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

Huang, Ying
2024-03-04T03:31:09Z

Byungchul Park <byungchul@sk.com> writes:

> 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.

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.

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

--
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