Re: [PATCH v2] mm, vmscan: don't turn on cache_trim_mode at high scan priorities

[Huang, Ying]

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