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Date: Thu, 22 Feb 2024 18:49:00 +0900
From: Byungchul Park <byungchul@sk.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, kernel_team@skhynix.com, yuzhao@google.com
Subject: Re: [PATCH v2] mm, vmscan: don't turn on cache_trim_mode at high
 scan priorities
Message-ID: <20240222094900.GA13076@system.software.com>
References: <20240222070817.70515-1-byungchul@sk.com>
 <87sf1kj3nn.fsf@yhuang6-desk2.ccr.corp.intel.com>
 <20240222092042.GA33967@system.software.com>
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On Thu, Feb 22, 2024 at 06:20:42PM +0900, Byungchul Park wrote:
> On Thu, Feb 22, 2024 at 04:37:16PM +0800, Huang, Ying wrote:
> > Byungchul Park <byungchul@sk.com> writes:
> > 
> > > 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 07e0baab368160e50b6ca35d95745168aa60e217 Mon Sep 17 00:00:00 2001
> > > From: Byungchul Park <byungchul@sk.com>
> > > 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 <byungchul@sk.com>
> > > ---
> > >  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++;
---

	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