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

[Huang, Ying]

Byungchul Park <byungchul@xxxxxx> writes:

> On Tue, Mar 05, 2024 at 03:04:48PM +0800, Huang, Ying wrote:
>> Byungchul Park <byungchul@xxxxxx> writes:
>> 
>> > On Tue, Mar 05, 2024 at 02:18:33PM +0800, Huang, Ying wrote:
>> >> Byungchul Park <byungchul@xxxxxx> writes:
>> >> 
>> >> > On Tue, Mar 05, 2024 at 11:43:45AM +0900, Byungchul Park wrote:
>> >> >> On Tue, Mar 05, 2024 at 11:37:08AM +0900, Byungchul Park wrote:
>> >> >> > 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.
>> >> >> 
>> >> >> 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?

--
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