[PATCH 4/6] mm/page_alloc: Scale the number of pages that are batch freed

Mel Gorman
2021-05-21T10:31:15Z

When a task is freeing a large number of order-0 pages, it may acquire
the zone->lock multiple times freeing pages in batches. This may
unnecessarily contend on the zone lock when freeing very large number
of pages. This patch adapts the size of the batch based on the recent
pattern to scale the batch size for subsequent frees.

As the machines I used were not large enough to test this are not large
enough to illustrate a problem, a debugging patch shows patterns like
the following (slightly editted for clarity)

Baseline vanilla kernel
time-unmap-14426 [...] free_pcppages_bulk: free 63 count 378 high 378
time-unmap-14426 [...] free_pcppages_bulk: free 63 count 378 high 378
time-unmap-14426 [...] free_pcppages_bulk: free 63 count 378 high 378
time-unmap-14426 [...] free_pcppages_bulk: free 63 count 378 high 378
time-unmap-14426 [...] free_pcppages_bulk: free 63 count 378 high 378

With patches
time-unmap-7724 [...] free_pcppages_bulk: free 126 count 814 high 814
time-unmap-7724 [...] free_pcppages_bulk: free 252 count 814 high 814
time-unmap-7724 [...] free_pcppages_bulk: free 504 count 814 high 814
time-unmap-7724 [...] free_pcppages_bulk: free 751 count 814 high 814
time-unmap-7724 [...] free_pcppages_bulk: free 751 count 814 high 814

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
include/linux/mmzone.h | 3 ++-
mm/page_alloc.c | 30 ++++++++++++++++++++++++++++--
2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index b449151745d7..92182e0299b2 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -343,8 +343,9 @@ struct per_cpu_pages {
int count; /* number of pages in the list */
int high; /* high watermark, emptying needed */
int batch; /* chunk size for buddy add/remove */
+ short free_factor; /* batch scaling factor during free */
#ifdef CONFIG_NUMA
- int expire; /* When 0, remote pagesets are drained */
+ short expire; /* When 0, remote pagesets are drained */
#endif

/* Lists of pages, one per migrate type stored on the pcp-lists */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2761b03b3a44..c3da6401f138 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3267,18 +3267,42 @@ static bool free_unref_page_prepare(struct page *page, unsigned long pfn)
return true;
}

+static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch)
+{
+ int min_nr_free, max_nr_free;
+
+ /* Check for PCP disabled or boot pageset */
+ if (unlikely(high < batch))
+ return 1;
+
+ min_nr_free = batch;
+ max_nr_free = high - batch;
+
+ batch <<= pcp->free_factor;
+ if (batch < max_nr_free)
+ pcp->free_factor++;
+ batch = clamp(batch, min_nr_free, max_nr_free);
+
+ return batch;
+}
+
static void free_unref_page_commit(struct page *page, unsigned long pfn,
int migratetype)
{
struct zone *zone = page_zone(page);
struct per_cpu_pages *pcp;
+ int high;

__count_vm_event(PGFREE);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
list_add(&page->lru, &pcp->lists[migratetype]);
pcp->count++;
- if (pcp->count >= READ_ONCE(pcp->high))
- free_pcppages_bulk(zone, READ_ONCE(pcp->batch), pcp);
+ high = READ_ONCE(pcp->high);
+ if (pcp->count >= high) {
+ int batch = READ_ONCE(pcp->batch);
+
+ free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch), pcp);
+ }
}

/*
@@ -3531,6 +3555,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,

local_lock_irqsave(&pagesets.lock, flags);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
+ pcp->free_factor >>= 1;
list = &pcp->lists[migratetype];
page = __rmqueue_pcplist(zone, migratetype, alloc_flags, pcp, list);
local_unlock_irqrestore(&pagesets.lock, flags);
@@ -6690,6 +6715,7 @@ static void per_cpu_pages_init(struct per_cpu_pages *pcp, struct per_cpu_zonesta
*/
pcp->high = BOOT_PAGESET_HIGH;
pcp->batch = BOOT_PAGESET_BATCH;
+ pcp->free_factor = 0;
}

static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long high,
--
2.26.2