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

From: Byungchul Park
Date: Sun Mar 03 2024 - 22:36:33 EST

On Mon, Mar 04, 2024 at 11:29:06AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@xxxxxx> writes:
>
> > On Mon, Mar 04, 2024 at 10:53:11AM +0800, Huang, Ying wrote:
> >> Byungchul Park <byungchul@xxxxxx> 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@xxxxxx>
> >> > 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@xxxxxx>
> >> > ---
> >> > 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.
>
> We should only restart without cache_trim_mode if cache_trim_mode causes
> issue. If it isn't enabled with highest priority (lowest value), it
> doesn't help to disable cache_trim_mode.

Yes, right. Lemme think it more and apply the consideration.

> And, please take care of other "break" in the loop, for example, if
> kthread_should_stop(), etc.

I will. Thank you.

Byungchul

> --
> Best Regards,
> Huang, Ying
>
> > 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