The patch titled
Subject: mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure
has been added to the -mm mm-unstable branch. Its filename is
mm-vmscan-retry-kswapds-priority-loop-with-cache_trim_mode-off-on-failure.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmscan-retry-kswapds-priority-loop-with-cache_trim_mode-off-on-failure.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Byungchul Park <byungchul@xxxxxx>
Subject: mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure
Date: Mon, 4 Mar 2024 15:27:37 +0900
With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
pages. However, it should be more careful to use 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 kswapd needs to retry its scan priority loop with cache_trim_mode off
again if the mode doesn't work for 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 | 1664772 |
| nr_active_anon | 1780700 | 437834 |
| nr_inactive_file | 30425 | 40882 |
| nr_active_file | 14961 | 3012 |
| pgpromote_success | 356 | 1293122 |
| pgpromote_candidate | 21953245 | 1824148 |
| pgactivate | 1844523 | 3311907 |
| pgdeactivate | 50634 | 1554069 |
| pgfault | 31100294 | 6518806 |
| pgdemote_kswapd | 30856 | 2230821 |
| pgscan_kswapd | 1861981 | 7667629 |
| pgscan_anon | 1822930 | 7610583 |
| pgscan_file | 39051 | 57046 |
| pgsteal_anon | 386 | 2192033 |
| pgsteal_file | 30470 | 38788 |
| pageoutrun | 30 | 412 |
| numa_hint_faults | 27418279 | 2875955 |
| numa_pages_migrated | 356 | 1293122 |
+-----------------------+-------------------------------+
Link: https://lkml.kernel.org/r/20240304082118.20499-1-byungchul@xxxxxx
Signed-off-by: Byungchul Park <byungchul@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
--- a/mm/vmscan.c~mm-vmscan-retry-kswapds-priority-loop-with-cache_trim_mode-off-on-failure
+++ a/mm/vmscan.c
@@ -108,6 +108,12 @@ struct scan_control {
/* Can folios be swapped as part of reclaim? */
unsigned int may_swap:1;
+ /* Not allow cache_trim_mode to be turned on as part of reclaim? */
+ unsigned int no_cache_trim_mode:1;
+
+ /* Has cache_trim_mode failed at least once? */
+ unsigned int cache_trim_mode_failed:1;
+
/* Proactive reclaim invoked by userspace through memory.reclaim */
unsigned int proactive:1;
@@ -2265,7 +2271,8 @@ static void prepare_scan_control(pg_data
* anonymous pages.
*/
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->no_cache_trim_mode)
sc->cache_trim_mode = 1;
else
sc->cache_trim_mode = 0;
@@ -5980,6 +5987,8 @@ again:
*/
if (reclaimable)
pgdat->kswapd_failures = 0;
+ else if (sc->cache_trim_mode)
+ sc->cache_trim_mode_failed = 1;
}
/*
@@ -6915,6 +6924,16 @@ restart:
sc.priority--;
} while (sc.priority >= 1);
+ /*
+ * Restart only if it went through the priority loop all the way,
+ * but cache_trim_mode didn't work.
+ */
+ if (!sc.nr_reclaimed && sc.priority < 1 &&
+ !sc.no_cache_trim_mode && sc.cache_trim_mode_failed) {
+ sc.no_cache_trim_mode = 1;
+ goto restart;
+ }
+
if (!sc.nr_reclaimed)
pgdat->kswapd_failures++;
_
Patches currently in -mm which might be from byungchul@xxxxxx are
sched-numa-mm-do-not-try-to-migrate-memory-to-memoryless-nodes.patch
mm-vmscan-retry-kswapds-priority-loop-with-cache_trim_mode-off-on-failure.patch
