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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 07e0baab368160e50b6ca35d95745168aa60e217 Mon Sep 17 00:00:00 2001 > >> > > > From: Byungchul Park > >> > > > Date: Thu, 22 Feb 2024 14:50:17 +0900 > >> > > > Subject: [PATCH v2] mm, vmscan: don't turn on cache_trim_mode at high scan priorities > >> > > > > >> > > > 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 can lead kswapd_failures to reach MAX_RECLAIM_RETRIES and > >> > > > stopping kswapd until direct reclaim eventually works to resume kswapd. > >> > > > So this is more like a bug fix than a performance improvement. > >> > > > > >> > > > 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 kswapd stops. > >> > > > > >> > > > With this, you could eventually 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: > >> > > > > >> > > > -nr_inactive_anon 321935 > >> > > > -nr_active_anon 1780700 > >> > > > -nr_inactive_file 30425 > >> > > > -nr_active_file 14961 > >> > > > -pgpromote_success 356 > >> > > > -pgpromote_candidate 21953245 > >> > > > -pgactivate 1844523 > >> > > > -pgdeactivate 50634 > >> > > > -pgfault 31100294 > >> > > > -pgdemote_kswapd 30856 > >> > > > -pgscan_kswapd 1861981 > >> > > > -pgscan_anon 1822930 > >> > > > -pgscan_file 39051 > >> > > > -pgsteal_anon 386 > >> > > > -pgsteal_file 30470 > >> > > > -pageoutrun 30 > >> > > > -numa_hint_faults 27418279 > >> > > > -numa_pages_migrated 356 > >> > > > > >> > > > +nr_inactive_anon 1662306 > >> > > > +nr_active_anon 440303 > >> > > > +nr_inactive_file 27669 > >> > > > +nr_active_file 1654 > >> > > > +pgpromote_success 1314102 > >> > > > +pgpromote_candidate 1892525 > >> > > > +pgactivate 3284457 > >> > > > +pgdeactivate 1527504 > >> > > > +pgfault 6847775 > >> > > > +pgdemote_kswapd 2142047 > >> > > > +pgscan_kswapd 7496588 > >> > > > +pgscan_anon 7462488 > >> > > > +pgscan_file 34100 > >> > > > +pgsteal_anon 2115661 > >> > > > +pgsteal_file 26386 > >> > > > +pageoutrun 378 > >> > > > +numa_hint_faults 3220891 > >> > > > +numa_pages_migrated 1314102 > >> > > > > >> > > > where -: before this patch, +: after this patch > >> > > > > >> > > > Signed-off-by: Byungchul Park > >> > > > --- > >> > > > mm/vmscan.c | 10 +++++++++- > >> > > > 1 file changed, 9 insertions(+), 1 deletion(-) > >> > > > > >> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > >> > > > index bba207f41b14..6eda59fce5ee 100644 > >> > > > --- a/mm/vmscan.c > >> > > > +++ b/mm/vmscan.c > >> > > > @@ -2266,9 +2266,17 @@ 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. > >> > > > + * > >> > > > + * However, the condition 'sc->cache_trim_mode == 1' all through > >> > > > + * the scan priorties might lead reclaim failure. If it keeps > >> > > > + * MAX_RECLAIM_RETRIES times, then kswapd would get stopped even > >> > > > + * if there are still plenty anon pages to reclaim, which is not > >> > > > + * desirable. So do not use cache_trim_mode when reclaim is not > >> > > > + * smooth e.i. high scan priority. > >> > > > */ > >> > > > file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE); > >> > > > - if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE)) > >> > > > + if (sc->priority > 1 && file >> sc->priority && > >> > > > + !(sc->may_deactivate & DEACTIVATE_FILE)) > >> > > > sc->cache_trim_mode = 1; > >> > > > else > >> > > > sc->cache_trim_mode = 0; > >> > > > >> > > In get_scan_count(), there's following code, > >> > > > >> > > /* > >> > > * Do not apply any pressure balancing cleverness when the > >> > > * system is close to OOM, scan both anon and file equally > >> > > * (unless the swappiness setting disagrees with swapping). > >> > > */ > >> > > if (!sc->priority && swappiness) { > >> > > scan_balance = SCAN_EQUAL; > >> > > goto out; > >> > > } > >> > > > >> > > So, swappiness is 0 in you system? Please check it. If it's not 0, > >> > > please check why this doesn't help. > >> > > >> > Nice information! Then the change should be: > >> > > >> > diff --git a/mm/vmscan.c b/mm/vmscan.c > >> > index bba207f41b14..91f9bab86e92 100644 > >> > --- a/mm/vmscan.c > >> > +++ b/mm/vmscan.c > >> > @@ -2357,7 +2357,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc, > >> > * system is close to OOM, scan both anon and file equally > >> > * (unless the swappiness setting disagrees with swapping). > >> > */ > >> > - if (!sc->priority && swappiness) { > >> > + if (sc->priority <= 1 && swappiness) { > >> > scan_balance = SCAN_EQUAL; > >> > goto out; > >> > } > >> > >> Or: > >> > >> diff --git a/mm/vmscan.c b/mm/vmscan.c > >> index bba207f41b14..c54371a398b1 100644 > >> --- a/mm/vmscan.c > >> +++ b/mm/vmscan.c > >> @@ -6896,7 +6896,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx) > >> > >> if (raise_priority || !nr_reclaimed) > >> sc.priority--; > >> - } while (sc.priority >= 1); > >> + } while (sc.priority >= 0); > >> > >> if (!sc.nr_reclaimed) > >> pgdat->kswapd_failures++; > > > > +cc Mel Gorman > > > > I just found this was intended. See commit 9aa41348a8d11 ("mm: vmscan: > > do not allow kswapd to scan at maximum priority"). Mel Gorman didn't want > > to make kswapd too much aggressive. However, does it make sense to stop > > kswapd even if there are plenty cold anon pages to reclaim and make the > > system wait for direct reclaim? > > Maybe we can play with cache_trim_mode, for example, if sc.nr_reclaimed > == 0 and sc.cache_trim_mode == true, force disabling cache_trim_mode in > the next round? Looks reasonable to me. I will try. Byungchul > -- > Best Regards, > Huang, Ying > > > Thoughts? > > > > Byungchul > > > >> --- > >> > >> Byungchul > >> > >> > Worth noting that the priority goes from DEF_PRIORITY to 1 in > >> > balance_pgdat() of kswapd. I will change how to fix to this if this > >> > looks more reasonable. > >> > > >> > Byungchul