[PATCH 1/3] blk-iolatency: clear delay state when freeing policy data

[Usama Arif <usama.arif@linux.dev>]

io.latency can throttle a group which has no latency target of its own.
When a sibling misses its target, check_scale_change() scales down its
peers, and a peer that reaches queue depth one gets blkcg_use_delay()
called on it on every further scale-down, even with min_lat_nsec == 0.

iolatency_pd_offline() resets the target through
iolatency_set_min_lat_nsec(), which clears the delay only on a nonzero
to zero transition, so it never clears such a peer.  Freeing the policy
data then leaves blkg->use_delay set and blkcg->congestion_count
elevated with nothing left that can drop it.

blk_cgroup_congested() then returns true for every task in that cgroup
and its descendants for as long as the cgroup lives: page_cache_sync_ra()
cuts readahead to a single page, page_cache_async_ra() skips it
altogether, and __folio_throttle_swaprate() takes swap_avail_lock and
schedules a throttle on anonymous folio allocation.

Clear the delay in iolatency_pd_free().  By then bio-held blkg
references have drained, or the queue is frozen for policy
deactivation, so check_scale_change() cannot re-arm it.  The free
callback can also see policy data which was never attached to a blkg,
hence the pd->blkg check.

Fixes: d70675121546 ("block: introduce blk-iolatency io controller")
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 block/blk-iolatency.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 9eb69010c34e6..2caa79a008ad1 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -1043,6 +1043,15 @@ static void iolat_release(struct rcu_head *rcu)
 
 static void iolatency_pd_free(struct blkg_policy_data *pd)
 {
+	struct blkcg_gq *blkg = pd_to_blkg(pd);
+
+	/*
+	 * Groups throttled as collateral have min_lat_nsec == 0, so
+	 * iolatency_pd_offline() leaves their delay set.  Drop it here, where
+	 * no in-flight bio can re-arm it via check_scale_change().
+	 */
+	if (blkg)
+		blkcg_clear_delay(blkg);
 	call_rcu(&pd->rcu_head, iolat_release);
 }
 
-- 
2.53.0-Meta