[PATCH 1/5] iomap: simplify logic for when a dio can get completed inline

[Jens Axboe <axboe@kernel.dk>]

Currently iomap gates this on !IOMAP_DIO_WRITE, but this isn't entirely
accurate. Some writes can complete just fine inline. One such example is
polled IO, where the completion always happens in task context.

Add IOMAP_DIO_INLINE_COMP which tells the completion side if we can
complete this dio inline, or if it needs punting to a workqueue. We set
this flag by default for any dio, and turn it off for unwritten extents
or blocks that require a sync at completion time.

Gate the inline completion on whether we're in a task or not as well.
This will always be true for polled IO, but for IRQ driven IO, the
completion context may not allow for inline completions.

Testing a basic QD 1..8 dio random write with polled IO with the
following fio job:

fio --name=polled-dio-write --filename=/data1/file --time_based=1 \
--runtime=10 --bs=4096 --rw=randwrite --norandommap --buffered=0 \
--cpus_allowed=4 --ioengine=io_uring --iodepth=$depth --hipri=1

yields:

        Stock   Patched         Diff
=======================================
QD1     180K    201K            +11%
QD2     356K    394K            +10%
QD4     608K    650K            +7%
QD8     827K    831K            +0.5%

which shows a nice win, particularly for lower queue depth writes.
This is expected, as higher queue depths will be busy polling
completions while the offloaded workqueue completions can happen in
parallel.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/iomap/direct-io.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index ea3b868c8355..6fa77094cf0a 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -20,6 +20,7 @@
  * Private flags for iomap_dio, must not overlap with the public ones in
  * iomap.h:
  */
+#define IOMAP_DIO_INLINE_COMP	(1 << 27)
 #define IOMAP_DIO_WRITE_FUA	(1 << 28)
 #define IOMAP_DIO_NEED_SYNC	(1 << 29)
 #define IOMAP_DIO_WRITE		(1 << 30)
@@ -161,15 +162,15 @@ void iomap_dio_bio_end_io(struct bio *bio)
 			struct task_struct *waiter = dio->submit.waiter;
 			WRITE_ONCE(dio->submit.waiter, NULL);
 			blk_wake_io_task(waiter);
-		} else if (dio->flags & IOMAP_DIO_WRITE) {
+		} else if ((dio->flags & IOMAP_DIO_INLINE_COMP) && in_task()) {
+			WRITE_ONCE(dio->iocb->private, NULL);
+			iomap_dio_complete_work(&dio->aio.work);
+		} else {
 			struct inode *inode = file_inode(dio->iocb->ki_filp);
 
 			WRITE_ONCE(dio->iocb->private, NULL);
 			INIT_WORK(&dio->aio.work, iomap_dio_complete_work);
 			queue_work(inode->i_sb->s_dio_done_wq, &dio->aio.work);
-		} else {
-			WRITE_ONCE(dio->iocb->private, NULL);
-			iomap_dio_complete_work(&dio->aio.work);
 		}
 	}
 
@@ -244,6 +245,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
 
 	if (iomap->type == IOMAP_UNWRITTEN) {
 		dio->flags |= IOMAP_DIO_UNWRITTEN;
+		dio->flags &= ~IOMAP_DIO_INLINE_COMP;
 		need_zeroout = true;
 	}
 
@@ -500,7 +502,8 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 	dio->i_size = i_size_read(inode);
 	dio->dops = dops;
 	dio->error = 0;
-	dio->flags = 0;
+	/* default to inline completion, turned off when not supported */
+	dio->flags = IOMAP_DIO_INLINE_COMP;
 	dio->done_before = done_before;
 
 	dio->submit.iter = iter;
@@ -535,6 +538,7 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		/* for data sync or sync, we need sync completion processing */
 		if (iocb_is_dsync(iocb)) {
 			dio->flags |= IOMAP_DIO_NEED_SYNC;
+			dio->flags &= ~IOMAP_DIO_INLINE_COMP;
 
 		       /*
 			* For datasync only writes, we optimistically try
-- 
2.40.1