_queue(rdev->bdev));
      }
  +    /* Set the NOWAIT flags if all underlying devices support it */
+    if (nowait)
+        blk_queue_flag_set(QUEUE_FLAG_NOWAIT, mddev->queue);
+
      if (!bioset_initialized(&mddev->bio_set)) {
          err = bioset_init(&mddev->bio_set, BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS);
          if (err)

@@ -7007,6 +7013,14 @@ static int hot_add_disk(struct mddev *mddev, dev_t dev)

      set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
      if (!mddev->thread)
          md_update_sb(mddev, 1);
+    /* If the new disk does not support REQ_NOWAIT,
+     * disable on the whole MD.
+     */

The comment style is

/*
 * xxx
 */
Ack, will fix it.

quoted

+    if (!blk_queue_nowait(bdev_get_queue(rdev->bdev))) {
+        pr_info("%s: Disabling nowait because %s does not support nowait\n",
+            mdname(mddev), bdevname(rdev->bdev, b));

Use %pg to print block device name will be more popular though md has lots of legacy code
with bdevname.

Thanks for pointing, will fix it!

Thanks,
Guoqing