[PATCH] mm: Enable setting -1 for vm.percpu_pagelist_high_fraction to set the minimum pagelist

[Yafang Shao <laoar.shao@gmail.com>]

Currently, we're encountering latency spikes in our container environment
when a specific container with multiple Python-based tasks exits. These
tasks may hold the zone->lock for an extended period, significantly
impacting latency for other containers attempting to allocate memory.

As a workaround, we've found that minimizing the pagelist size, such as
setting it to 4 times the batch size, can help mitigate these spikes.
However, managing vm.percpu_pagelist_high_fraction across a large fleet of
servers poses challenges due to variations in CPU counts, NUMA nodes, and
physical memory capacities.

To enhance practicality, we propose allowing the setting of -1 for
vm.percpu_pagelist_high_fraction to designate a minimum pagelist size.

Furthermore, considering the challenges associated with utilizing
vm.percpu_pagelist_high_fraction, it would be beneficial to introduce a
more intuitive parameter, vm.percpu_pagelist_high_size, that would permit
direct specification of the pagelist size as a multiple of the batch size.
This methodology would mirror the functionality of vm.dirty_ratio and
vm.dirty_bytes, providing users with greater flexibility and control.

We have discussed the possibility of introducing multiple small zones to
mitigate the contention on the zone->lock[0], but this approach is likely
to require a longer-term implementation effort.

Link: https://lore.kernel.org/linux-mm/ZnTrZ9mcAIRodnjx@casper.infradead.org/ [0]
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
---
 Documentation/admin-guide/sysctl/vm.rst | 4 ++++
 mm/page_alloc.c                         | 8 ++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index e86c968a7a0e..1f535d022cda 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -856,6 +856,10 @@ on per-cpu page lists. This entry only changes the value of hot per-cpu
 page lists. A user can specify a number like 100 to allocate 1/100th of
 each zone between per-cpu lists.
 
+The minimum number of pages that can be stored in per-CPU page lists is
+four times the batch value. By writing '-1' to this sysctl, you can set
+this minimum value.
+
 The batch value of each per-cpu page list remains the same regardless of
 the value of the high fraction so allocation latencies are unaffected.
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2e22ce5675ca..e7313f9d704b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5486,6 +5486,10 @@ static int zone_highsize(struct zone *zone, int batch, int cpu_online,
 	int nr_split_cpus;
 	unsigned long total_pages;
 
+	/* Setting -1 to set the minimum pagelist size, four times the batch size */
+	if (high_fraction == -1)
+		return batch << 2;
+
 	if (!high_fraction) {
 		/*
 		 * By default, the high value of the pcp is based on the zone
@@ -6192,7 +6196,8 @@ static int percpu_pagelist_high_fraction_sysctl_handler(struct ctl_table *table,
 
 	/* Sanity checking to avoid pcp imbalance */
 	if (percpu_pagelist_high_fraction &&
-	    percpu_pagelist_high_fraction < MIN_PERCPU_PAGELIST_HIGH_FRACTION) {
+	    percpu_pagelist_high_fraction < MIN_PERCPU_PAGELIST_HIGH_FRACTION &&
+	    percpu_pagelist_high_fraction != -1) {
 		percpu_pagelist_high_fraction = old_percpu_pagelist_high_fraction;
 		ret = -EINVAL;
 		goto out;
@@ -6241,7 +6246,6 @@ static struct ctl_table page_alloc_sysctl_table[] = {
 		.maxlen		= sizeof(percpu_pagelist_high_fraction),
 		.mode		= 0644,
 		.proc_handler	= percpu_pagelist_high_fraction_sysctl_handler,
-		.extra1		= SYSCTL_ZERO,
 	},
 	{
 		.procname	= "lowmem_reserve_ratio",
-- 
2.43.5