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

["Huang, Ying" <ying.huang@intel.com>]

Byungchul Park <byungchul@sk.com> writes:

> On Tue, Mar 05, 2024 at 09:54:19AM +0800, Huang, Ying wrote:
>> Byungchul Park <byungchul@sk.com> 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@sk.com>
>> > 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@sk.com>
>> > ---
>> >  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.

For direct reclaim, this is true.  But, balance_pgdat() works for one
node only.

--
Best Regards,
Huang, Ying

> 	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