lightnvm: pblk: avoid being reported as hung on rated GC

The amount of GC I/O on the write buffer is managed by the rate-limiter,
which is calculated as a function of the number of available free
blocks. When reaching the stable point, we risk having scheduled more
I/Os for GC than are allowed on the write buffer. This would result on
the GC semaphore balancing the outstanding read GC I/Os to be reported
as "hung", though the behavior is normal.

Solve this by allowing to schedule when we detect that the read GC path
is not moving forward.
Signed-off-by: NJavier González <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk-gc.c

@@ -218,7 +218,13 @@ static void pblk_gc_line_prepare_ws(struct work_struct *work)
 	gc_rq_ws->line = line;
 	gc_rq_ws->priv = gc_rq;
 
-	down(&gc->gc_sem);
+	/* The write GC path can be much slower than the read GC one due to
+	 * the budget imposed by the rate-limiter. Balance in case that we get
+	 * back pressure from the write GC path.
+	 */
+	while (down_timeout(&gc->gc_sem, msecs_to_jiffies(30000)))
+		io_schedule();
+
 	kref_get(&line->ref);
 
 	INIT_WORK(&gc_rq_ws->ws, pblk_gc_line_ws);