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From: Byungchul Park <byungchul@sk.com>
To: akpm@linux-foundation.org
Cc: linux-kernel@vger.kernel.org,
	linux-mm@kvack.org,
	kernel_team@skhynix.com,
	yuzhao@google.com,
	ying.huang@intel.com
Subject: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work
Date: Fri, 23 Feb 2024 14:44:07 +0900
Message-Id: <20240223054407.14829-1-byungchul@sk.com>
Precedence: bulk

Changes from v2:
	1. Change the condition to stop cache_trim_mode.

	   From - Stop it if it's at high scan priorities, 0 or 1.
	   To   - Stop it if it's at high scan priorities, 0 or 1, and
	          the mode didn't work in the previous turn.

	   (feedbacked by Huang Ying)

	2. Change the test result in the commit message after testing
	   with the new logic.

Changes from v1:
	1. Add a comment describing why this change is necessary in code
	   and rewrite the commit message with how to reproduce and what
	   the result is using vmstat. (feedbacked by Andrew Morton and
	   Yu Zhao)
	2. Change the condition to avoid cache_trim_mode from
	   'sc->priority != 1' to 'sc->priority > 1' to reflect cases
	   where the priority goes to zero all the way. (feedbacked by
	   Yu Zhao)

--->8---
From 05846e34bf02ac9b3e254324dc2d7afd97a025d9 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul@sk.com>
Date: Fri, 23 Feb 2024 13:47:16 +0900
Subject: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work

With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
pages.  However, it should be more careful to turn on the mode because
it's going to prevent anon pages from being reclaimed even if there are
a huge number of anon pages that are cold and should be reclaimed.  Even
worse, that leads kswapd_failures to reach MAX_RECLAIM_RETRIES and
stopping kswapd from functioning until direct reclaim eventually works
to resume kswapd.

So do not turn on cache_trim_mode if the mode doesn't work, especially
while the sytem is struggling against reclaim.

The problematic behavior can be reproduced by:

   CONFIG_NUMA_BALANCING enabled
   sysctl_numa_balancing_mode set to NUMA_BALANCING_MEMORY_TIERING
   numa node0 (8GB local memory, 16 CPUs)
   numa node1 (8GB slow tier memory, no CPUs)

   Sequence:

   1) echo 3 > /proc/sys/vm/drop_caches
   2) To emulate the system with full of cold memory in local DRAM, run
      the following dummy program and never touch the region:

         mmap(0, 8 * 1024 * 1024 * 1024, PROT_READ | PROT_WRITE,
	      MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1, 0);

   3) Run any memory intensive work e.g. XSBench.
   4) Check if numa balancing is working e.i. promotion/demotion.
   5) Iterate 1) ~ 4) until numa balancing stops.

With this, you could see that promotion/demotion are not working because
kswapd has stopped due to ->kswapd_failures >= MAX_RECLAIM_RETRIES.

Interesting vmstat delta's differences between before and after are like:

   +-----------------------+-------------------------------+
   | interesting vmstat	   | before	   | after	   |
   +-----------------------+-------------------------------+
   | nr_inactive_anon	   | 321935	   | 1636737	   |
   | nr_active_anon	   | 1780700	   | 465913	   |
   | nr_inactive_file	   | 30425	   | 35711	   |
   | nr_active_file	   | 14961	   | 8698	   |
   | pgpromote_success	   | 356	   | 1267785	   |
   | pgpromote_candidate   | 21953245	   | 1745631	   |
   | pgactivate		   | 1844523	   | 3309867	   |
   | pgdeactivate	   | 50634	   | 1545041	   |
   | pgfault		   | 31100294	   | 6411036	   |
   | pgdemote_kswapd	   | 30856	   | 2267467	   |
   | pgscan_kswapd	   | 1861981	   | 7729231	   |
   | pgscan_anon	   | 1822930	   | 7667544	   |
   | pgscan_file	   | 39051	   | 61687	   |
   | pgsteal_anon	   | 386	   | 2227217	   |
   | pgsteal_file	   | 30470	   | 40250	   |
   | pageoutrun		   | 30		   | 457	   |
   | numa_hint_faults	   | 27418279	   | 2752289	   |
   | numa_pages_migrated   | 356	   | 1267785 	   |
   +-----------------------+-------------------------------+

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 mm/vmscan.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bba207f41b14..f7312d831fed 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -127,6 +127,9 @@ struct scan_control {
 	/* One of the zones is ready for compaction */
 	unsigned int compaction_ready:1;
 
+	/* If the last try was reclaimable */
+	unsigned int reclaimable:1;
+
 	/* There is easily reclaimable cold cache in the current node */
 	unsigned int cache_trim_mode:1;
 
@@ -2266,9 +2269,14 @@ static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
 	 * If we have plenty of inactive file pages that aren't
 	 * thrashing, try to reclaim those first before touching
 	 * anonymous pages.
+	 *
+	 * It doesn't make sense to keep cache_trim_mode on if the mode
+	 * is not working while struggling against reclaim. So do not
+	 * turn it on if so. Note the highest priority of kswapd is 1.
 	 */
 	file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE);
-	if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE))
+	if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE) &&
+	    !(sc->cache_trim_mode && !sc->reclaimable && sc->priority <= 1))
 		sc->cache_trim_mode = 1;
 	else
 		sc->cache_trim_mode = 0;
@@ -5862,7 +5870,6 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 {
 	unsigned long nr_reclaimed, nr_scanned, nr_node_reclaimed;
 	struct lruvec *target_lruvec;
-	bool reclaimable = false;
 
 	if (lru_gen_enabled() && root_reclaim(sc)) {
 		lru_gen_shrink_node(pgdat, sc);
@@ -5877,6 +5884,14 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 	nr_reclaimed = sc->nr_reclaimed;
 	nr_scanned = sc->nr_scanned;
 
+	/*
+	 * Reset to the default values at the start.
+	 */
+	if (sc->priority == DEF_PRIORITY) {
+		sc->reclaimable = 1;
+		sc->cache_trim_mode = 0;
+	}
+
 	prepare_scan_control(pgdat, sc);
 
 	shrink_node_memcgs(pgdat, sc);
@@ -5890,8 +5905,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
 			   sc->nr_scanned - nr_scanned, nr_node_reclaimed);
 
-	if (nr_node_reclaimed)
-		reclaimable = true;
+	sc->reclaimable = !!nr_node_reclaimed;
 
 	if (current_is_kswapd()) {
 		/*
@@ -5965,7 +5979,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 	 * sleep. On reclaim progress, reset the failure counter. A
 	 * successful direct reclaim run will revive a dormant kswapd.
 	 */
-	if (reclaimable)
+	if (sc->reclaimable)
 		pgdat->kswapd_failures = 0;
 }
 
-- 
2.17.1