Byungchul Park <byungchul@xxxxxx> writes:
> 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 07e0baab368160e50b6ca35d95745168aa60e217 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@xxxxxx>
> Date: Thu, 22 Feb 2024 14:50:17 +0900
> Subject: [PATCH v2] mm, vmscan: don't turn on cache_trim_mode at high scan priorities
>
> 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 can lead kswapd_failures to reach MAX_RECLAIM_RETRIES and
> stopping kswapd until direct reclaim eventually works to resume kswapd.
> So this is more like a bug fix than a performance improvement.
>
> 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 kswapd stops.
>
> With this, you could eventually 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:
>
> -nr_inactive_anon 321935
> -nr_active_anon 1780700
> -nr_inactive_file 30425
> -nr_active_file 14961
> -pgpromote_success 356
> -pgpromote_candidate 21953245
> -pgactivate 1844523
> -pgdeactivate 50634
> -pgfault 31100294
> -pgdemote_kswapd 30856
> -pgscan_kswapd 1861981
> -pgscan_anon 1822930
> -pgscan_file 39051
> -pgsteal_anon 386
> -pgsteal_file 30470
> -pageoutrun 30
> -numa_hint_faults 27418279
> -numa_pages_migrated 356
>
> +nr_inactive_anon 1662306
> +nr_active_anon 440303
> +nr_inactive_file 27669
> +nr_active_file 1654
> +pgpromote_success 1314102
> +pgpromote_candidate 1892525
> +pgactivate 3284457
> +pgdeactivate 1527504
> +pgfault 6847775
> +pgdemote_kswapd 2142047
> +pgscan_kswapd 7496588
> +pgscan_anon 7462488
> +pgscan_file 34100
> +pgsteal_anon 2115661
> +pgsteal_file 26386
> +pageoutrun 378
> +numa_hint_faults 3220891
> +numa_pages_migrated 1314102
>
> where -: before this patch, +: after this patch
>
> Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> ---
> mm/vmscan.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bba207f41b14..6eda59fce5ee 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2266,9 +2266,17 @@ 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.
> + *
> + * However, the condition 'sc->cache_trim_mode == 1' all through
> + * the scan priorties might lead reclaim failure. If it keeps
> + * MAX_RECLAIM_RETRIES times, then kswapd would get stopped even
> + * if there are still plenty anon pages to reclaim, which is not
> + * desirable. So do not use cache_trim_mode when reclaim is not
> + * smooth e.i. high scan priority.
> */
> file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE);
> - if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE))
> + if (sc->priority > 1 && file >> sc->priority &&
> + !(sc->may_deactivate & DEACTIVATE_FILE))
> sc->cache_trim_mode = 1;
> else
> sc->cache_trim_mode = 0;
In get_scan_count(), there's following code,
/*
* Do not apply any pressure balancing cleverness when the
* system is close to OOM, scan both anon and file equally
* (unless the swappiness setting disagrees with swapping).
*/
if (!sc->priority && swappiness) {
scan_balance = SCAN_EQUAL;
goto out;
}
So, swappiness is 0 in you system? Please check it. If it's not 0,
please check why this doesn't help.
--
Best Regards,
Huang, Ying
