Re: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work

From: Byungchul Park
Date: Thu Feb 29 2024 - 03:40:45 EST

On Thu, Feb 29, 2024 at 02:21:15PM +0800, Huang, Ying wrote:
> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
>
> > On Mon, Feb 26, 2024 at 02:06:30PM +0100, Michal Hocko wrote:
> >> [CC Mel, Vlastimil and Johannes for awareness]
> >>
> >> On Fri 23-02-24 14:44:07, Byungchul Park wrote:
> >> > 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 05846e34bf02ac9b3e254324dc2d7afd97a025d9 Mon Sep 17 00:00:00 2001
> >> > From: Byungchul Park <byungchul@xxxxxx>
> >> > Date: Fri, 23 Feb 2024 13:47:16 +0900
> >> > Subject: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work
> >> >
> >> > With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
> >> > pages. However, it should be more careful to turn on 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 do not turn on cache_trim_mode if the mode doesn't work, especially
> >> > while the sytem is struggling against 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 | 1636737 |
> >> > | nr_active_anon | 1780700 | 465913 |
> >> > | nr_inactive_file | 30425 | 35711 |
> >> > | nr_active_file | 14961 | 8698 |
> >> > | pgpromote_success | 356 | 1267785 |
> >> > | pgpromote_candidate | 21953245 | 1745631 |
> >> > | pgactivate | 1844523 | 3309867 |
> >> > | pgdeactivate | 50634 | 1545041 |
> >> > | pgfault | 31100294 | 6411036 |
> >> > | pgdemote_kswapd | 30856 | 2267467 |
> >> > | pgscan_kswapd | 1861981 | 7729231 |
> >> > | pgscan_anon | 1822930 | 7667544 |
> >> > | pgscan_file | 39051 | 61687 |
> >> > | pgsteal_anon | 386 | 2227217 |
> >> > | pgsteal_file | 30470 | 40250 |
> >> > | pageoutrun | 30 | 457 |
> >> > | numa_hint_faults | 27418279 | 2752289 |
> >> > | numa_pages_migrated | 356 | 1267785 |
> >> > +-----------------------+-------------------------------+
> >> >
> >> > Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> >> > ---
> >> > mm/vmscan.c | 24 +++++++++++++++++++-----
> >> > 1 file changed, 19 insertions(+), 5 deletions(-)
> >> >
> >> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> > index bba207f41b14..f7312d831fed 100644
> >> > --- a/mm/vmscan.c
> >> > +++ b/mm/vmscan.c
> >> > @@ -127,6 +127,9 @@ struct scan_control {
> >> > /* One of the zones is ready for compaction */
> >> > unsigned int compaction_ready:1;
> >> >
> >> > + /* If the last try was reclaimable */
> >> > + unsigned int reclaimable:1;
> >> > +
> >> > /* There is easily reclaimable cold cache in the current node */
> >> > unsigned int cache_trim_mode:1;
> >> >
> >> > @@ -2266,9 +2269,14 @@ static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
> >> > * If we have plenty of inactive file pages that aren't
> >> > * thrashing, try to reclaim those first before touching
> >> > * anonymous pages.
> >> > + *
> >> > + * It doesn't make sense to keep cache_trim_mode on if the mode
> >> > + * is not working while struggling against reclaim. So do not
> >> > + * turn it on if so. Note the highest priority of kswapd is 1.
> >> > */
> >> > 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->cache_trim_mode && !sc->reclaimable && sc->priority <= 1))
> >> > sc->cache_trim_mode = 1;
> >> > else
> >> > sc->cache_trim_mode = 0;
> >
> > The overall goal makes sense to me.
> >
> > file >> priority is just a heuristic that there are enough potential
> > candidate pages, not a guarantee that any forward progress will
> > happen. So it makes sense to retry without before failing.
> >
> > The way you wrote this conditional kind of hurts my head,
> > though. Please don't write negations of complex terms like this.
> >
> > It expands to this:
> >
> > !sc->cache_trim_mode || sc->reclaimable || sc->priority > 1
> >
> > which I'm not sure makes sense. Surely it should be something like
> >
> > !sc->cache_trim_mode && sc->reclaimable && sc->priority > 1
> >
> > instead?
> >
> > Also
> >
> > if (!sc->cache_trim_mode)
> > sc->cache_trim_mode = 1
> > else
> > sc->cache_trim_mode = 0
> >
> > will toggle on every loop. So if direct reclaim runs through a
> > zonelist, it'll cache trim every other numa node...?
> >
> >> > @@ -5862,7 +5870,6 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >> > {
> >> > unsigned long nr_reclaimed, nr_scanned, nr_node_reclaimed;
> >> > struct lruvec *target_lruvec;
> >> > - bool reclaimable = false;
> >> >
> >> > if (lru_gen_enabled() && root_reclaim(sc)) {
> >> > lru_gen_shrink_node(pgdat, sc);
> >> > @@ -5877,6 +5884,14 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >> > nr_reclaimed = sc->nr_reclaimed;
> >> > nr_scanned = sc->nr_scanned;
> >> >
> >> > + /*
> >> > + * Reset to the default values at the start.
> >> > + */
> >> > + if (sc->priority == DEF_PRIORITY) {
> >> > + sc->reclaimable = 1;
> >> > + sc->cache_trim_mode = 0;
> >> > + }
> >> > +
> >> > prepare_scan_control(pgdat, sc);
> >> >
> >> > shrink_node_memcgs(pgdat, sc);
> >> > @@ -5890,8 +5905,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >> > vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
> >> > sc->nr_scanned - nr_scanned, nr_node_reclaimed);
> >> >
> >> > - if (nr_node_reclaimed)
> >> > - reclaimable = true;
> >> > + sc->reclaimable = !!nr_node_reclaimed;
> >
> > The scope of this doesn't quite make sense. If direct reclaim scans
> > multiple nodes, reclaim failure on the first node would disable cache
> > trim mode on the second node, which is totally unrelated.
> >
> > I think it needs separate paths for direct reclaim and kswapd. For
> > direct reclaim, the retry should be before these similar retry catches
> > in do_try_to_free_pages(), after all zones have been considered:
> >
> > /*
> > * We make inactive:active ratio decisions based on the node's
> > * composition of memory, but a restrictive reclaim_idx or a
> > * memory.low cgroup setting can exempt large amounts of
> > * memory from reclaim. Neither of which are very common, so
> > * instead of doing costly eligibility calculations of the
> > * entire cgroup subtree up front, we assume the estimates are
> > * good, and retry with forcible deactivation if that fails.
> > */
> > if (sc->skipped_deactivate) {
> > sc->priority = initial_priority;
> > sc->force_deactivate = 1;
> > sc->skipped_deactivate = 0;
> > goto retry;
> > }
> >
> > /* Untapped cgroup reserves? Don't OOM, retry. */
> > if (sc->memcg_low_skipped) {
> > sc->priority = initial_priority;
> > sc->force_deactivate = 0;
> > sc->memcg_low_reclaim = 1;
> > sc->memcg_low_skipped = 0;
> > goto retry;
> > }
>
> In get_scan_count(), we have
>
> if (!sc->priority && swappiness) {
> scan_balance = SCAN_EQUAL;
> goto out;
> }

Even though this can mitigate the issue for direct reclaim, it's still
suffering from the problem while direct reclaim goes from DEF_PRIORITY
to 1. What we need is not a mitigation but making things right.

Thoughts?

Byungchul

> So, we don't really need the heuristics in direct reclaim path. So, one
> choice is to constrain this in kswapd reclaim only.
>
> --
> Best Regards,
> Huang, Ying
>
>
> > and for kswapd it looks like it should be in balance_pgdat(), after
> > the priority loop, before increasing kswapd_failures.
> >
> > Instead of sc->reclaimable, which is very broad, it would be better to
> > call it sc->may_cache_trim_mode. This is in line with a bunch of other
> > such mechanisms in scan_control.