Re: [PATCH v2 4/4] block/mq-deadline: Prioritize high-priority requests

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On 9/28/21 12:03 AM, Bart Van Assche wrote:
In addition to reverting commit 7b05bf771084 ("Revert "block/mq-deadline:
Prioritize high-priority requests""), this patch uses 'jiffies' instead
of ktime_get() in the code for aging lower priority requests.

This patch has been tested as follows:

Measured QD=1/jobs=1 IOPS for nullb with the mq-deadline scheduler.
Result without and with this patch: 555 K IOPS.

Measured QD=1/jobs=8 IOPS for nullb with the mq-deadline scheduler.
Result without and with this patch: about 380 K IOPS.

Ran the following script:

set -e
scriptdir=$(dirname "$0")
if [ -e /sys/module/scsi_debug ]; then modprobe -r scsi_debug; fi
modprobe scsi_debug ndelay=1000000 max_queue=16
sd=''
while [ -z "$sd" ]; do
   sd=$(basename /sys/bus/pseudo/drivers/scsi_debug/adapter*/host*/target*/*/block/*)
done
echo $((100*1000)) > "/sys/block/$sd/queue/iosched/prio_aging_expire"
if [ -e /sys/fs/cgroup/io.prio.class ]; then
   cd /sys/fs/cgroup
   echo restrict-to-be >io.prio.class
   echo +io > cgroup.subtree_control
else
   cd /sys/fs/cgroup/blkio/
   echo restrict-to-be >blkio.prio.class
fi
echo $$ >cgroup.procs
mkdir -p hipri
cd hipri
if [ -e io.prio.class ]; then
   echo none-to-rt >io.prio.class
else
   echo none-to-rt >blkio.prio.class
fi
{ "${scriptdir}/max-iops" -a1 -d32 -j1 -e mq-deadline "/dev/$sd" >& ~/low-pri.txt & }
echo $$ >cgroup.procs
"${scriptdir}/max-iops" -a1 -d32 -j1 -e mq-deadline "/dev/$sd" >& ~/hi-pri.txt

Result:
* 11000 IOPS for the high-priority job
*    40 IOPS for the low-priority job

If the prio aging expiry time is changed from 100s into 0, the IOPS results
change into 6712 and 6796 IOPS.

The max-iops script is a script that runs fio with the following arguments:
--bs=4K --gtod_reduce=1 --ioengine=libaio --ioscheduler=${arg_e} --runtime=60
--norandommap --rw=read --thread --buffered=0 --numjobs=${arg_j}
--iodepth=${arg_d} --iodepth_batch_submit=${arg_a}
--iodepth_batch_complete=$((arg_d / 2)) --name=${positional_argument_1}
--filename=${positional_argument_1}

Cc: Damien Le Moal <damien.lemoal@xxxxxxx>
Cc: Niklas Cassel <Niklas.Cassel@xxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
  block/mq-deadline.c | 77 ++++++++++++++++++++++++++++++++++++++++++---
  1 file changed, 73 insertions(+), 4 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index b262f40f32c0..bb723478baf1 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -31,6 +31,11 @@
   */
  static const int read_expire = HZ / 2;  /* max time before a read is submitted. */
  static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
+/*
+ * Time after which to dispatch lower priority requests even if higher
+ * priority requests are pending.
+ */
+static const int prio_aging_expire = 10 * HZ;
  static const int writes_starved = 2;    /* max times reads can starve a write */
  static const int fifo_batch = 16;       /* # of sequential requests treated as one
  				     by the above parameters. For throughput. */
@@ -96,6 +101,7 @@ struct deadline_data {
  	int writes_starved;
  	int front_merges;
  	u32 async_depth;
+	int prio_aging_expire;
spinlock_t lock;
  	spinlock_t zone_lock;
@@ -338,12 +344,27 @@ deadline_next_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
  	return rq;
  }
+/*
+ * Returns true if and only if @rq started after @latest_start where
+ * @latest_start is in jiffies.
+ */
+static bool started_after(struct deadline_data *dd, struct request *rq,
+			  unsigned long latest_start)
+{
+	unsigned long start_time = (unsigned long)rq->fifo_time;
+
+	start_time -= dd->fifo_expire[rq_data_dir(rq)];
+
+	return time_after(start_time, latest_start);
+}
+
  /*
   * deadline_dispatch_requests selects the best request according to
- * read/write expire, fifo_batch, etc
+ * read/write expire, fifo_batch, etc and with a start time <= @latest.
   */
  static struct request *__dd_dispatch_request(struct deadline_data *dd,
-					     struct dd_per_prio *per_prio)
+					     struct dd_per_prio *per_prio,
+					     unsigned long latest_start)
  {
  	struct request *rq, *next_rq;
  	enum dd_data_dir data_dir;
@@ -355,6 +376,8 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
  	if (!list_empty(&per_prio->dispatch)) {
  		rq = list_first_entry(&per_prio->dispatch, struct request,
  				      queuelist);
+		if (started_after(dd, rq, latest_start))
+			return NULL;
  		list_del_init(&rq->queuelist);
  		goto done;
  	}
@@ -432,6 +455,9 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
  	dd->batching = 0;
dispatch_request:
+	if (started_after(dd, rq, latest_start))
+		return NULL;
+
  	/*
  	 * rq is the selected appropriate request.
  	 */
@@ -449,6 +475,34 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
  	return rq;
  }
+/*
+ * Check whether there are any requests with priority other than DD_RT_PRIO
+ * that were inserted more than prio_aging_expire jiffies ago.
+ */
+static struct request *dd_dispatch_prio_aged_requests(struct deadline_data *dd,
+						      unsigned long now)
+{
+	struct request *rq;
+	enum dd_prio prio;
+	int prio_cnt;
+
+	lockdep_assert_held(&dd->lock);
+
+	prio_cnt = !!dd_queued(dd, DD_RT_PRIO) + !!dd_queued(dd, DD_BE_PRIO) +
+		   !!dd_queued(dd, DD_IDLE_PRIO);
+	if (prio_cnt < 2)
+		return NULL;
+
+	for (prio = DD_BE_PRIO; prio <= DD_PRIO_MAX; prio++) {
+		rq = __dd_dispatch_request(dd, &dd->per_prio[prio],
+					   now - dd->prio_aging_expire);
+		if (rq)
+			return rq;
+	}
+
+	return NULL;
+}
+
  /*
   * Called from blk_mq_run_hw_queue() -> __blk_mq_sched_dispatch_requests().
   *
@@ -460,15 +514,26 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
  static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
  {
  	struct deadline_data *dd = hctx->queue->elevator->elevator_data;
+	const unsigned long now = jiffies;
  	struct request *rq;
  	enum dd_prio prio;
spin_lock(&dd->lock);
+	rq = dd_dispatch_prio_aged_requests(dd, now);
+	if (rq)
+		goto unlock;
+
+	/*
+	 * Next, dispatch requests in priority order. Ignore lower priority
+	 * requests if any higher priority requests are pending.
+	 */
  	for (prio = 0; prio <= DD_PRIO_MAX; prio++) {
-		rq = __dd_dispatch_request(dd, &dd->per_prio[prio]);
-		if (rq)
+		rq = __dd_dispatch_request(dd, &dd->per_prio[prio], now);
+		if (rq || dd_queued(dd, prio))
  			break;
  	}
+
+unlock:
  	spin_unlock(&dd->lock);
return rq;
@@ -573,6 +638,7 @@ static int dd_init_sched(struct request_queue *q, struct elevator_type *e)
  	dd->front_merges = 1;
  	dd->last_dir = DD_WRITE;
  	dd->fifo_batch = fifo_batch;
+	dd->prio_aging_expire = prio_aging_expire;
  	spin_lock_init(&dd->lock);
  	spin_lock_init(&dd->zone_lock);
@@ -796,6 +862,7 @@ static ssize_t __FUNC(struct elevator_queue *e, char *page) \
  #define SHOW_JIFFIES(__FUNC, __VAR) SHOW_INT(__FUNC, jiffies_to_msecs(__VAR))
  SHOW_JIFFIES(deadline_read_expire_show, dd->fifo_expire[DD_READ]);
  SHOW_JIFFIES(deadline_write_expire_show, dd->fifo_expire[DD_WRITE]);
+SHOW_JIFFIES(deadline_prio_aging_expire_show, dd->prio_aging_expire);
  SHOW_INT(deadline_writes_starved_show, dd->writes_starved);
  SHOW_INT(deadline_front_merges_show, dd->front_merges);
  SHOW_INT(deadline_async_depth_show, dd->front_merges);
@@ -825,6 +892,7 @@ static ssize_t __FUNC(struct elevator_queue *e, const char *page, size_t count)
  	STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, msecs_to_jiffies)
  STORE_JIFFIES(deadline_read_expire_store, &dd->fifo_expire[DD_READ], 0, INT_MAX);
  STORE_JIFFIES(deadline_write_expire_store, &dd->fifo_expire[DD_WRITE], 0, INT_MAX);
+STORE_JIFFIES(deadline_prio_aging_expire_store, &dd->prio_aging_expire, 0, INT_MAX);
  STORE_INT(deadline_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX);
  STORE_INT(deadline_front_merges_store, &dd->front_merges, 0, 1);
  STORE_INT(deadline_async_depth_store, &dd->front_merges, 1, INT_MAX);
@@ -843,6 +911,7 @@ static struct elv_fs_entry deadline_attrs[] = {
  	DD_ATTR(front_merges),
  	DD_ATTR(async_depth),
  	DD_ATTR(fifo_batch),
+	DD_ATTR(prio_aging_expire),
  	__ATTR_NULL
  };
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>

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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