Re: [PATCH V3 for 5.11 08/12] blk-mq: return budget token from .get_budget callback

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On 9/23/20 3:33 AM, Ming Lei wrote:
SCSI uses one global atomic variable to track queue depth for each
LUN/request queue.

This way doesn't scale well when there is lots of CPU cores and the
disk is very fast. It has been observed that IOPS is affected a lot
by tracking queue depth via sdev->device_busy in IO path.

Return budget token from .get_budget callback, and the budget token
can be passed to driver, so that we can replace the atomic variable
with sbitmap_queue, then the scale issue can be fixed.

Cc: Omar Sandoval <osandov@xxxxxx>
Cc: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Cc: Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
Cc: Ewan D. Milne <emilne@xxxxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
  block/blk-mq-sched.c    | 17 +++++++++++++----
  block/blk-mq.c          | 36 +++++++++++++++++++++++++-----------
  block/blk-mq.h          | 25 +++++++++++++++++++++----
  drivers/scsi/scsi_lib.c | 16 +++++++++++-----
  include/linux/blk-mq.h  |  4 ++--
  5 files changed, 72 insertions(+), 26 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 3e9596738852..1c04a58c5f99 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -131,6 +131,7 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
do {
  		struct request *rq;
+		int budget_token;
if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
  			break;
@@ -140,12 +141,13 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
  			break;
  		}
- if (!blk_mq_get_dispatch_budget(q))
+		budget_token = blk_mq_get_dispatch_budget(q);
+		if (budget_token < 0)
  			break;
rq = e->type->ops.dispatch_request(hctx);
  		if (!rq) {
-			blk_mq_put_dispatch_budget(q);
+			blk_mq_put_dispatch_budget(q, budget_token);
  			/*
  			 * We're releasing without dispatching. Holding the
  			 * budget could have blocked any "hctx"s with the
@@ -157,6 +159,8 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
  			break;
  		}
+ blk_mq_set_rq_budget_token(rq, budget_token);
+
  		/*
  		 * Now this rq owns the budget which has to be released
  		 * if this rq won't be queued to driver via .queue_rq()
@@ -230,6 +234,8 @@ static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
  	struct request *rq;
do {
+		int budget_token;
+
  		if (!list_empty_careful(&hctx->dispatch)) {
  			ret = -EAGAIN;
  			break;
@@ -238,12 +244,13 @@ static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
  		if (!sbitmap_any_bit_set(&hctx->ctx_map))
  			break;
- if (!blk_mq_get_dispatch_budget(q))
+		budget_token = blk_mq_get_dispatch_budget(q);
+		if (budget_token < 0)
  			break;
rq = blk_mq_dequeue_from_ctx(hctx, ctx);
  		if (!rq) {
-			blk_mq_put_dispatch_budget(q);
+			blk_mq_put_dispatch_budget(q, budget_token);
  			/*
  			 * We're releasing without dispatching. Holding the
  			 * budget could have blocked any "hctx"s with the
@@ -255,6 +262,8 @@ static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
  			break;
  		}
+ blk_mq_set_rq_budget_token(rq, budget_token);
+
  		/*
  		 * Now this rq owns the budget which has to be released
  		 * if this rq won't be queued to driver via .queue_rq()
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 88154cea83d8..7743cacb02b4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1296,10 +1296,15 @@ static enum prep_dispatch blk_mq_prep_dispatch_rq(struct request *rq,
  						  bool need_budget)
  {
  	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
+	int budget_token = -1;
- if (need_budget && !blk_mq_get_dispatch_budget(rq->q)) {
-		blk_mq_put_driver_tag(rq);
-		return PREP_DISPATCH_NO_BUDGET;
+	if (need_budget) {
+		budget_token = blk_mq_get_dispatch_budget(rq->q);
+		if (budget_token < 0) {
+			blk_mq_put_driver_tag(rq);
+			return PREP_DISPATCH_NO_BUDGET;
+		}
+		blk_mq_set_rq_budget_token(rq, budget_token);
  	}
if (!blk_mq_get_driver_tag(rq)) {
@@ -1316,7 +1321,7 @@ static enum prep_dispatch blk_mq_prep_dispatch_rq(struct request *rq,
  			 * together during handling partial dispatch
  			 */
  			if (need_budget)
-				blk_mq_put_dispatch_budget(rq->q);
+				blk_mq_put_dispatch_budget(rq->q, budget_token);
  			return PREP_DISPATCH_NO_TAG;
  		}
  	}
@@ -1326,12 +1331,16 @@ static enum prep_dispatch blk_mq_prep_dispatch_rq(struct request *rq,
/* release all allocated budgets before calling to blk_mq_dispatch_rq_list */
  static void blk_mq_release_budgets(struct request_queue *q,
-		unsigned int nr_budgets)
+		struct list_head *list)
  {
-	int i;
+	struct request *rq;
- for (i = 0; i < nr_budgets; i++)
-		blk_mq_put_dispatch_budget(q);
+	list_for_each_entry(rq, list, queuelist) {
+		int budget_token = blk_mq_get_rq_budget_token(rq);
+
+		if (budget_token >= 0)
+			blk_mq_put_dispatch_budget(q, budget_token);
+	}
  }
/*
@@ -1424,7 +1433,8 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
  			(hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED);
  		bool no_budget_avail = prep == PREP_DISPATCH_NO_BUDGET;
- blk_mq_release_budgets(q, nr_budgets);
+		if (nr_budgets)
+			blk_mq_release_budgets(q, list);
/*
  		 * If we didn't flush the entire list, we could have told
@@ -1997,6 +2007,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
  {
  	struct request_queue *q = rq->q;
  	bool run_queue = true;
+	int budget_token;
/*
  	 * RCU or SRCU read lock is needed before checking quiesced flag.
@@ -2014,11 +2025,14 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
  	if (q->elevator && !bypass_insert)
  		goto insert;
- if (!blk_mq_get_dispatch_budget(q))
+	budget_token = blk_mq_get_dispatch_budget(q);
+	if (budget_token < 0)
  		goto insert;
+ blk_mq_set_rq_budget_token(rq, budget_token);
+
  	if (!blk_mq_get_driver_tag(rq)) {
-		blk_mq_put_dispatch_budget(q);
+		blk_mq_put_dispatch_budget(q, budget_token);
  		goto insert;
  	}
diff --git a/block/blk-mq.h b/block/blk-mq.h
index a52703c98b77..bb59a3e54c3b 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -186,17 +186,34 @@ unsigned int blk_mq_in_flight(struct request_queue *q, struct hd_struct *part);
  void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
  			 unsigned int inflight[2]);
-static inline void blk_mq_put_dispatch_budget(struct request_queue *q)
+static inline void blk_mq_put_dispatch_budget(struct request_queue *q,
+					      int budget_token)
  {
  	if (q->mq_ops->put_budget)
-		q->mq_ops->put_budget(q);
+		q->mq_ops->put_budget(q, budget_token);
  }
-static inline bool blk_mq_get_dispatch_budget(struct request_queue *q)
+static inline int blk_mq_get_dispatch_budget(struct request_queue *q)
  {
  	if (q->mq_ops->get_budget)
  		return q->mq_ops->get_budget(q);
-	return true;
+	return 0;
+}
+
+static inline void blk_mq_set_rq_budget_token(struct request *rq, int token)
+{
+	if (token < 0)
+		return;
+
+	if (rq->q->mq_ops->set_rq_budget_token)
+		rq->q->mq_ops->set_rq_budget_token(rq, token);
+}
+
+static inline int blk_mq_get_rq_budget_token(struct request *rq)
+{
+	if (rq->q->mq_ops->get_rq_budget_token)
+		return rq->q->mq_ops->get_rq_budget_token(rq);
+	return -1;
  }
static inline void __blk_mq_inc_active_requests(struct blk_mq_hw_ctx *hctx)
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index bfcccc73fd01..41380e200925 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -343,6 +343,7 @@ void scsi_device_unbusy(struct scsi_device *sdev, struct scsi_cmnd *cmd)
  		atomic_dec(&starget->target_busy);
atomic_dec(&sdev->device_busy);
+	cmd->budget_token = -1;
  }
static void scsi_kick_queue(struct request_queue *q)
@@ -1128,6 +1129,7 @@ void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd)
  	unsigned long jiffies_at_alloc;
  	int retries, to_clear;
  	bool in_flight;
+	int budget_token = cmd->budget_token;
if (!blk_rq_is_scsi(rq) && !(flags & SCMD_INITIALIZED)) {
  		flags |= SCMD_INITIALIZED;
@@ -1156,6 +1158,7 @@ void scsi_init_command(struct scsi_device *dev, struct scsi_cmnd *cmd)
  	cmd->retries = retries;
  	if (in_flight)
  		__set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
+	cmd->budget_token = budget_token;
} @@ -1618,19 +1621,19 @@ static void scsi_mq_done(struct scsi_cmnd *cmd)
  	blk_mq_complete_request(cmd->request);
  }
-static void scsi_mq_put_budget(struct request_queue *q)
+static void scsi_mq_put_budget(struct request_queue *q, int budget_token)
  {
  	struct scsi_device *sdev = q->queuedata;
atomic_dec(&sdev->device_busy);
  }
-static bool scsi_mq_get_budget(struct request_queue *q)
+static int scsi_mq_get_budget(struct request_queue *q)
  {
  	struct scsi_device *sdev = q->queuedata;
if (scsi_dev_queue_ready(q, sdev))
-		return true;
+		return 0;
atomic_inc(&sdev->restarts); @@ -1652,7 +1655,7 @@ static bool scsi_mq_get_budget(struct request_queue *q)
  	if (unlikely(atomic_read(&sdev->device_busy) == 0 &&
  				!scsi_device_blocked(sdev)))
  		blk_mq_delay_run_hw_queues(sdev->request_queue, SCSI_QUEUE_DELAY);
-	return false;
+	return -1;
  }
static void scsi_mq_set_rq_budget_token(struct request *req, int token)
@@ -1680,6 +1683,8 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
  	blk_status_t ret;
  	int reason;
+ WARN_ON_ONCE(cmd->budget_token < 0);
+
  	/*
  	 * If the device is not in running state we will reject some or all
  	 * commands.
@@ -1730,7 +1735,8 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
  	if (scsi_target(sdev)->can_queue > 0)
  		atomic_dec(&scsi_target(sdev)->target_busy);
  out_put_budget:
-	scsi_mq_put_budget(q);
+	scsi_mq_put_budget(q, cmd->budget_token);
+	cmd->budget_token = -1;
  	switch (ret) {
  	case BLK_STS_OK:
  		break;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 71060c6a84b1..7b4d4b23403a 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -307,12 +307,12 @@ struct blk_mq_ops {
  	 * reserved budget. Also we have to handle failure case
  	 * of .get_budget for avoiding I/O deadlock.
  	 */
-	bool (*get_budget)(struct request_queue *);
+	int (*get_budget)(struct request_queue *);
/**
  	 * @put_budget: Release the reserved budget.
  	 */
-	void (*put_budget)(struct request_queue *);
+	void (*put_budget)(struct request_queue *, int);
/*
  	 * @set_rq_budget_toekn: store rq's budget token

In principle, yes. But I would like to have some more clarification about the values the 'budget_token' can have. From what I gather -1 means 'no budget', but what exactly is the difference between '0' and, say, '5'?
And indeed I would prefer to have the budget token to be unsigned,
such that '0' means 'no budget', and any positive number would indicate the budget itself.
Can we have a bit more documentation here?

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer



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