Thanks Vlastimil for your inputs!!

On 6/16/2021 5:29 PM, Vlastimil Babka wrote:

quoted

This triggering of proactive compaction is done on a write to
sysctl.compaction_proactiveness by user.

[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=facdaa917c4d5a376d09d25865f5a863f906234a

Signed-off-by: Charan Teja Reddy <redacted>
---
changes in V2:

You forgot to also summarize the changes. Please do in next version.

Sure. Will take care this in the next version.

quoted

  */
 unsigned int __read_mostly sysctl_compaction_proactiveness = 20;
 
+int compaction_proactiveness_sysctl_handler(struct ctl_table *table, int write,
+		void *buffer, size_t *length, loff_t *ppos)
+{
+	int rc, nid;
+
+	rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
+	if (rc)
+		return rc;
+
+	if (write && sysctl_compaction_proactiveness) {
+		for_each_online_node(nid) {
+			pg_data_t *pgdat = NODE_DATA(nid);
+
+			if (pgdat->proactive_compact_trigger)
+				continue;
+
+			pgdat->proactive_compact_trigger = true;

I don't like the new variable. I wish we could do without it. I understand this
is added to ignore proactive_defer.
We could instead expose proactive_defer in pgdat and reset it to 0 before wakeup
(instead being a thread variable in kcompactd). But that would be racy with the
decreases done by kcompactd.
But I like the patch 2/2 and the idea could be extended to proactive_defer
handling. If there's no proactive_defer, timeout is
HPAGE_FRAG_CHECK_INTERVAL_MSEC. If kcompactd decides to defer, timeout would be
HPAGE_FRAG_CHECK_INTERVAL_MSEC << COMPACT_MAX_DEFER_SHIFT. Thus, no more waking
up just to decrease proactive_defer, we can then get rid of the counter. On
writing new proactiveness just wake up and that's it, regardless of which
timeout there was at the moment.

I think we can get rid off 'proactive_defer' thread variable with the
timeout approach you suggested. But it is still requires to have one
additional variable 'proactive_compact_trigger', which main purpose is
to decide if the kcompactd wakeup is for proactive compaction or not.
Please see below code:
   if (wait_event_freezable_timeout() && !proactive_compact_trigger) {
	// do the non-proactive work
	continue
   }
   // do the proactive work
     .................

Thus I feel that on writing new proactiveness, it is required to do
wakeup_kcomppactd() + set a flag that this wakeup is for proactive work.

Am I failed to get your point here?

The only change is, if we get woken up to do non-proactive work, by
wakeup_kcompactd(), the proactive_defer value would be now be effectively lost.
I think it's OK as wakeup_kcompactd() means the condition of the zone changed
substantionally anyway and carrying on with previous defer makes not much sense.
What do you think?

Agree.

quoted

+			wake_up_interruptible(&pgdat->kcompactd_wait);
+		}
+	}
+
+	return 0;
+}
+
 /*
  * This is the entry point for compacting all nodes via
  * /proc/sys/vm/compact_memory

@@ -2752,7 +2776,8 @@ void compaction_unregister_node(struct node *node)
 
 static inline bool kcompactd_work_requested(pg_data_t *pgdat)
 {
-	return pgdat->kcompactd_max_order > 0 || kthread_should_stop();
+	return pgdat->kcompactd_max_order > 0 || kthread_should_stop() ||
+		pgdat->proactive_compact_trigger;
 }
 
 static bool kcompactd_node_suitable(pg_data_t *pgdat)

@@ -2905,7 +2930,8 @@ 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))) {
+			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC)) &&
+			!pgdat->proactive_compact_trigger) {
 
 			psi_memstall_enter(&pflags);
 			kcompactd_do_work(pgdat);

@@ -2917,10 +2943,20 @@ static int kcompactd(void *p)
 		if (should_proactive_compact_node(pgdat)) {
 			unsigned int prev_score, score;
 
-			if (proactive_defer) {
+			/*
+			 * On wakeup of proactive compaction by sysctl
+			 * write, ignore the accumulated defer score.
+			 * Anyway, if the proactive compaction didn't
+			 * make any progress for the new value, it will
+			 * be further deferred by 2^COMPACT_MAX_DEFER_SHIFT
+			 * times.
+			 */
+			if (proactive_defer &&
+				!pgdat->proactive_compact_trigger) {
 				proactive_defer--;
 				continue;
 			}
+
 			prev_score = fragmentation_score_node(pgdat);
 			proactive_compact_node(pgdat);
 			score = fragmentation_score_node(pgdat);

@@ -2931,6 +2967,8 @@ static int kcompactd(void *p)
 			proactive_defer = score < prev_score ?
 					0 : 1 << COMPACT_MAX_DEFER_SHIFT;
 		}
+		if (pgdat->proactive_compact_trigger)
+			pgdat->proactive_compact_trigger = false;
 	}
 
 	return 0;

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