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Message-ID: <20240305024345.GB60719@system.software.com>
Date: Tue, 5 Mar 2024 11:43:45 +0900
From: Byungchul Park <byungchul@...com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, kernel_team@...ynix.com, yuzhao@...gle.com,
hannes@...xchg.org
Subject: Re: [PATCH v6] mm, vmscan: retry kswapd's priority loop with
cache_trim_mode off on failure
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@...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@...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@...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.
Ah, okay. Confining to kswapd, that might make sense. I will apply it if
there's no objection to it. Thanks.
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
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