Re: [PATCH v5] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

Huang, Ying
2024-03-04T02:55:13Z

Byungchul Park <byungchul@sk.com> writes:

> Sorry for noise. I should've applied v5's change in v4.
>
> 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 58f1a0e41b9feea72d7fd4bd7bed1ace592e6e4c Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Mon, 4 Mar 2024 11:24:40 +0900
> Subject: [PATCH v5] 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 | 1646193 |
> | nr_active_anon | 1780700 | 456388 |
> | nr_inactive_file | 30425 | 27836 |
> | nr_active_file | 14961 | 1217 |
> | pgpromote_success | 356 | 1310120 |
> | pgpromote_candidate | 21953245 | 1736872 |
> | pgactivate | 1844523 | 3292443 |
> | pgdeactivate | 50634 | 1526701 |
> | pgfault | 31100294 | 6715375 |
> | pgdemote_kswapd | 30856 | 1954199 |
> | pgscan_kswapd | 1861981 | 7100099 |
> | pgscan_anon | 1822930 | 7061135 |
> | pgscan_file | 39051 | 38964 |
> | pgsteal_anon | 386 | 1925214 |
> | pgsteal_file | 30470 | 28985 |
> | pageoutrun | 30 | 500 |
> | numa_hint_faults | 27418279 | 3090773 |
> | numa_pages_migrated | 356 | 1310120 |
> +-----------------------+-------------------------------+
>
> Signed-off-by: Byungchul Park <byungchul@sk.com>
> ---
> mm/vmscan.c | 23 +++++++++++++++++++++--
> 1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bba207f41b14..77948b0f8b5b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -108,6 +108,9 @@ struct scan_control {
> /* Can folios be swapped as part of reclaim? */
> unsigned int may_swap:1;
>
> + /* Can cache_trim_mode be turned on as part of reclaim? */
> + unsigned int may_cache_trim_mode:1;
> +

Although it's generally not good to use negative logic, I think that
it's better to name the flag as something like "no_cache_trim_mode" to
make it easier to initialize the flag to its default value ("0").

> /* Proactive reclaim invoked by userspace through memory.reclaim */
> unsigned int proactive:1;
>
> @@ -1500,6 +1503,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .may_unmap = 1,
> + .may_cache_trim_mode = 1,
> };
> struct reclaim_stat stat;
> unsigned int nr_reclaimed;
> @@ -2094,6 +2098,7 @@ static unsigned int reclaim_folio_list(struct list_head *folio_list,
> .may_writepage = 1,
> .may_unmap = 1,
> .may_swap = 1,
> + .may_cache_trim_mode = 1,
> .no_demotion = 1,
> };
>
> @@ -2268,7 +2273,8 @@ 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->may_cache_trim_mode)
> sc->cache_trim_mode = 1;
> else
> sc->cache_trim_mode = 0;
> @@ -5435,6 +5441,7 @@ static ssize_t lru_gen_seq_write(struct file *file, const char __user *src,
> .may_writepage = true,
> .may_unmap = true,
> .may_swap = true,
> + .may_cache_trim_mode = 1,
> .reclaim_idx = MAX_NR_ZONES - 1,
> .gfp_mask = GFP_KERNEL,
> };
> @@ -6394,6 +6401,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> .may_writepage = !laptop_mode,
> .may_unmap = 1,
> .may_swap = 1,
> + .may_cache_trim_mode = 1,
> };
>
> /*
> @@ -6439,6 +6447,7 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
> .may_unmap = 1,
> .reclaim_idx = MAX_NR_ZONES - 1,
> .may_swap = !noswap,
> + .may_cache_trim_mode = 1,
> };
>
> WARN_ON_ONCE(!current->reclaim_state);
> @@ -6482,6 +6491,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> .may_writepage = !laptop_mode,
> .may_unmap = 1,
> .may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
> + .may_cache_trim_mode = 1,
> .proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
> };
> /*
> @@ -6744,6 +6754,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
> .gfp_mask = GFP_KERNEL,
> .order = order,
> .may_unmap = 1,
> + .may_cache_trim_mode = 1,
> };
>
> set_task_reclaim_state(current, &sc.reclaim_state);
> @@ -6898,8 +6909,14 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
> sc.priority--;
> } while (sc.priority >= 1);
>
> - if (!sc.nr_reclaimed)
> + if (!sc.nr_reclaimed) {
> + if (sc.may_cache_trim_mode) {

sc.may_cache_trim_mode && cache_trim_mode ?

> + sc.may_cache_trim_mode = 0;
> + goto restart;
> + }
> +
> pgdat->kswapd_failures++;
> + }
>
> out:
> clear_reclaim_active(pgdat, highest_zoneidx);
> @@ -7202,6 +7219,7 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
> .may_writepage = 1,
> .may_unmap = 1,
> .may_swap = 1,
> + .may_cache_trim_mode = 1,
> .hibernation_mode = 1,
> };
> struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
> @@ -7360,6 +7378,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> .may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
> .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
> .may_swap = 1,
> + .may_cache_trim_mode = 1,
> .reclaim_idx = gfp_zone(gfp_mask),
> };
> unsigned long pflags;

--
Best Regards,
Huang, Ying