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Subject: Re: [PATCH 22/22] mm, compaction: Capture a page under direct
 compaction
To:     Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
References: <20190118175136.31341-1-mgorman@techsingularity.net>
 <20190118175136.31341-23-mgorman@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Thu, 31 Jan 2019 17:11:04 +0100
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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);
>