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Make it retry the kswapd's scan priority loop with > >> >> >> > > > cache_trim_mode off *only if* the mode didn't work in the > >> >> >> > > > previous loop. (feedbacked by Huang Ying) > >> >> >> > > > 2. Take into account 'break's from the priority loop when making > >> >> >> > > > the decision whether to retry. (feedbacked by Huang Ying) > >> >> >> > > > 3. Update the test result in the commit message. > >> >> >> > > > > >> >> >> > > > 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 f811ee583158fd53d0e94d32ce5948fac4b17cfe Mon Sep 17 00:00:00 2001 > >> >> >> > > > From: Byungchul Park > >> >> >> > > > Date: Mon, 4 Mar 2024 15:27:37 +0900 > >> >> >> > > > Subject: [PATCH v6] 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 | 1664772 | > >> >> >> > > > | nr_active_anon | 1780700 | 437834 | > >> >> >> > > > | nr_inactive_file | 30425 | 40882 | > >> >> >> > > > | nr_active_file | 14961 | 3012 | > >> >> >> > > > | pgpromote_success | 356 | 1293122 | > >> >> >> > > > | pgpromote_candidate | 21953245 | 1824148 | > >> >> >> > > > | pgactivate | 1844523 | 3311907 | > >> >> >> > > > | pgdeactivate | 50634 | 1554069 | > >> >> >> > > > | pgfault | 31100294 | 6518806 | > >> >> >> > > > | pgdemote_kswapd | 30856 | 2230821 | > >> >> >> > > > | pgscan_kswapd | 1861981 | 7667629 | > >> >> >> > > > | pgscan_anon | 1822930 | 7610583 | > >> >> >> > > > | pgscan_file | 39051 | 57046 | > >> >> >> > > > | pgsteal_anon | 386 | 2192033 | > >> >> >> > > > | pgsteal_file | 30470 | 38788 | > >> >> >> > > > | pageoutrun | 30 | 412 | > >> >> >> > > > | numa_hint_faults | 27418279 | 2875955 | > >> >> >> > > > | numa_pages_migrated | 356 | 1293122 | > >> >> >> > > > +-----------------------+-------------------------------+ > >> >> >> > > > > >> >> >> > > > Signed-off-by: Byungchul Park > >> >> >> > > > --- > >> >> >> > > > mm/vmscan.c | 21 ++++++++++++++++++++- > >> >> >> > > > 1 file changed, 20 insertions(+), 1 deletion(-) > >> >> >> > > > > >> >> >> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > >> >> >> > > > index bba207f41b14..6fe45eca7766 100644 > >> >> >> > > > --- a/mm/vmscan.c > >> >> >> > > > +++ b/mm/vmscan.c > >> >> >> > > > @@ -108,6 +108,12 @@ struct scan_control { > >> >> >> > > > /* Can folios be swapped as part of reclaim? */ > >> >> >> > > > unsigned int may_swap:1; > >> >> >> > > > > >> >> >> > > > + /* Not allow cache_trim_mode to be turned on as part of reclaim? */ > >> >> >> > > > + unsigned int no_cache_trim_mode:1; > >> >> >> > > > + > >> >> >> > > > + /* Has cache_trim_mode failed at least once? */ > >> >> >> > > > + unsigned int cache_trim_mode_failed:1; > >> >> >> > > > + > >> >> >> > > > /* Proactive reclaim invoked by userspace through memory.reclaim */ > >> >> >> > > > unsigned int proactive:1; > >> >> >> > > > > >> >> >> > > > @@ -2268,7 +2274,8 @@ 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->no_cache_trim_mode) > >> >> >> > > > sc->cache_trim_mode = 1; > >> >> >> > > > else > >> >> >> > > > sc->cache_trim_mode = 0; > >> >> >> > > > @@ -5967,6 +5974,8 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc) > >> >> >> > > > */ > >> >> >> > > > if (reclaimable) > >> >> >> > > > pgdat->kswapd_failures = 0; > >> >> >> > > > + else if (sc->cache_trim_mode) > >> >> >> > > > + sc->cache_trim_mode_failed = 1; > >> >> >> > > > } > >> >> >> > > > > >> >> >> > > > /* > >> >> >> > > > @@ -6898,6 +6907,16 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx) > >> >> >> > > > sc.priority--; > >> >> >> > > > } while (sc.priority >= 1); > >> >> >> > > > > >> >> >> > > > + /* > >> >> >> > > > + * Restart only if it went through the priority loop all the way, > >> >> >> > > > + * but cache_trim_mode didn't work. > >> >> >> > > > + */ > >> >> >> > > > + if (!sc.nr_reclaimed && sc.priority < 1 && > >> >> >> > > > + !sc.no_cache_trim_mode && sc.cache_trim_mode_failed) { > >> >> >> > > > >> >> >> > > Can we just use sc.cache_trim_mode (instead of > >> >> >> > > sc.cache_trim_mode_failed) here? That is, if cache_trim_mode is enabled > >> >> >> > > >> >> >> > As Johannes mentioned, within a priority scan, all the numa nodes are > >> >> >> > scanned each with its own value of cache_trim_mode. So we cannot use > >> >> >> > cache_trim_mode for that purpose. > >> >> >> > >> >> >> Ah, okay. Confining to kswapd, that might make sense. I will apply it if > >> >> >> there's no objection to it. Thanks. > >> >> > > >> >> > I didn't want to introduce two additional flags either, but it was > >> >> > possible to make it do exactly what we want it to do thanks to the flags. > >> >> > I'd like to keep this version if possible unless there are any other > >> >> > objections on it. > >> >> > >> >> Sorry, I'm confused. Whether does "cache_trim_mode == 1" do the trick? > >> >> If so, why not? If not, why? > >> > > >> > kswapd might happen to go through: > >> > > >> > priority 12(== DEF_PRIORITY) + cache_trim_mode on -> fail > >> > priority 11 + cache_trim_mode on -> fail > >> > priority 10 + cache_trim_mode on -> fail > >> > priority 9 + cache_trim_mode on -> fail > >> > priority 8 + cache_trim_mode on -> fail > >> > priority 7 + cache_trim_mode on -> fail > >> > priority 6 + cache_trim_mode on -> fail > >> > priority 5 + cache_trim_mode on -> fail > >> > priority 4 + cache_trim_mode on -> fail > >> > priority 3 + cache_trim_mode on -> fail > >> > priority 2 + cache_trim_mode on -> fail > >> > priority 1 + cache_trim_mode off -> fail > >> > > >> > I'd like to retry even in this case. > >> > >> I don't think that we should retry in this case. If the following case > >> fails, > >> > >> > priority 1 + cache_trim_mode off -> fail > >> > >> Why will we succeed after retrying? > > > > At priority 1, anon pages will be partially scanned. However, there > > might be anon pages that have never been scanned but can be reclaimed. > > > > Do I get it wrong? > > Yes. In theory, that's possible. But do you think that will be some > practical issue? So that, pgdat->kswapd_failures will reach max value? v6 is based on what Johannes suggested. I thought it's more right way to fix the issue. Yeah, I also think checking cache_trim_mode only at the kswapd's highest priorty would manage to work for the issue. I'd like to listen to Johannes's opinion or others. Byungchul > -- > Best Regards, > Huang, Ying > > > Byungchul > > > >> -- > >> Best Regards, > >> Huang, Ying > >> > >> > Am I missing something? > >> > > >> > Byungchul > >> > > >> >> -- > >> >> Best Regards, > >> >> Huang, Ying > >> >> > >> >> > Byungchul > >> >> > > >> >> >> Byungchul > >> >> >> > > >> >> >> > Byungchul > >> >> >> > > >> >> >> > > for priority == 1 and failed to reclaim, we will restart. If this > >> >> >> > > works, we can avoid to add another flag. > >> >> >> > > > >> >> >> > > > + sc.no_cache_trim_mode = 1; > >> >> >> > > > + goto restart; > >> >> >> > > > + } > >> >> >> > > > + > >> >> >> > > > if (!sc.nr_reclaimed) > >> >> >> > > > pgdat->kswapd_failures++; > >> >> >> > > > >> >> >> > > -- > >> >> >> > > Best Regards, > >> >> >> > > Huang, Ying