Re: [PATCH v6] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

[Byungchul Park <byungchul@sk.com>]

On Mon, Mar 04, 2024 at 05:21:18PM +0900, Byungchul Park wrote:
> Changes from v5:
> 	1. Make it retry the kswapd's scan priority loop with
> 	   cache_trim_mode off *only if* the mode didn't work in the
> 	   previous loop. (feedbacked by Huang Ying)
> 	2. Take into account 'break's from the priority loop when making
> 	   the decision whether to retry. (feedbacked by Huang Ying)
> 	3. Update the test result in the commit message.
> 
> Changes from v4:
> 	1. Make other scans start with may_cache_trim_mode = 1.
> 
> Changes from v3:
> 	1. Update the test result in the commit message with v4.
> 	2. Retry the whole priority loop with cache_trim_mode off again,
> 	   rather than forcing the mode off at the highest priority,
> 	   when the mode doesn't work. (feedbacked by Johannes Weiner)
> 
> 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 f811ee583158fd53d0e94d32ce5948fac4b17cfe Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Mon, 4 Mar 2024 15:27:37 +0900
> Subject: [PATCH v6] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure
> 
> 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       |
>    +-----------------------+-------------------------------+
> 
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> ---
>  mm/vmscan.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bba207f41b14..6fe45eca7766 100644
> --- a/mm/vmscan.c
> +++ b/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;
>  
> @@ -2268,7 +2274,8 @@ static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
>  	 * 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;
> @@ -5967,6 +5974,8 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  	 */
>  	if (reclaimable)
>  		pgdat->kswapd_failures = 0;
> +	else if (sc->cache_trim_mode)
> +		sc->cache_trim_mode_failed = 1;
>  }
>  
>  /*
> @@ -6898,6 +6907,16 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>  			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++;

Or 's/cache_trim_mode_failed/balancing_cleverness_failed' so that any
balancing cleverness can be surpressed when needed?

Even though I faced an issue by cache_trim_mode and I'm trying to resolve
it this time, but I'm still not sure if kswapd's reclaim is okay with
other cleverness(?) including SCAN_FRACT at its highest priority.

Just grumbling. You can ignore the second paragrph :(

	Byungchul