Re: [PATCH RFC 3/7] block: add iopoll method for non-mq device

On Thu, Jan 07 2021 at 10:24pm -0500,
JeffleXu [off-list ref] wrote:

On 1/8/21 5:47 AM, Mike Snitzer wrote:

quoted

On Wed, Dec 23 2020 at  6:26am -0500,
Jeffle Xu [off-list ref] wrote:

quoted

->poll_fn is introduced in commit ea435e1b9392 ("block: add a poll_fn
callback to struct request_queue") for supporting non-mq queues such as
nvme multipath, but removed in commit 529262d56dbe ("block: remove
->poll_fn").

To add support of IO polling for non-mq device, this method need to be
back. Since commit c62b37d96b6e ("block: move ->make_request_fn to
struct block_device_operations") has moved all callbacks into struct
block_device_operations in gendisk, we also add the new method named
->iopoll in block_device_operations.

Please update patch subject and header to:

block: add iopoll method to support bio-based IO polling

->poll_fn was introduced in commit ea435e1b9392 ("block: add a poll_fn
callback to struct request_queue") to support bio-based queues such as
nvme multipath, but was later removed in commit 529262d56dbe ("block:
remove ->poll_fn").

Given commit c62b37d96b6e ("block: move ->make_request_fn to struct
block_device_operations") restore the possibility of bio-based IO
polling support by adding an ->iopoll method to gendisk->fops.
Elevate bulk of blk_mq_poll() implementation to blk_poll() and reduce
blk_mq_poll() to blk-mq specific code that is called from blk_poll().

quoted

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
 block/blk-core.c       | 79 ++++++++++++++++++++++++++++++++++++++++++
 block/blk-mq.c         | 70 +++++--------------------------------
 include/linux/blk-mq.h |  3 ++
 include/linux/blkdev.h |  1 +
 4 files changed, 92 insertions(+), 61 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 96e5fcd7f071..2f5c51ce32e3 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1131,6 +1131,85 @@ blk_qc_t submit_bio(struct bio *bio)
 }
 EXPORT_SYMBOL(submit_bio);
 
+static bool blk_poll_hybrid(struct request_queue *q, blk_qc_t cookie)
+{
+	struct blk_mq_hw_ctx *hctx;
+
+	/* TODO: bio-based device doesn't support hybrid poll. */
+	if (!queue_is_mq(q))
+		return false;
+
+	hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
+	if (blk_mq_poll_hybrid(q, hctx, cookie))
+		return true;
+
+	hctx->poll_considered++;
+	return false;
+}

I don't see where you ever backfill bio-based hybrid support (in
the following patches in this series, so it is lingering TODO).

Yes the hybrid polling is not implemented and thus bypassed for
bio-based device currently.

quoted

quoted

+
+/**
+ * blk_poll - poll for IO completions
+ * @q:  the queue
+ * @cookie: cookie passed back at IO submission time
+ * @spin: whether to spin for completions
+ *
+ * Description:
+ *    Poll for completions on the passed in queue. Returns number of
+ *    completed entries found. If @spin is true, then blk_poll will continue
+ *    looping until at least one completion is found, unless the task is
+ *    otherwise marked running (or we need to reschedule).
+ */
+int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
+{
+	long state;
+
+	if (!blk_qc_t_valid(cookie) ||
+	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
+		return 0;
+
+	if (current->plug)
+		blk_flush_plug_list(current->plug, false);
+
+	/*
+	 * If we sleep, have the caller restart the poll loop to reset
+	 * the state. Like for the other success return cases, the
+	 * caller is responsible for checking if the IO completed. If
+	 * the IO isn't complete, we'll get called again and will go
+	 * straight to the busy poll loop. If specified not to spin,
+	 * we also should not sleep.
+	 */
+	if (spin && blk_poll_hybrid(q, cookie))
+		return 1;
+
+	state = current->state;
+	do {
+		int ret;
+		struct gendisk *disk = queue_to_disk(q);
+
+		if (disk->fops->iopoll)
+			ret = disk->fops->iopoll(q, cookie);
+		else
+			ret = blk_mq_poll(q, cookie);

The original code is indeed buggy. For bio-based device, ->iopoll() may
not be implemented while QUEUE_FLAG_POLL flag is still set, in which
case blk_mq_poll() will be called for this bio-based device.

Yes, here is the patch I created to capture my suggestions.  Provided it
looks good to you, please fold it into patch 3 when you rebase for
posting a v2 of your patchset:

From: Mike Snitzer <redacted>
Date: Thu, 7 Jan 2021 20:45:17 -0500
Subject: [PATCH] fixup patch 3

---
 block/blk-core.c       | 51 +++++++++++++++++++++-----------------------------
 block/blk-mq.c         |  6 ++----
 include/linux/blk-mq.h |  3 ++-
 3 files changed, 25 insertions(+), 35 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index e6671f6ce1a4..44f62dc0fa9f 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1212,22 +1212,6 @@ int blk_bio_poll(struct request_queue *q, blk_qc_t cookie)
 }
 EXPORT_SYMBOL(blk_bio_poll);
 
-static bool blk_poll_hybrid(struct request_queue *q, blk_qc_t cookie)
-{
-	struct blk_mq_hw_ctx *hctx;
-
-	/* TODO: bio-based device doesn't support hybrid poll. */
-	if (!queue_is_mq(q))
-		return false;
-
-	hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
-	if (blk_mq_poll_hybrid(q, hctx, cookie))
-		return true;
-
-	hctx->poll_considered++;
-	return false;
-}
-
 /**
  * blk_poll - poll for IO completions
  * @q:  the queue

@@ -1243,6 +1227,8 @@ static bool blk_poll_hybrid(struct request_queue *q, blk_qc_t cookie)
 int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
 {
 	long state;
+	struct blk_mq_hw_ctx *hctx = NULL;
+	struct gendisk *disk = NULL;
 
 	if (!blk_qc_t_valid(cookie) ||
 	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))

@@ -1251,26 +1237,31 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
 	if (current->plug)
 		blk_flush_plug_list(current->plug, false);
 
-	/*
-	 * If we sleep, have the caller restart the poll loop to reset
-	 * the state. Like for the other success return cases, the
-	 * caller is responsible for checking if the IO completed. If
-	 * the IO isn't complete, we'll get called again and will go
-	 * straight to the busy poll loop. If specified not to spin,
-	 * we also should not sleep.
-	 */
-	if (spin && blk_poll_hybrid(q, cookie))
-		return 1;
+	if (queue_is_mq(q)) {
+		/*
+		 * If we sleep, have the caller restart the poll loop to reset
+		 * the state. Like for the other success return cases, the
+		 * caller is responsible for checking if the IO completed. If
+		 * the IO isn't complete, we'll get called again and will go
+		 * straight to the busy poll loop. If specified not to spin,
+		 * we also should not sleep.
+		 */
+		hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
+		if (spin && blk_mq_poll_hybrid(q, hctx, cookie))
+			return 1;
+		hctx->poll_considered++;
+	} else
+		disk = queue_to_disk(q);
 
 	state = current->state;
 	do {
 		int ret;
-		struct gendisk *disk = queue_to_disk(q);
 
-		if (disk->fops->iopoll)
+		if (hctx)
+			ret = blk_mq_poll(q, hctx, cookie);
+		else if (disk->fops->iopoll)
 			ret = disk->fops->iopoll(q, cookie);
-		else
-			ret = blk_mq_poll(q, cookie);
+
 		if (ret > 0) {
 			__set_current_state(TASK_RUNNING);
 			return ret;

diff --git a/block/blk-mq.c b/block/blk-mq.c
index fcb44604f806..90d8dead1da5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -3826,12 +3826,10 @@ bool blk_mq_poll_hybrid(struct request_queue *q,
 	return blk_mq_poll_hybrid_sleep(q, rq);
 }
 
-int blk_mq_poll(struct request_queue *q, blk_qc_t cookie)
+int blk_mq_poll(struct request_queue *q,
+		struct blk_mq_hw_ctx *hctx, blk_qc_t cookie)
 {
 	int ret;
-	struct blk_mq_hw_ctx *hctx;
-
-	hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
 
 	hctx->poll_invoked++;
 	ret = q->mq_ops->poll(hctx);

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 2f3742207df5..b95f2ffa866f 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h

@@ -607,7 +607,8 @@ static inline void blk_rq_bio_prep(struct request *rq, struct bio *bio,
 }
 
 blk_qc_t blk_mq_submit_bio(struct bio *bio);
-int blk_mq_poll(struct request_queue *q, blk_qc_t cookie);
+int blk_mq_poll(struct request_queue *q,
+		struct blk_mq_hw_ctx *hctx, blk_qc_t cookie);
 bool blk_mq_poll_hybrid(struct request_queue *q,
 		struct blk_mq_hw_ctx *hctx, blk_qc_t cookie);
 void blk_mq_hctx_set_fq_lock_class(struct blk_mq_hw_ctx *hctx,

-- 
2.15.0