Re: [PATCH v6] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

[Byungchul Park]

On Tue, Mar 05, 2024 at 09:54:19AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@xxxxxx> writes:
> 
> > Changes from v5:
> > 	1. 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 <byungchul@xxxxxx>
> > 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 <byungchul@xxxxxx>
> > ---
> >  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.

	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