[PATCH] mm/vmscan: Centralise timeout values for reclaim_throttle

[Mel Gorman]

Neil Brown raised concerns about callers of reclaim_throttle specifying
a timeout value. The original timeout values to congestion_wait() were
probably pulled out of thin air or copy&pasted from somewhere else.
This patch centralises the timeout values and selects a timeout based
on the reason for reclaim throttling. These figures are also pulled
out of the same thin air but better values may be derived

Running a workload that is throttling for inappropriate periods
and tracing mm_vmscan_throttled can be used to pick a more appropriate
value. Excessive throttling would pick a lower timeout where as
excessive CPU usage in reclaim context would select a larger timeout.
Ideally a large value would always be used and the wakeups would
occur before a timeout but that requires careful testing.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/compaction.c     |  2 +-
 mm/internal.h       |  3 +--
 mm/page-writeback.c |  2 +-
 mm/vmscan.c         | 39 +++++++++++++++++++++++++++++++--------
 4 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 7359093d8ac0..151b04c4dab3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -828,7 +828,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (cc->mode == MIGRATE_ASYNC)
 			return -EAGAIN;
 
-		reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED, HZ/10);
+		reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
 
 		if (fatal_signal_pending(current))
 			return -EINTR;
diff --git a/mm/internal.h b/mm/internal.h
index 06d0c376efcd..f8d203cfd4e1 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -129,8 +129,7 @@ extern unsigned long highest_memmap_pfn;
  */
 extern int isolate_lru_page(struct page *page);
 extern void putback_lru_page(struct page *page);
-extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
-								long timeout);
+extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason);
 
 /*
  * in mm/rmap.c:
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f34f54fcd5b4..7d08706c541a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2374,7 +2374,7 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 		 * guess as any.
 		 */
 		reclaim_throttle(NODE_DATA(numa_node_id()),
-			VMSCAN_THROTTLE_WRITEBACK, HZ/50);
+						VMSCAN_THROTTLE_WRITEBACK);
 	}
 	/*
 	 * Usually few pages are written by now from those we've just submitted
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b0012f9536e1..36b21549a3a4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1006,14 +1006,37 @@ static void handle_write_error(struct address_space *mapping,
 	unlock_page(page);
 }
 
-void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
-							long timeout)
+void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
 {
 	wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
 	unsigned long start = jiffies;
-	long ret;
+	long timeout, ret;
 	DEFINE_WAIT(wait);
 
+	/*
+	 * These figures are pulled out of thin air.
+	 * VMSCAN_THROTTLE_ISOLATED is a transient condition based on too many
+	 * parallel reclaimers which is a short-lived event so the timeout is
+	 * short. Failing to make progress or waiting on writeback are
+	 * potentially long-lived events so use a longer timeout. This is shaky
+	 * logic as a failure to make progress could be due to anything from
+	 * writeback to a slow device to excessive references pages at the tail
+	 * of the inactive LRU.
+	 */
+	switch(reason) {
+	case VMSCAN_THROTTLE_NOPROGRESS:
+	case VMSCAN_THROTTLE_WRITEBACK:
+		timeout = HZ/10;
+		break;
+	case VMSCAN_THROTTLE_ISOLATED:
+		timeout = HZ/50;
+		break;
+	default:
+		WARN_ON_ONCE(1);
+		timeout = HZ;
+		break;
+	}
+
 	atomic_inc(&pgdat->nr_reclaim_throttled);
 	WRITE_ONCE(pgdat->nr_reclaim_start,
 		 node_page_state(pgdat, NR_THROTTLED_WRITTEN));
@@ -2298,7 +2321,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 
 		/* wait a bit for the reclaimer. */
 		stalled = true;
-		reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED, HZ/10);
+		reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
 
 		/* We are about to die and free our memory. Return now. */
 		if (fatal_signal_pending(current))
@@ -3230,7 +3253,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 		 * until some pages complete writeback.
 		 */
 		if (sc->nr.immediate)
-			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
+			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
 	}
 
 	/*
@@ -3254,7 +3277,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 	if (!current_is_kswapd() && current_may_throttle() &&
 	    !sc->hibernation_mode &&
 	    test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
-		reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
+		reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
 
 	if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
 				    sc))
@@ -3326,7 +3349,7 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
 
 	/* Throttle if making no progress at high prioities. */
 	if (sc->priority < DEF_PRIORITY - 2)
-		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
+		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
 }
 
 /*
@@ -3795,7 +3818,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 		z = first_zones_zonelist(zonelist, sc.reclaim_idx, sc.nodemask);
 		pgdat = zonelist_zone(z)->zone_pgdat;
 
-		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
+		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
 	}
 
 	return nr_reclaimed;