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

Byungchul Park
2024-03-05T04:33:21Z

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