iocost: don't reset the inuse weight of under-weighted debtors

When an iocg is in debt, its inuse weight is owned by debt handling and
should stay at 1. This invariant was broken when determining the amount of
surpluses at the beginning of donation calculation - when an iocg's
hierarchical weight is too low, the iocg is excluded from donation
calculation and its inuse is reset to its active regardless of its
indebtedness, triggering warnings like the following:

 WARNING: CPU: 5 PID: 0 at block/blk-iocost.c:1416 iocg_kick_waitq+0x392/0x3a0
 ...
 RIP: 0010:iocg_kick_waitq+0x392/0x3a0
 Code: 00 00 be ff ff ff ff 48 89 4d a8 e8 98 b2 70 00 48 8b 4d a8 85 c0 0f 85 4a fe ff ff 0f 0b e9 43 fe ff ff 0f 0b e9 4d fe ff ff <0f> 0b e9 50 fe ff ff e8 a2 ae 70 00 66 90 0f 1f 44 00 00 55 48 89
 RSP: 0018:ffffc90000200d08 EFLAGS: 00010016
 ...
  <IRQ>
  ioc_timer_fn+0x2e0/0x1470
  call_timer_fn+0xa1/0x2c0
 ...

As this happens only when an iocg's hierarchical weight is negligible, its
impact likely is limited to triggering the warnings. Fix it by skipping
resetting inuse of under-weighted debtors.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Rik van Riel <riel@surriel.com>
Fixes: c421a3eb2e ("blk-iocost: revamp debt handling")
Cc: stable@vger.kernel.org # v5.10+
Link: https://lore.kernel.org/r/YmjODd4aif9BzFuO@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -2322,7 +2322,17 @@ static void ioc_timer_fn(struct timer_list *timer)
 				iocg->hweight_donating = hwa;
 				iocg->hweight_after_donation = new_hwi;
 				list_add(&iocg->surplus_list, &surpluses);
-			} else {
+			} else if (!iocg->abs_vdebt) {
+				/*
+				 * @iocg doesn't have enough to donate. Reset
+				 * its inuse to active.
+				 *
+				 * Don't reset debtors as their inuse's are
+				 * owned by debt handling. This shouldn't affect
+				 * donation calculuation in any meaningful way
+				 * as @iocg doesn't have a meaningful amount of
+				 * share anyway.
+				 */
 				TRACE_IOCG_PATH(inuse_shortage, iocg, &now,
 						iocg->inuse, iocg->active,
 						iocg->hweight_inuse, new_hwi);