Re: [PATCH 4/6] mm, compaction: always skip compound pages by order in migrate scanner

Michal Nazarewicz
2015-06-12T10:11:44Z

On Wed, Jun 10 2015, Vlastimil Babka wrote:
> The compaction migrate scanner tries to skip compound pages by their order, to
> reduce number of iterations for pages it cannot isolate. The check is only done
> if PageLRU() is true, which means it applies to THP pages, but not e.g.
> hugetlbfs pages or any other non-LRU compound pages, which we have to iterate
> by base pages.
>
> This limitation comes from the assumption that it's only safe to read
> compound_order() when we have the zone's lru_lock and THP cannot be split under
> us. But the only danger (after filtering out order values that are not below
> MAX_ORDER, to prevent overflows) is that we skip too much or too little after
> reading a bogus compound_order() due to a rare race. This is the same reasoning
> as patch 99c0fd5e51c4 ("mm, compaction: skip buddy pages by their order in the
> migrate scanner") introduced for unsafely reading PageBuddy() order.
>
> After this patch, all pages are tested for PageCompound() and we skip them by
> compound_order(). The test is done after the test for balloon_page_movable()
> as we don't want to assume if balloon pages (or other pages with own isolation
> and migration implementation if a generic API gets implemented) are compound
> or not.
>
> When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs
> pages, the vmstat compact_migrate_scanned count decreased by 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 | 36 +++++++++++++++++-------------------
> 1 file changed, 17 insertions(+), 19 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index d334bb3..e37d361 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -680,6 +680,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>
> /* Time to isolate some pages for migration */
> for (; low_pfn < end_pfn; low_pfn++) {
> + bool is_lru;
> +
> /*
> * Periodically drop the lock (if held) regardless of its
> * contention, to give chance to IRQs. Abort async compaction
> @@ -723,39 +725,35 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> * It's possible to migrate LRU pages and balloon pages
> * Skip any other type of page
> */
> - if (!PageLRU(page)) {
> + is_lru = PageLRU(page);
> + if (!is_lru) {
> if (unlikely(balloon_page_movable(page))) {
> if (balloon_page_isolate(page)) {
> /* Successfully isolated */
> goto isolate_success;
> }
> }
> - continue;
> }
>
> /*
> - * PageLRU is set. lru_lock normally excludes isolation
> - * splitting and collapsing (collapsing has already happened
> - * if PageLRU is set) but the lock is not necessarily taken
> - * here and it is wasteful to take it just to check transhuge.
> - * Check PageCompound without lock and skip the whole pageblock
> - * if it's a transhuge page, as calling compound_order()
> - * without preventing THP from splitting the page underneath us
> - * may return surprising results.
> - * If we happen to check a THP tail page, compound_order()
> - * returns 0. It should be rare enough to not bother with
> - * using compound_head() in that case.
> + * Regardless of being on LRU, compound pages such as THP and
> + * hugetlbfs are not to be compacted. We can potentially save
> + * 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)) {
> - int nr;
> - if (locked)
> - nr = 1 << compound_order(page);
> - else
> - nr = pageblock_nr_pages;
> - low_pfn += nr - 1;
> + unsigned int comp_order = compound_order(page);
> +
> + if (comp_order > 0 && comp_order < MAX_ORDER)
> + low_pfn += (1UL << comp_order) - 1;
> +
> continue;
> }
>
> + if (!is_lru)
> + continue;
> +
> /*
> * Migration will fail if an anonymous page is pinned in memory,
> * so avoid taking lru_lock and isolating it unnecessarily in an
> --
> 2.1.4
>

--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>--ooO--(_)--Ooo--