The patch titled
Subject: mm/free_pcppages_bulk: prefetch buddy while not holding lock
has been removed from the -mm tree. Its filename was
mm-free_pcppages_bulk-prefetch-buddy-while-not-holding-lock.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Aaron Lu <aaron.lu@xxxxxxxxx>
Subject: mm/free_pcppages_bulk: prefetch buddy while not holding lock
When a page is freed back to the global pool, its buddy will be checked to
see if it's possible to do a merge. This requires accessing buddy's page
structure and that access could take a long time if it's cache cold.
This patch adds a prefetch to the to-be-freed page's buddy outside of
zone->lock in hope of accessing buddy's page structure later under
zone->lock will be faster. Since we *always* do buddy merging and check
an order-0 page's buddy to try to merge it when it goes into the main
allocator, the cacheline will always come in, i.e. the prefetched data
will never be unused.
Normally, the number of prefetch will be pcp->batch(default=31 and has an
upper limit of (PAGE_SHIFT * 8)=96 on x86_64) but in the case of pcp's
pages get all drained, it will be pcp->count which has an upper limit of
pcp->high. pcp->high, although has a default value of 186 (pcp->batch=31
* 6), can be changed by user through /proc/sys/vm/percpu_pagelist_fraction
and there is no software upper limit so could be large, like several
thousand. For this reason, only the first pcp->batch number of page's
buddy structure is prefetched to avoid excessive prefetching.
In the meantime, there are two concerns:
1 the prefetch could potentially evict existing cachelines, especially
for L1D cache since it is not huge;
2 there is some additional instruction overhead, namely calculating
buddy pfn twice.
For 1, it's hard to say, this microbenchmark though shows good result but
the actual benefit of this patch will be workload/CPU dependant;
For 2, since the calculation is a XOR on two local variables, it's
expected in many cases that cycles spent will be offset by reduced memory
latency later. This is especially true for NUMA machines where multiple
CPUs are contending on zone->lock and the most time consuming part under
zone->lock is the wait of 'struct page' cacheline of the to-be-freed pages
and their buddies.
Test with will-it-scale/page_fault1 full load:
kernel Broadwell(2S) Skylake(2S) Broadwell(4S) Skylake(4S)
v4.16-rc2+ 9034215 7971818 13667135 15677465
patch2/3 9536374 +5.6% 8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
this patch 10180856 +6.8% 8506369 +2.3% 14756865 +4.9% 17325324 +3.9%
Note: this patch's performance improvement percent is against patch2/3.
(Changelog stolen from Dave Hansen and Mel Gorman's comments at
http://lkml.kernel.org/r/148a42d8-8306-2f2f-7f7c-86bc118f8ccd@xxxxxxxxx)
[aaron.lu@xxxxxxxxx: use helper function, avoid disordering pages]
Link: http://lkml.kernel.org/r/20180301062845.26038-4-aaron.lu@xxxxxxxxx
Link: http://lkml.kernel.org/r/20180320113146.GB24737@xxxxxxxxx
[aaron.lu@xxxxxxxxx: v4]
Link: http://lkml.kernel.org/r/20180301062845.26038-4-aaron.lu@xxxxxxxxx
Link: http://lkml.kernel.org/r/20180309082431.GB30868@xxxxxxxxx
Link: http://lkml.kernel.org/r/20180301062845.26038-4-aaron.lu@xxxxxxxxx
Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
Suggested-by: Ying Huang <ying.huang@xxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Kemi Wang <kemi.wang@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff -puN mm/page_alloc.c~mm-free_pcppages_bulk-prefetch-buddy-while-not-holding-lock mm/page_alloc.c
--- a/mm/page_alloc.c~mm-free_pcppages_bulk-prefetch-buddy-while-not-holding-lock
+++ a/mm/page_alloc.c
@@ -1063,6 +1063,15 @@ static bool bulkfree_pcp_prepare(struct
}
#endif /* CONFIG_DEBUG_VM */
+static inline void prefetch_buddy(struct page *page)
+{
+ unsigned long pfn = page_to_pfn(page);
+ unsigned long buddy_pfn = __find_buddy_pfn(pfn, 0);
+ struct page *buddy = page + (buddy_pfn - pfn);
+
+ prefetch(buddy);
+}
+
/*
* Frees a number of pages from the PCP lists
* Assumes all pages on list are in same zone, and of same order.
@@ -1079,6 +1088,7 @@ static void free_pcppages_bulk(struct zo
{
int migratetype = 0;
int batch_free = 0;
+ int prefetch_nr = 0;
bool isolated_pageblocks;
struct page *page, *tmp;
LIST_HEAD(head);
@@ -1114,6 +1124,18 @@ static void free_pcppages_bulk(struct zo
continue;
list_add_tail(&page->lru, &head);
+
+ /*
+ * We are going to put the page back to the global
+ * pool, prefetch its buddy to speed up later access
+ * under zone->lock. It is believed the overhead of
+ * an additional test and calculating buddy_pfn here
+ * can be offset by reduced memory latency later. To
+ * avoid excessive prefetching due to large count, only
+ * prefetch buddy for the first pcp->batch nr of pages.
+ */
+ if (prefetch_nr++ < pcp->batch)
+ prefetch_buddy(page);
} while (--count && --batch_free && !list_empty(list));
}
_
Patches currently in -mm which might be from aaron.lu@xxxxxxxxx are
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