[CC Mel, Vlastimil and Johannes for awareness]
On Fri 23-02-24 14:44:07, Byungchul Park wrote:
> 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@xxxxxx>
> 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@xxxxxx>
> ---
> 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
>
--
Michal Hocko
SUSE Labs
