Re: [PATCH 5/6] mm, compaction: skip compound pages by order in free scanner

[Michal Nazarewicz]

On Wed, Jun 10 2015, Vlastimil Babka wrote:
> The compaction free scanner is looking for PageBuddy() pages and skipping all
> others.  For large compound pages such as THP or hugetlbfs, we can save a lot
> of iterations if we skip them at once using their compound_order(). This is
> generally unsafe and we can read a bogus value of order due to a race, but if
> we are careful, the only danger is skipping too much.
>
> When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs
> pages, the vmstat compact_free_scanned count decreased by at least 15%.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>

Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>

> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  mm/compaction.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e37d361..4a14084 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -437,6 +437,24 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>  
>  		if (!valid_page)
>  			valid_page = page;
> +
> +		/*
> +		 * For compound pages such as THP and hugetlbfs, we can save
> +		 * potentially a lot of iterations if we skip them at once.
> +		 * The check is racy, but we can consider only valid values
> +		 * and the only danger is skipping too much.
> +		 */
> +		if (PageCompound(page)) {
> +			unsigned int comp_order = compound_order(page);
> +
> +			if (comp_order > 0 && comp_order < MAX_ORDER) {

+			if (comp_order < MAX_ORDER) {

Might produce shorter/faster code.  Dunno.  Maybe.  So much
micro-optimisations.  Applies to the previous patch as well.

> +				blockpfn += (1UL << comp_order) - 1;
> +				cursor += (1UL << comp_order) - 1;
> +			}
> +
> +			goto isolate_fail;
> +		}
> +
>  		if (!PageBuddy(page))
>  			goto isolate_fail;
>  
> @@ -496,6 +514,13 @@ isolate_fail:
>  
>  	}
>  
> +	/*
> +	 * There is a tiny chance that we have read bogus compound_order(),
> +	 * so be careful to not go outside of the pageblock.
> +	 */
> +	if (unlikely(blockpfn > end_pfn))
> +		blockpfn = end_pfn;
> +
>  	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
>  					nr_scanned, total_isolated);
>  
> -- 
> 2.1.4
>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  MichaÅ âmina86â Nazarewicz    (o o)
ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>--ooO--(_)--Ooo--
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