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

Huang, Ying
2024-03-05T02:42:21Z

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