Re: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work

[Huang, Ying]

Johannes Weiner <hannes@xxxxxxxxxxx> writes:

> On Mon, Feb 26, 2024 at 02:06:30PM +0100, Michal Hocko wrote:
>> [CC Mel, Vlastimil and Johannes for awareness]
>> 
>> On Fri 23-02-24 14:44:07, Byungchul Park wrote:
>> > 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 05846e34bf02ac9b3e254324dc2d7afd97a025d9 Mon Sep 17 00:00:00 2001
>> > From: Byungchul Park <byungchul@xxxxxx>
>> > Date: Fri, 23 Feb 2024 13:47:16 +0900
>> > Subject: [PATCH v3] mm, vmscan: do not turn on cache_trim_mode if it doesn't work
>> > 
>> > 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 leads kswapd_failures to reach MAX_RECLAIM_RETRIES and
>> > stopping kswapd from functioning until direct reclaim eventually works
>> > to resume kswapd.
>> > 
>> > So do not turn on cache_trim_mode if the mode doesn't work, especially
>> > while the sytem is struggling against 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	   | 1636737	   |
>> >    | nr_active_anon	   | 1780700	   | 465913	   |
>> >    | nr_inactive_file	   | 30425	   | 35711	   |
>> >    | nr_active_file	   | 14961	   | 8698	   |
>> >    | pgpromote_success	   | 356	   | 1267785	   |
>> >    | pgpromote_candidate   | 21953245	   | 1745631	   |
>> >    | pgactivate		   | 1844523	   | 3309867	   |
>> >    | pgdeactivate	   | 50634	   | 1545041	   |
>> >    | pgfault		   | 31100294	   | 6411036	   |
>> >    | pgdemote_kswapd	   | 30856	   | 2267467	   |
>> >    | pgscan_kswapd	   | 1861981	   | 7729231	   |
>> >    | pgscan_anon	   | 1822930	   | 7667544	   |
>> >    | pgscan_file	   | 39051	   | 61687	   |
>> >    | pgsteal_anon	   | 386	   | 2227217	   |
>> >    | pgsteal_file	   | 30470	   | 40250	   |
>> >    | pageoutrun		   | 30		   | 457	   |
>> >    | numa_hint_faults	   | 27418279	   | 2752289	   |
>> >    | numa_pages_migrated   | 356	   | 1267785 	   |
>> >    +-----------------------+-------------------------------+
>> > 
>> > Signed-off-by: Byungchul Park <byungchul@xxxxxx>
>> > ---
>> >  mm/vmscan.c | 24 +++++++++++++++++++-----
>> >  1 file changed, 19 insertions(+), 5 deletions(-)
>> > 
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index bba207f41b14..f7312d831fed 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -127,6 +127,9 @@ struct scan_control {
>> >  	/* One of the zones is ready for compaction */
>> >  	unsigned int compaction_ready:1;
>> >  
>> > +	/* If the last try was reclaimable */
>> > +	unsigned int reclaimable:1;
>> > +
>> >  	/* There is easily reclaimable cold cache in the current node */
>> >  	unsigned int cache_trim_mode:1;
>> >  
>> > @@ -2266,9 +2269,14 @@ 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.
>> > +	 *
>> > +	 * It doesn't make sense to keep cache_trim_mode on if the mode
>> > +	 * is not working while struggling against reclaim. So do not
>> > +	 * turn it on if so. Note the highest priority of kswapd is 1.
>> >  	 */
>> >  	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->cache_trim_mode && !sc->reclaimable && sc->priority <= 1))
>> >  		sc->cache_trim_mode = 1;
>> >  	else
>> >  		sc->cache_trim_mode = 0;
>
> The overall goal makes sense to me.
>
> file >> priority is just a heuristic that there are enough potential
> candidate pages, not a guarantee that any forward progress will
> happen. So it makes sense to retry without before failing.
>
> The way you wrote this conditional kind of hurts my head,
> though. Please don't write negations of complex terms like this.
>
> It expands to this:
>
> 	!sc->cache_trim_mode || sc->reclaimable || sc->priority > 1
>
> which I'm not sure makes sense. Surely it should be something like
>
> 	!sc->cache_trim_mode && sc->reclaimable && sc->priority > 1
>
> instead?
>
> Also
>
> 	if (!sc->cache_trim_mode)
> 		sc->cache_trim_mode = 1
> 	else
> 		sc->cache_trim_mode = 0
>
> will toggle on every loop. So if direct reclaim runs through a
> zonelist, it'll cache trim every other numa node...?
>
>> > @@ -5862,7 +5870,6 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>> >  {
>> >  	unsigned long nr_reclaimed, nr_scanned, nr_node_reclaimed;
>> >  	struct lruvec *target_lruvec;
>> > -	bool reclaimable = false;
>> >  
>> >  	if (lru_gen_enabled() && root_reclaim(sc)) {
>> >  		lru_gen_shrink_node(pgdat, sc);
>> > @@ -5877,6 +5884,14 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>> >  	nr_reclaimed = sc->nr_reclaimed;
>> >  	nr_scanned = sc->nr_scanned;
>> >  
>> > +	/*
>> > +	 * Reset to the default values at the start.
>> > +	 */
>> > +	if (sc->priority == DEF_PRIORITY) {
>> > +		sc->reclaimable = 1;
>> > +		sc->cache_trim_mode = 0;
>> > +	}
>> > +
>> >  	prepare_scan_control(pgdat, sc);
>> >  
>> >  	shrink_node_memcgs(pgdat, sc);
>> > @@ -5890,8 +5905,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>> >  		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
>> >  			   sc->nr_scanned - nr_scanned, nr_node_reclaimed);
>> >  
>> > -	if (nr_node_reclaimed)
>> > -		reclaimable = true;
>> > +	sc->reclaimable = !!nr_node_reclaimed;
>
> The scope of this doesn't quite make sense. If direct reclaim scans
> multiple nodes, reclaim failure on the first node would disable cache
> trim mode on the second node, which is totally unrelated.
>
> I think it needs separate paths for direct reclaim and kswapd. For
> direct reclaim, the retry should be before these similar retry catches
> in do_try_to_free_pages(), after all zones have been considered:
>
> 	/*
> 	 * We make inactive:active ratio decisions based on the node's
> 	 * composition of memory, but a restrictive reclaim_idx or a
> 	 * memory.low cgroup setting can exempt large amounts of
> 	 * memory from reclaim. Neither of which are very common, so
> 	 * instead of doing costly eligibility calculations of the
> 	 * entire cgroup subtree up front, we assume the estimates are
> 	 * good, and retry with forcible deactivation if that fails.
> 	 */
> 	if (sc->skipped_deactivate) {
> 		sc->priority = initial_priority;
> 		sc->force_deactivate = 1;
> 		sc->skipped_deactivate = 0;
> 		goto retry;
> 	}
>
> 	/* Untapped cgroup reserves?  Don't OOM, retry. */
> 	if (sc->memcg_low_skipped) {
> 		sc->priority = initial_priority;
> 		sc->force_deactivate = 0;
> 		sc->memcg_low_reclaim = 1;
> 		sc->memcg_low_skipped = 0;
> 		goto retry;
> 	}

In get_scan_count(), we have

	if (!sc->priority && swappiness) {
		scan_balance = SCAN_EQUAL;
		goto out;
	}

So, we don't really need the heuristics in direct reclaim path.  So, one
choice is to constrain this in kswapd reclaim only.

--
Best Regards,
Huang, Ying


> and for kswapd it looks like it should be in balance_pgdat(), after
> the priority loop, before increasing kswapd_failures.
>
> Instead of sc->reclaimable, which is very broad, it would be better to
> call it sc->may_cache_trim_mode. This is in line with a bunch of other
> such mechanisms in scan_control.