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Message-ID: <20240223093441.GA1612@system.software.com>
Date: Fri, 23 Feb 2024 18:34:41 +0900
From: Byungchul Park <byungchul@...com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, kernel_team@...ynix.com, yuzhao@...gle.com
Subject: Re: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it
doesn't work
On Fri, Feb 23, 2024 at 05:25:54PM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@...com> writes:
>
> > 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@...com>
> > 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@...com>
> > ---
> > 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))
>
> It's a little hard to digest the logic above for me, is it better to use
> the following logic?
>
> if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE) &&
> (!sc->cache_trim_mode || sc->reclaimable || sc->priority > 1))
Looks fine to me. Thanks.
Byungchul
> Otherwise, this looks good to me, feel free to add,
>
> Acked-by: "Huang, Ying" <ying.huang@...el.com>
>
> > sc->cache_trim_mode = 1;
> > else
> > sc->cache_trim_mode = 0;
> > @@ -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;
> >
> > if (current_is_kswapd()) {
> > /*
> > @@ -5965,7 +5979,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> > * sleep. On reclaim progress, reset the failure counter. A
> > * successful direct reclaim run will revive a dormant kswapd.
> > */
> > - if (reclaimable)
> > + if (sc->reclaimable)
> > pgdat->kswapd_failures = 0;
> > }
>
> --
> Best Regards,
> Huang, Ying
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