[RFC PATCH 24/26] mm: page_alloc: kill watermark boosting

From: Johannes Weiner
Date: Tue Apr 18 2023 - 15:15:39 EST

Watermark boosting is meant to increase the chances of pageblock
production when fallbacks are observed. Since reclaim/compaction now
produce neutral pageblocks per default, this is no longer needed.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
Documentation/admin-guide/sysctl/vm.rst | 21 -----
include/linux/mm.h | 1 -
include/linux/mmzone.h | 12 +--
kernel/sysctl.c | 8 --
mm/page_alloc.c | 67 --------------
mm/vmscan.c | 111 +-----------------------
mm/vmstat.c | 2 -
7 files changed, 7 insertions(+), 215 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 988f6a4c8084..498655c322bc 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -72,7 +72,6 @@ files can be found in mm/swap.c.
- unprivileged_userfaultfd
- user_reserve_kbytes
- vfs_cache_pressure
-- watermark_boost_factor
- watermark_scale_factor
- zone_reclaim_mode

@@ -968,26 +967,6 @@ directory and inode objects. With vfs_cache_pressure=1000, it will look for
ten times more freeable objects than there are.


-watermark_boost_factor
-======================
-
-This factor controls the level of reclaim when memory is being fragmented.
-It defines the percentage of the high watermark of a zone that will be
-reclaimed if pages of different mobility are being mixed within pageblocks.
-The intent is that compaction has less work to do in the future and to
-increase the success rate of future high-order allocations such as SLUB
-allocations, THP and hugetlbfs pages.
-
-To make it sensible with respect to the watermark_scale_factor
-parameter, the unit is in fractions of 10,000. The default value of
-15,000 means that up to 150% of the high watermark will be reclaimed in the
-event of a pageblock being mixed due to fragmentation. The level of reclaim
-is determined by the number of fragmentation events that occurred in the
-recent past. If this value is smaller than a pageblock then a pageblocks
-worth of pages will be reclaimed (e.g. 2MB on 64-bit x86). A boost factor
-of 0 will disable the feature.
-
-
watermark_scale_factor
======================

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f13f20258ce9..e7c2631848ed 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2746,7 +2746,6 @@ extern void setup_per_cpu_pageset(void);

/* page_alloc.c */
extern int min_free_kbytes;
-extern int watermark_boost_factor;
extern int watermark_scale_factor;
extern bool arch_has_descending_max_zone_pfns(void);

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index c705f2f7c829..1363ff6caff3 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -567,10 +567,10 @@ enum zone_watermarks {
#define NR_LOWORDER_PCP_LISTS (MIGRATE_PCPTYPES * (PAGE_ALLOC_COSTLY_ORDER + 1))
#define NR_PCP_LISTS (NR_LOWORDER_PCP_LISTS + NR_PCP_THP)

-#define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost)
-#define low_wmark_pages(z) (z->_watermark[WMARK_LOW] + z->watermark_boost)
-#define high_wmark_pages(z) (z->_watermark[WMARK_HIGH] + z->watermark_boost)
-#define wmark_pages(z, i) (z->_watermark[i] + z->watermark_boost)
+#define min_wmark_pages(z) (z->_watermark[WMARK_MIN])
+#define low_wmark_pages(z) (z->_watermark[WMARK_LOW])
+#define high_wmark_pages(z) (z->_watermark[WMARK_HIGH])
+#define wmark_pages(z, i) (z->_watermark[i])

/* Fields and list protected by pagesets local_lock in page_alloc.c */
struct per_cpu_pages {
@@ -709,7 +709,6 @@ struct zone {

/* zone watermarks, access with *_wmark_pages(zone) macros */
unsigned long _watermark[NR_WMARK];
- unsigned long watermark_boost;

/*
* We don't know if the memory that we're going to allocate will be
@@ -884,9 +883,6 @@ enum pgdat_flags {
};

enum zone_flags {
- ZONE_BOOSTED_WATERMARK, /* zone recently boosted watermarks.
- * Cleared when kswapd is woken.
- */
ZONE_RECLAIM_ACTIVE, /* kswapd may be scanning the zone. */
};

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 137d4abe3eda..68bcd3a7c9c6 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2229,14 +2229,6 @@ static struct ctl_table vm_table[] = {
.proc_handler = min_free_kbytes_sysctl_handler,
.extra1 = SYSCTL_ZERO,
},
- {
- .procname = "watermark_boost_factor",
- .data = &watermark_boost_factor,
- .maxlen = sizeof(watermark_boost_factor),
- .mode = 0644,
- .proc_handler = proc_dointvec_minmax,
- .extra1 = SYSCTL_ZERO,
- },
{
.procname = "watermark_scale_factor",
.data = &watermark_scale_factor,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e8ae04feb1bd..f835a5548164 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -401,7 +401,6 @@ compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {

int min_free_kbytes = 1024;
int user_min_free_kbytes = -1;
-int watermark_boost_factor __read_mostly = 15000;
int watermark_scale_factor = 10;

static unsigned long nr_kernel_pages __initdata;
@@ -2742,43 +2741,6 @@ static bool can_steal_fallback(unsigned int order, int start_mt,
return false;
}

-static inline bool boost_watermark(struct zone *zone)
-{
- unsigned long max_boost;
-
- if (!watermark_boost_factor)
- return false;
- /*
- * Don't bother in zones that are unlikely to produce results.
- * On small machines, including kdump capture kernels running
- * in a small area, boosting the watermark can cause an out of
- * memory situation immediately.
- */
- if ((pageblock_nr_pages * 4) > zone_managed_pages(zone))
- return false;
-
- max_boost = mult_frac(zone->_watermark[WMARK_HIGH],
- watermark_boost_factor, 10000);
-
- /*
- * high watermark may be uninitialised if fragmentation occurs
- * very early in boot so do not boost. We do not fall
- * through and boost by pageblock_nr_pages as failing
- * allocations that early means that reclaim is not going
- * to help and it may even be impossible to reclaim the
- * boosted watermark resulting in a hang.
- */
- if (!max_boost)
- return false;
-
- max_boost = max(pageblock_nr_pages, max_boost);
-
- zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,
- max_boost);
-
- return true;
-}
-
/*
* This function implements actual steal behaviour. If order is large enough,
* we can steal whole pageblock. If not, we first move freepages in this
@@ -2802,14 +2764,6 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
goto single_page;
}

- /*
- * Boost watermarks to increase reclaim pressure to reduce the
- * likelihood of future fallbacks. Wake kswapd now as the node
- * may be balanced overall and kswapd will not wake naturally.
- */
- if (boost_watermark(zone) && (alloc_flags & ALLOC_KSWAPD))
- set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
-
/* We are not allowed to try stealing from the whole block */
if (!whole_block)
goto single_page;
@@ -3738,12 +3692,6 @@ struct page *rmqueue(struct zone *preferred_zone,
migratetype);

out:
- /* Separate test+clear to avoid unnecessary atomics */
- if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
- clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
- wakeup_kswapd(zone, 0, 0, zone_idx(zone));
- }
-
VM_BUG_ON_PAGE(page && bad_range(zone, page), page);
return page;
}
@@ -3976,18 +3924,6 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
if (__zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
free_pages))
return true;
- /*
- * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
- * when checking the min watermark. The min watermark is the
- * point where boosting is ignored so that kswapd is woken up
- * when below the low watermark.
- */
- if (unlikely(!order && (gfp_mask & __GFP_ATOMIC) && z->watermark_boost
- && ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN))) {
- mark = z->_watermark[WMARK_MIN];
- return __zone_watermark_ok(z, order, mark, highest_zoneidx,
- alloc_flags, free_pages);
- }

return false;
}
@@ -6137,7 +6073,6 @@ void __show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_zone_i
" free_movable:%lukB"
" free_reclaimable:%lukB"
" free_free:%lukB"
- " boost:%lukB"
" min:%lukB"
" low:%lukB"
" high:%lukB"
@@ -6161,7 +6096,6 @@ void __show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_zone_i
K(zone_page_state(zone, NR_FREE_MOVABLE)),
K(zone_page_state(zone, NR_FREE_RECLAIMABLE)),
K(zone_page_state(zone, NR_FREE_FREE)),
- K(zone->watermark_boost),
K(min_wmark_pages(zone)),
K(low_wmark_pages(zone)),
K(high_wmark_pages(zone)),
@@ -8701,7 +8635,6 @@ static void __setup_per_zone_wmarks(void)
if (IS_ENABLED(CONFIG_COMPACTION))
tmp = ALIGN(tmp, 1 << pageblock_order);

- zone->watermark_boost = 0;
zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
zone->_watermark[WMARK_HIGH] = low_wmark_pages(zone) + tmp;
zone->_watermark[WMARK_PROMO] = high_wmark_pages(zone) + tmp;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a7374cd6fe91..5586be6997cd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6827,30 +6827,6 @@ static void kswapd_age_node(struct pglist_data *pgdat, struct scan_control *sc)
} while (memcg);
}

-static bool pgdat_watermark_boosted(pg_data_t *pgdat, int highest_zoneidx)
-{
- int i;
- struct zone *zone;
-
- /*
- * Check for watermark boosts top-down as the higher zones
- * are more likely to be boosted. Both watermarks and boosts
- * should not be checked at the same time as reclaim would
- * start prematurely when there is no boosting and a lower
- * zone is balanced.
- */
- for (i = highest_zoneidx; i >= 0; i--) {
- zone = pgdat->node_zones + i;
- if (!managed_zone(zone))
- continue;
-
- if (zone->watermark_boost)
- return true;
- }
-
- return false;
-}
-
/*
* Returns true if there is an eligible zone balanced for the request order
* and highest_zoneidx
@@ -7025,14 +7001,13 @@ static void balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
unsigned long nr_soft_reclaimed;
unsigned long nr_soft_scanned;
unsigned long pflags;
- unsigned long nr_boost_reclaim;
- unsigned long zone_boosts[MAX_NR_ZONES] = { 0, };
- bool boosted;
struct zone *zone;
struct scan_control sc = {
.gfp_mask = GFP_KERNEL,
.order = order,
.may_unmap = 1,
+ .may_swap = 1,
+ .may_writepage = !laptop_mode,
};

set_task_reclaim_state(current, &sc.reclaim_state);
@@ -7041,29 +7016,11 @@ static void balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)

count_vm_event(PAGEOUTRUN);

- /*
- * Account for the reclaim boost. Note that the zone boost is left in
- * place so that parallel allocations that are near the watermark will
- * stall or direct reclaim until kswapd is finished.
- */
- nr_boost_reclaim = 0;
- for (i = 0; i <= highest_zoneidx; i++) {
- zone = pgdat->node_zones + i;
- if (!managed_zone(zone))
- continue;
-
- nr_boost_reclaim += zone->watermark_boost;
- zone_boosts[i] = zone->watermark_boost;
- }
- boosted = nr_boost_reclaim;
-
-restart:
set_reclaim_active(pgdat, highest_zoneidx);
sc.priority = DEF_PRIORITY;
do {
unsigned long nr_reclaimed = sc.nr_reclaimed;
bool raise_priority = true;
- bool balanced;
bool ret;

sc.reclaim_idx = highest_zoneidx;
@@ -7089,40 +7046,9 @@ static void balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
}
}

- /*
- * If the pgdat is imbalanced then ignore boosting and preserve
- * the watermarks for a later time and restart. Note that the
- * zone watermarks will be still reset at the end of balancing
- * on the grounds that the normal reclaim should be enough to
- * re-evaluate if boosting is required when kswapd next wakes.
- */
- balanced = pgdat_balanced(pgdat, sc.order, highest_zoneidx);
- if (!balanced && nr_boost_reclaim) {
- nr_boost_reclaim = 0;
- goto restart;
- }
-
- /*
- * If boosting is not active then only reclaim if there are no
- * eligible zones. Note that sc.reclaim_idx is not used as
- * buffer_heads_over_limit may have adjusted it.
- */
- if (!nr_boost_reclaim && balanced)
+ if (pgdat_balanced(pgdat, sc.order, highest_zoneidx))
goto out;

- /* Limit the priority of boosting to avoid reclaim writeback */
- if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2)
- raise_priority = false;
-
- /*
- * Do not writeback or swap pages for boosted reclaim. The
- * intent is to relieve pressure not issue sub-optimal IO
- * from reclaim context. If no pages are reclaimed, the
- * reclaim will be aborted.
- */
- sc.may_writepage = !laptop_mode && !nr_boost_reclaim;
- sc.may_swap = !nr_boost_reclaim;
-
/*
* Do some background aging, to give pages a chance to be
* referenced before reclaiming. All pages are rotated
@@ -7173,15 +7099,6 @@ static void balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
* progress in reclaiming pages
*/
nr_reclaimed = sc.nr_reclaimed - nr_reclaimed;
- nr_boost_reclaim -= min(nr_boost_reclaim, nr_reclaimed);
-
- /*
- * If reclaim made no progress for a boost, stop reclaim as
- * IO cannot be queued and it could be an infinite loop in
- * extreme circumstances.
- */
- if (nr_boost_reclaim && !nr_reclaimed)
- break;

if (raise_priority || !nr_reclaimed)
sc.priority--;
@@ -7193,28 +7110,6 @@ static void balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
out:
clear_reclaim_active(pgdat, highest_zoneidx);

- /* If reclaim was boosted, account for the reclaim done in this pass */
- if (boosted) {
- unsigned long flags;
-
- for (i = 0; i <= highest_zoneidx; i++) {
- if (!zone_boosts[i])
- continue;
-
- /* Increments are under the zone lock */
- zone = pgdat->node_zones + i;
- spin_lock_irqsave(&zone->lock, flags);
- zone->watermark_boost -= min(zone->watermark_boost, zone_boosts[i]);
- spin_unlock_irqrestore(&zone->lock, flags);
- }
-
- /*
- * As there is now likely space, wakeup kcompact to defragment
- * pageblocks.
- */
- wakeup_kcompactd(pgdat, pageblock_order, highest_zoneidx);
- }
-
snapshot_refaults(NULL, pgdat);
__fs_reclaim_release(_THIS_IP_);
psi_memstall_leave(&pflags);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a2f7b41564df..80ee26588242 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1682,7 +1682,6 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
}
seq_printf(m,
"\n pages free %lu"
- "\n boost %lu"
"\n min %lu"
"\n low %lu"
"\n high %lu"
@@ -1691,7 +1690,6 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
"\n managed %lu"
"\n cma %lu",
zone_page_state(zone, NR_FREE_PAGES),
- zone->watermark_boost,
min_wmark_pages(zone),
low_wmark_pages(zone),
high_wmark_pages(zone),
--
2.39.2