On Tue, Sep 21, 2021 at 10:13:17AM +1000, NeilBrown wrote:

On Mon, 20 Sep 2021, Mel Gorman wrote:

quoted

-long wait_iff_congested(int sync, long timeout)
-{
-	long ret;
-	unsigned long start = jiffies;
-	DEFINE_WAIT(wait);
-	wait_queue_head_t *wqh = &congestion_wqh[sync];
-
-	/*
-	 * If there is no congestion, yield if necessary instead
-	 * of sleeping on the congestion queue
-	 */
-	if (atomic_read(&nr_wb_congested[sync]) == 0) {
-		cond_resched();
-
-		/* In case we scheduled, work out time remaining */
-		ret = timeout - (jiffies - start);
-		if (ret < 0)
-			ret = 0;
-
-		goto out;
-	}
-
-	/* Sleep until uncongested or a write happens */
-	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);

Uninterruptible wait.

....

quoted

+static void
+reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
+							long timeout)
+{
+	wait_queue_head_t *wqh = &pgdat->reclaim_wait;
+	unsigned long start = jiffies;
+	long ret;
+	DEFINE_WAIT(wait);
+
+	atomic_inc(&pgdat->nr_reclaim_throttled);
+	WRITE_ONCE(pgdat->nr_reclaim_start,
+		 node_page_state(pgdat, NR_THROTTLED_WRITTEN));
+
+	prepare_to_wait(wqh, &wait, TASK_INTERRUPTIBLE);

Interruptible wait.

Why the change?  I think these waits really need to be TASK_UNINTERRUPTIBLE.

Because from mm/ context, I saw no reason why the task *should* be
uninterruptible. It's waiting on other tasks to complete IO and it is not
protecting device state, filesystem state or anything else. If it gets
a signal, it's safe to wake up, particularly if that signal is KILL and
the context is a direct reclaimer.

The original TASK_UNINTERRUPTIBLE is almost certainly a copy&paste from
congestion_wait which may be called because a filesystem operation must
complete before it can return to userspace so a signal waking it up is
pointless.

-- 
Mel Gorman
SUSE Labs