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

Michal Nazarewicz
2015-06-12T10:18:15Z

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@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>

Acked-by: Michal Nazarewicz <mina86@mina86.com>

> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> ---
> 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, Michał “mina86” Nazarewicz (o o)
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