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

From: Byungchul Park
Date: Tue Mar 05 2024 - 02:57:15 EST

On Tue, Mar 05, 2024 at 03:35:35PM +0800, Huang, Ying wrote:
> 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?

v6 is based on what Johannes suggested. I thought it's more right way to
fix the issue.

Yeah, I also think checking cache_trim_mode only at the kswapd's highest
priorty would manage to work for the issue.

I'd like to listen to Johannes's opinion or others.

Byungchul

> --
> 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