[PATCH v5 4/4] dm zoned: do not busy-loop idle reclaim when no zone needs reclaim

[Kyrylo Biryuk <illofspeed@gmail.com>]

An idle target whose random (and cache) zones are all unmapped keeps its
reclaim work running without pause:

  - since commit 90a9b8693f1b ("dm zoned: reclaim random zones when
    idle"), dmz_should_reclaim() returns true when the target is idle and
    the total number of random (and cache) zones is non-zero, whether or
    not any of them holds data;
  - dmz_do_reclaim() then finds no candidate and returns -EBUSY;
  - dmz_reclaim_work() runs dmz_check_dev(), which checks every device
    for media events (a TEST UNIT READY on SCSI disks), and
    dmz_schedule_reclaim() requeues the work with no delay because
    dmz_should_reclaim() is still true.

Without the previous patch, a pass ending while the target is busy and
has enough free zones can leave reclaim unqueued, which ends the loop;
keeping the idle poll armed makes it recur whenever the target becomes
idle. On three 27 TB host-managed drives used
without cache devices (all random zones free) it kept about three CPUs
busy and sent about 155,000 TEST UNIT READY commands per second to each
drive while the target was idle.

Only reclaim on idle when a random or cache zone actually holds data (the
intent of the "p_unmap < 100" test that commit replaced), and
handle -EBUSY like -ENOSPC: nothing can progress and no I/O failed, so
poll again after the idle period instead of rescheduling at once, and
skip the device check. Use queue_delayed_work() for that poll so work an
allocation has already queued is not postponed.

Fixes: 90a9b8693f1b ("dm zoned: reclaim random zones when idle")
Assisted-by: LLM
Signed-off-by: Kyrylo Biryuk <illofspeed@gmail.com>
---
 drivers/md/dm-zoned-reclaim.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
index f1f9adf6039..0d62ade1466 100644
--- a/drivers/md/dm-zoned-reclaim.c
+++ b/drivers/md/dm-zoned-reclaim.c
@@ -470,9 +470,10 @@ static unsigned int dmz_reclaim_percentage(struct dmz_reclaim *zrc)
  */
 static bool dmz_should_reclaim(struct dmz_reclaim *zrc, unsigned int p_unmap)
 {
-	unsigned int nr_reclaim;
+	unsigned int nr_reclaim, nr_unmap;
 
 	nr_reclaim = dmz_nr_rnd_zones(zrc->metadata, zrc->dev_idx);
+	nr_unmap = dmz_nr_unmap_rnd_zones(zrc->metadata, zrc->dev_idx);
 
 	if (dmz_nr_cache_zones(zrc->metadata)) {
 		/*
@@ -483,10 +484,11 @@ static bool dmz_should_reclaim(struct dmz_reclaim *zrc, unsigned int p_unmap)
 		if (zrc->dev_idx == 0)
 			return false;
 		nr_reclaim += dmz_nr_cache_zones(zrc->metadata);
+		nr_unmap += dmz_nr_unmap_cache_zones(zrc->metadata);
 	}
 
-	/* Reclaim when idle */
-	if (dmz_target_idle(zrc) && nr_reclaim)
+	/* Reclaim when idle, but only if a random or cache zone holds data */
+	if (dmz_target_idle(zrc) && nr_reclaim > nr_unmap)
 		return true;
 
 	/* If there are still plenty of cache zones, do not reclaim */
@@ -543,17 +545,19 @@ static void dmz_reclaim_work(struct work_struct *work)
 		dmz_nr_rnd_zones(zmd, zrc->dev_idx));
 
 	ret = dmz_do_reclaim(zrc);
+	if (ret == -ENOSPC || ret == -EBUSY) {
+		/*
+		 * No zone to copy into (-ENOSPC) or no zone to reclaim
+		 * (-EBUSY): nothing can progress now and no I/O failed.
+		 * Rescheduling immediately would spin; poll instead, without
+		 * postponing work that an allocation has already queued.
+		 */
+		queue_delayed_work(zrc->wq, &zrc->work, DMZ_IDLE_PERIOD);
+		return;
+	}
 	if (ret && ret != -EINTR) {
 		if (!dmz_check_dev(zmd))
 			return;
-		if (ret == -ENOSPC) {
-			/*
-			 * No zone to copy into: nothing can progress now.
-			 * Rescheduling immediately would spin; poll instead.
-			 */
-			mod_delayed_work(zrc->wq, &zrc->work, DMZ_IDLE_PERIOD);
-			return;
-		}
 	}
 
 	dmz_schedule_reclaim(zrc);
-- 
2.43.0