Re: [PATCH v5] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

[Huang, Ying]

Byungchul Park <byungchul@xxxxxx> writes:

> On Mon, Mar 04, 2024 at 10:53:11AM +0800, Huang, Ying wrote:
>> Byungchul Park <byungchul@xxxxxx> writes:
>> 
>> > Sorry for noise. I should've applied v5's change in v4.
>> >
>> > 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 58f1a0e41b9feea72d7fd4bd7bed1ace592e6e4c Mon Sep 17 00:00:00 2001
>> > From: Byungchul Park <byungchul@xxxxxx>
>> > Date: Mon, 4 Mar 2024 11:24:40 +0900
>> > Subject: [PATCH v5] 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        | 1646193       |
>> >    | nr_active_anon        | 1780700       | 456388        |
>> >    | nr_inactive_file      | 30425         | 27836         |
>> >    | nr_active_file        | 14961         | 1217          |
>> >    | pgpromote_success     | 356           | 1310120       |
>> >    | pgpromote_candidate   | 21953245      | 1736872       |
>> >    | pgactivate            | 1844523       | 3292443       |
>> >    | pgdeactivate          | 50634         | 1526701       |
>> >    | pgfault               | 31100294      | 6715375       |
>> >    | pgdemote_kswapd       | 30856         | 1954199       |
>> >    | pgscan_kswapd         | 1861981       | 7100099       |
>> >    | pgscan_anon           | 1822930       | 7061135       |
>> >    | pgscan_file           | 39051         | 38964         |
>> >    | pgsteal_anon          | 386           | 1925214       |
>> >    | pgsteal_file          | 30470         | 28985         |
>> >    | pageoutrun            | 30            | 500           |
>> >    | numa_hint_faults      | 27418279      | 3090773       |
>> >    | numa_pages_migrated   | 356           | 1310120       |
>> >    +-----------------------+-------------------------------+
>> >
>> > Signed-off-by: Byungchul Park <byungchul@xxxxxx>
>> > ---
>> >  mm/vmscan.c | 23 +++++++++++++++++++++--
>> >  1 file changed, 21 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index bba207f41b14..77948b0f8b5b 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -108,6 +108,9 @@ struct scan_control {
>> >  	/* Can folios be swapped as part of reclaim? */
>> >  	unsigned int may_swap:1;
>> >  
>> > +	/* Can cache_trim_mode be turned on as part of reclaim? */
>> > +	unsigned int may_cache_trim_mode:1;
>> > +
>> 
>> Although it's generally not good to use negative logic, I think that
>> it's better to name the flag as something like "no_cache_trim_mode" to
>> make it easier to initialize the flag to its default value ("0").
>
> No preference to me. But don't think it's better to use another of may_*
> in scan_control as Johannes Weiner suggested?
>
>> >  	/* Proactive reclaim invoked by userspace through memory.reclaim */
>> >  	unsigned int proactive:1;
>> >  
>> > @@ -1500,6 +1503,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>> >  	struct scan_control sc = {
>> >  		.gfp_mask = GFP_KERNEL,
>> >  		.may_unmap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  	};
>> >  	struct reclaim_stat stat;
>> >  	unsigned int nr_reclaimed;
>> > @@ -2094,6 +2098,7 @@ static unsigned int reclaim_folio_list(struct list_head *folio_list,
>> >  		.may_writepage = 1,
>> >  		.may_unmap = 1,
>> >  		.may_swap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  		.no_demotion = 1,
>> >  	};
>> >  
>> > @@ -2268,7 +2273,8 @@ 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->may_cache_trim_mode)
>> >  		sc->cache_trim_mode = 1;
>> >  	else
>> >  		sc->cache_trim_mode = 0;
>> > @@ -5435,6 +5441,7 @@ static ssize_t lru_gen_seq_write(struct file *file, const char __user *src,
>> >  		.may_writepage = true,
>> >  		.may_unmap = true,
>> >  		.may_swap = true,
>> > +		.may_cache_trim_mode = 1,
>> >  		.reclaim_idx = MAX_NR_ZONES - 1,
>> >  		.gfp_mask = GFP_KERNEL,
>> >  	};
>> > @@ -6394,6 +6401,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>> >  		.may_writepage = !laptop_mode,
>> >  		.may_unmap = 1,
>> >  		.may_swap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  	};
>> >  
>> >  	/*
>> > @@ -6439,6 +6447,7 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
>> >  		.may_unmap = 1,
>> >  		.reclaim_idx = MAX_NR_ZONES - 1,
>> >  		.may_swap = !noswap,
>> > +		.may_cache_trim_mode = 1,
>> >  	};
>> >  
>> >  	WARN_ON_ONCE(!current->reclaim_state);
>> > @@ -6482,6 +6491,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>> >  		.may_writepage = !laptop_mode,
>> >  		.may_unmap = 1,
>> >  		.may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
>> > +		.may_cache_trim_mode = 1,
>> >  		.proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
>> >  	};
>> >  	/*
>> > @@ -6744,6 +6754,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>> >  		.gfp_mask = GFP_KERNEL,
>> >  		.order = order,
>> >  		.may_unmap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  	};
>> >  
>> >  	set_task_reclaim_state(current, &sc.reclaim_state);
>> > @@ -6898,8 +6909,14 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>> >  			sc.priority--;
>> >  	} while (sc.priority >= 1);
>> >  
>> > -	if (!sc.nr_reclaimed)
>> > +	if (!sc.nr_reclaimed) {
>> > +		if (sc.may_cache_trim_mode) {
>> 
>> sc.may_cache_trim_mode && cache_trim_mode ?
>
> I don't think so. cache_trim_mode has a chance to switch every
> prepare_scan_control() like:
>
>    if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE) &&
>        sc->may_cache_trim_mode)                                          
> 	sc->cache_trim_mode = 1;                                      
>    else                                                                  
> 	sc->cache_trim_mode = 0;                                      
>
> So referring to the last value is not a good idea.

We should only restart without cache_trim_mode if cache_trim_mode causes
issue.  If it isn't enabled with highest priority (lowest value), it
doesn't help to disable cache_trim_mode.

And, please take care of other "break" in the loop, for example, if
kthread_should_stop(), etc.

--
Best Regards,
Huang, Ying

> 	Byungchul
>
>> > +			sc.may_cache_trim_mode = 0;
>> > +			goto restart;
>> > +		}
>> > +
>> >  		pgdat->kswapd_failures++;
>> > +	}
>> >  
>> >  out:
>> >  	clear_reclaim_active(pgdat, highest_zoneidx);
>> > @@ -7202,6 +7219,7 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>> >  		.may_writepage = 1,
>> >  		.may_unmap = 1,
>> >  		.may_swap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  		.hibernation_mode = 1,
>> >  	};
>> >  	struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
>> > @@ -7360,6 +7378,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
>> >  		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
>> >  		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
>> >  		.may_swap = 1,
>> > +		.may_cache_trim_mode = 1,
>> >  		.reclaim_idx = gfp_zone(gfp_mask),
>> >  	};
>> >  	unsigned long pflags;
>> 
>> --
>> Best Regards,
>> Huang, Ying