From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Subject: mm/page_alloc: scale the number of pages that are batch freed 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 Link: https://lkml.kernel.org/r/20210525080119.5455-5-mgorman@xxxxxxxxxxxxxxxxxxx Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Hillf Danton <hdanton@xxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/mmzone.h | 3 +- mm/page_alloc.c | 41 +++++++++++++++++++++++++++++++++++++-- 2 files changed, 41 insertions(+), 3 deletions(-) --- a/include/linux/mmzone.h~mm-page_alloc-scale-the-number-of-pages-that-are-batch-freed +++ a/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 */ --- a/mm/page_alloc.c~mm-page_alloc-scale-the-number-of-pages-that-are-batch-freed +++ a/mm/page_alloc.c @@ -3278,18 +3278,47 @@ static bool free_unref_page_prepare(stru 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; + + /* Leave at least pcp->batch pages on the list */ + min_nr_free = batch; + max_nr_free = high - batch; + + /* + * Double the number of pages freed each time there is subsequent + * freeing of pages without any allocation. + */ + 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); + } } /* @@ -3541,7 +3570,14 @@ static struct page *rmqueue_pcplist(stru unsigned long flags; local_lock_irqsave(&pagesets.lock, flags); + + /* + * On allocation, reduce the number of pages that are batch freed. + * See nr_pcp_free() where free_factor is increased for subsequent + * frees. + */ 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); @@ -6737,6 +6773,7 @@ static void per_cpu_pages_init(struct pe */ 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, _
