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

Byungchul Park
2024-03-04T03:04:25Z

On Mon, Mar 04, 2024 at 10:53:11AM +0800, Huang, Ying wrote:
> 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").

No preference to me. But don't think it's better to use another of may_*
in scan_control as Johannes Weiner suggested?

> > /* 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 ?

I don't think so. cache_trim_mode has a chance to switch every
prepare_scan_control() like:

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;

So referring to the last value is not a good idea.

Byungchul

> > + 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