[PATCH resend] mm: compaction: optimize proactive compaction deferrals

[Charan Teja Reddy]

Vlastimil Babka figured out that when fragmentation score didn't go down
across the proactive compaction i.e. when no progress is made, next wake
up for proactive compaction is deferred for 1 <<
COMPACT_MAX_DEFER_SHIFT, i.e. 64 times, with each wakeup interval of
HPAGE_FRAG_CHECK_INTERVAL_MSEC(=500). In each of this wakeup, it just
decrement 'proactive_defer' counter and goes sleep i.e. it is getting
woken to just decrement a counter. The same deferral time can also
achieved by simply doing the HPAGE_FRAG_CHECK_INTERVAL_MSEC <<
COMPACT_MAX_DEFER_SHIFT thus unnecessary wakeup of kcompact thread is
avoided thus also removes the need of 'proactive_defer' thread counter.

Link: https://lore.kernel.org/linux-fsdevel/88abfdb6-2c13-b5a6-5b46-742d12d1c910@xxxxxxx/
Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
---
 Changes in V1:
    o Removed the 'proactive_defer' thread counter by optimizing proactive
    o This is a resend as earlier it was clubbed with other changes posted
      at https://lore.kernel.org/patchwork/patch/1448789/	

 mm/compaction.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 621508e..db00dbf 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2885,7 +2885,8 @@ static int kcompactd(void *p)
 {
 	pg_data_t *pgdat = (pg_data_t *)p;
 	struct task_struct *tsk = current;
-	unsigned int proactive_defer = 0;
+	long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC);
+	long timeout = default_timeout;
 
 	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
 
@@ -2902,23 +2903,30 @@ static int kcompactd(void *p)
 
 		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
 		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
-			kcompactd_work_requested(pgdat),
-			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
+			kcompactd_work_requested(pgdat), timeout)) {
 
 			psi_memstall_enter(&pflags);
 			kcompactd_do_work(pgdat);
 			psi_memstall_leave(&pflags);
+			/*
+			 * Reset the timeout value. The defer timeout by
+			 * proactive compaction can effectively lost
+			 * here but that is fine as the condition of the
+			 * zone changed substantionally and carrying on
+			 * with the previous defer is not useful.
+			 */
+			timeout = default_timeout;
 			continue;
 		}
 
-		/* kcompactd wait timeout */
+		/*
+		 * Start the proactive work with default timeout. Based
+		 * on the fragmentation score, this timeout is updated.
+		 */
+		timeout = default_timeout;
 		if (should_proactive_compact_node(pgdat)) {
 			unsigned int prev_score, score;
 
-			if (proactive_defer) {
-				proactive_defer--;
-				continue;
-			}
 			prev_score = fragmentation_score_node(pgdat);
 			proactive_compact_node(pgdat);
 			score = fragmentation_score_node(pgdat);
@@ -2926,8 +2934,9 @@ static int kcompactd(void *p)
 			 * Defer proactive compaction if the fragmentation
 			 * score did not go down i.e. no progress made.
 			 */
-			proactive_defer = score < prev_score ?
-					0 : 1 << COMPACT_MAX_DEFER_SHIFT;
+			if (unlikely(score >= prev_score))
+				timeout =
+				   default_timeout << COMPACT_MAX_DEFER_SHIFT;
 		}
 	}
 
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