[PATCH 1/6] mm/page_alloc: Delete vm.percpu_pagelist_fraction

Mel Gorman
2021-05-25T08:12:48Z

The vm.percpu_pagelist_fraction is used to increase the batch and high
limits for the per-cpu page allocator (PCP). The intent behind the sysctl
is to reduce zone lock acquisition when allocating/freeing pages but it has
a problem. While it can decrease contention, it can also increase latency
on the allocation side due to unreasonably large batch sizes. This leads
to games where an administrator adjusts percpu_pagelist_fraction on the
fly to work around contention and allocation latency problems.

This series aims to alleviate the problems with zone lock contention while
avoiding the allocation-side latency problems. For the purposes of review,
it's easier to remove this sysctl now and reintroduce a similar sysctl
later in the series that deals only with pcp->high.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
---
Documentation/admin-guide/sysctl/vm.rst | 19 ---------
include/linux/mmzone.h | 3 --
kernel/sysctl.c | 8 ----
mm/page_alloc.c | 55 ++-----------------------
4 files changed, 4 insertions(+), 81 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 586cd4b86428..2fcafccb53a8 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -64,7 +64,6 @@ files can be found in mm/swap.c.
- overcommit_ratio
- page-cluster
- panic_on_oom
-- percpu_pagelist_fraction
- stat_interval
- stat_refresh
- numa_stat
@@ -790,24 +789,6 @@ panic_on_oom=2+kdump gives you very strong tool to investigate
why oom happens. You can get snapshot.

-percpu_pagelist_fraction
-========================
-
-This is the fraction of pages at most (high mark pcp->high) in each zone that
-are allocated for each per cpu page list. The min value for this is 8. It
-means that we don't allow more than 1/8th of pages in each zone to be
-allocated in any single per_cpu_pagelist. This entry only changes the value
-of hot per cpu pagelists. User can specify a number like 100 to allocate
-1/100th of each zone to each per cpu page list.
-
-The batch value of each per cpu pagelist is also updated as a result. It is
-set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
-
-The initial value is zero. Kernel does not use this value at boot time to set
-the high water marks for each per cpu page list. If the user writes '0' to this
-sysctl, it will revert to this default behavior.
-
-
stat_interval
=============

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d7740c97b87e..b449151745d7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1038,15 +1038,12 @@ int watermark_scale_factor_sysctl_handler(struct ctl_table *, int, void *,
extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES];
int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int, void *,
size_t *, loff_t *);
-int percpu_pagelist_fraction_sysctl_handler(struct ctl_table *, int,
- void *, size_t *, loff_t *);
int sysctl_min_unmapped_ratio_sysctl_handler(struct ctl_table *, int,
void *, size_t *, loff_t *);
int sysctl_min_slab_ratio_sysctl_handler(struct ctl_table *, int,
void *, size_t *, loff_t *);
int numa_zonelist_order_handler(struct ctl_table *, int,
void *, size_t *, loff_t *);
-extern int percpu_pagelist_fraction;
extern char numa_zonelist_order[];
#define NUMA_ZONELIST_ORDER_LEN 16

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 14edf84cc571..4e5ac50a1af0 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2889,14 +2889,6 @@ static struct ctl_table vm_table[] = {
.extra1 = SYSCTL_ONE,
.extra2 = &one_thousand,
},
- {
- .procname = "percpu_pagelist_fraction",
- .data = &percpu_pagelist_fraction,
- .maxlen = sizeof(percpu_pagelist_fraction),
- .mode = 0644,
- .proc_handler = percpu_pagelist_fraction_sysctl_handler,
- .extra1 = SYSCTL_ZERO,
- },
{
.procname = "page_lock_unfairness",
.data = &sysctl_page_lock_unfairness,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ff8f706839ea..a48f305f0381 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -120,7 +120,6 @@ typedef int __bitwise fpi_t;

/* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
static DEFINE_MUTEX(pcp_batch_high_lock);
-#define MIN_PERCPU_PAGELIST_FRACTION (8)

struct pagesets {
local_lock_t lock;
@@ -182,7 +181,6 @@ EXPORT_SYMBOL(_totalram_pages);
unsigned long totalreserve_pages __read_mostly;
unsigned long totalcma_pages __read_mostly;

-int percpu_pagelist_fraction;
gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
DEFINE_STATIC_KEY_MAYBE(CONFIG_INIT_ON_ALLOC_DEFAULT_ON, init_on_alloc);
EXPORT_SYMBOL(init_on_alloc);
@@ -6696,22 +6694,15 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h

/*
* Calculate and set new high and batch values for all per-cpu pagesets of a
- * zone, based on the zone's size and the percpu_pagelist_fraction sysctl.
+ * zone based on the zone's size.
*/
static void zone_set_pageset_high_and_batch(struct zone *zone)
{
unsigned long new_high, new_batch;

- if (percpu_pagelist_fraction) {
- new_high = zone_managed_pages(zone) / percpu_pagelist_fraction;
- new_batch = max(1UL, new_high / 4);
- if ((new_high / 4) > (PAGE_SHIFT * 8))
- new_batch = PAGE_SHIFT * 8;
- } else {
- new_batch = zone_batchsize(zone);
- new_high = 6 * new_batch;
- new_batch = max(1UL, 1 * new_batch);
- }
+ new_batch = zone_batchsize(zone);
+ new_high = 6 * new_batch;
+ new_batch = max(1UL, 1 * new_batch);

if (zone->pageset_high == new_high &&
zone->pageset_batch == new_batch)
@@ -8377,44 +8368,6 @@ int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *table, int write,
return 0;
}

-/*
- * percpu_pagelist_fraction - changes the pcp->high for each zone on each
- * cpu. It is the fraction of total pages in each zone that a hot per cpu
- * pagelist can have before it gets flushed back to buddy allocator.
- */
-int percpu_pagelist_fraction_sysctl_handler(struct ctl_table *table, int write,
- void *buffer, size_t *length, loff_t *ppos)
-{
- struct zone *zone;
- int old_percpu_pagelist_fraction;
- int ret;
-
- mutex_lock(&pcp_batch_high_lock);
- old_percpu_pagelist_fraction = percpu_pagelist_fraction;
-
- ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
- if (!write || ret < 0)
- goto out;
-
- /* Sanity checking to avoid pcp imbalance */
- if (percpu_pagelist_fraction &&
- percpu_pagelist_fraction < MIN_PERCPU_PAGELIST_FRACTION) {
- percpu_pagelist_fraction = old_percpu_pagelist_fraction;
- ret = -EINVAL;
- goto out;
- }
-
- /* No change? */
- if (percpu_pagelist_fraction == old_percpu_pagelist_fraction)
- goto out;
-
- for_each_populated_zone(zone)
- zone_set_pageset_high_and_batch(zone);
-out:
- mutex_unlock(&pcp_batch_high_lock);
- return ret;
-}
-
#ifndef __HAVE_ARCH_RESERVED_KERNEL_PAGES
/*
* Returns the number of pages that arch has reserved but
--
2.26.2