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SubjectRe: [PATCH 22/22] mm, compaction: Capture a page under direct compaction
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On 1/18/19 6:51 PM, Mel Gorman wrote:
> Compaction is inherently race-prone as a suitable page freed during
> compaction can be allocated by any parallel task. This patch uses a
> capture_control structure to isolate a page immediately when it is freed
> by a direct compactor in the slow path of the page allocator. The intent
> is to avoid redundant scanning.
>
> 5.0.0-rc1 5.0.0-rc1
> selective-v3r17 capture-v3r19
> Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
> Amean fault-both-3 2582.11 ( 0.00%) 2563.68 ( 0.71%)
> Amean fault-both-5 4500.26 ( 0.00%) 4233.52 ( 5.93%)
> Amean fault-both-7 5819.53 ( 0.00%) 6333.65 ( -8.83%)
> Amean fault-both-12 9321.18 ( 0.00%) 9759.38 ( -4.70%)
> Amean fault-both-18 9782.76 ( 0.00%) 10338.76 ( -5.68%)
> Amean fault-both-24 15272.81 ( 0.00%) 13379.55 * 12.40%*
> Amean fault-both-30 15121.34 ( 0.00%) 16158.25 ( -6.86%)
> Amean fault-both-32 18466.67 ( 0.00%) 18971.21 ( -2.73%)
>
> Latency is only moderately affected but the devil is in the details.
> A closer examination indicates that base page fault latency is reduced
> but latency of huge pages is increased as it takes creater care to
> succeed. Part of the "problem" is that allocation success rates are close
> to 100% even when under pressure and compaction gets harder
>
> 5.0.0-rc1 5.0.0-rc1
> selective-v3r17 capture-v3r19
> Percentage huge-3 96.70 ( 0.00%) 98.23 ( 1.58%)
> Percentage huge-5 96.99 ( 0.00%) 95.30 ( -1.75%)
> Percentage huge-7 94.19 ( 0.00%) 97.24 ( 3.24%)
> Percentage huge-12 94.95 ( 0.00%) 97.35 ( 2.53%)
> Percentage huge-18 96.74 ( 0.00%) 97.30 ( 0.58%)
> Percentage huge-24 97.07 ( 0.00%) 97.55 ( 0.50%)
> Percentage huge-30 95.69 ( 0.00%) 98.50 ( 2.95%)
> Percentage huge-32 96.70 ( 0.00%) 99.27 ( 2.65%)
>
> And scan rates are reduced as expected by 6% for the migration scanner
> and 29% for the free scanner indicating that there is less redundant work.
>
> Compaction migrate scanned 20815362 19573286
> Compaction free scanned 16352612 11510663
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Nit below:

...

> @@ -819,6 +870,7 @@ static inline void __free_one_page(struct page *page,
> unsigned long uninitialized_var(buddy_pfn);
> struct page *buddy;
> unsigned int max_order;
> + struct capture_control *capc = task_capc(zone);
>
> max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
>
> @@ -834,6 +886,12 @@ static inline void __free_one_page(struct page *page,
>
> continue_merging:
> while (order < max_order - 1) {
> + if (compaction_capture(capc, page, order, migratetype)) {
> + if (likely(!is_migrate_isolate(migratetype)))

compaction_capture() won't act on isolated migratetype, so this check is
unnecessary?

> + __mod_zone_freepage_state(zone, -(1 << order),
> + migratetype);
> + return;
> + }
> buddy_pfn = __find_buddy_pfn(pfn, order);
> buddy = page + (buddy_pfn - pfn);
>

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