blk-throttle: fix zero wait time for iops throttled group

After commit 991f61fe ("Blk-throttle: reduce tail io latency when
iops limit is enforced") wait time could be zero even if group is
throttled and cannot issue requests right now. As a result
throtl_select_dispatch() turns into busy-loop under irq-safe queue
spinlock.

Fix is simple: always round up target time to the next throttle slice.

Fixes: 991f61fe ("Blk-throttle: reduce tail io latency when iops limit is enforced")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -881,13 +881,10 @@ static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio,
 	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
 	u64 tmp;
 
-	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
-
-	/* Slice has just started. Consider one slice interval */
-	if (!jiffy_elapsed)
-		jiffy_elapsed_rnd = tg->td->throtl_slice;
+	jiffy_elapsed = jiffies - tg->slice_start[rw];
 
-	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);
+	/* Round up to the next throttle slice, wait time must be nonzero */
+	jiffy_elapsed_rnd = roundup(jiffy_elapsed + 1, tg->td->throtl_slice);
 
 	/*
 	 * jiffy_elapsed_rnd should not be a big value as minimum iops can be