Re: [RFC 09/13] mm, compaction: make whole_zone flag ignore cached scanner positions

Michal Hocko
2016-05-13T13:23:13Z

On Tue 10-05-16 09:35:59, Vlastimil Babka wrote:
> A recent patch has added whole_zone flag that compaction sets when scanning
> starts from the zone boundary, in order to report that zone has been fully
> scanned in one attempt. For allocations that want to try really hard or cannot
> fail, we will want to introduce a mode where scanning whole zone is guaranteed
> regardless of the cached positions.
>
> This patch reuses the whole_zone flag in a way that if it's already passed true
> to compaction, the cached scanner positions are ignored. Employing this flag
> during reclaim/compaction loop will be done in the next patch. This patch
> however converts compaction invoked from userspace via procfs to use this flag.
> Before this patch, the cached positions were first reset to zone boundaries and
> then read back from struct zone, so there was a window where a parallel
> compaction could replace the reset values, making the manual compaction less
> effective. Using the flag instead of performing reset is more robust.

This makes perfect sense because user visible API should better have a
deterministic behavior. Playing scanner positions games which might race
with the system triggered compaction sounds quite unpredictable.

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

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/compaction.c | 15 +++++----------
> mm/internal.h | 2 +-
> 2 files changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index f649c7bc6de5..1ce6783d3ead 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1442,11 +1442,13 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
> */
> cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
> cc->free_pfn = zone->compact_cached_free_pfn;
> - if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
> + if (cc->whole_zone || cc->free_pfn < start_pfn ||
> + cc->free_pfn >= end_pfn) {
> cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
> zone->compact_cached_free_pfn = cc->free_pfn;
> }
> - if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
> + if (cc->whole_zone || cc->migrate_pfn < start_pfn ||
> + cc->migrate_pfn >= end_pfn) {
> cc->migrate_pfn = start_pfn;
> zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
> zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
> @@ -1693,14 +1695,6 @@ static void __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
> INIT_LIST_HEAD(&cc->freepages);
> INIT_LIST_HEAD(&cc->migratepages);
>
> - /*
> - * When called via /proc/sys/vm/compact_memory
> - * this makes sure we compact the whole zone regardless of
> - * cached scanner positions.
> - */
> - if (is_via_compact_memory(cc->order))
> - __reset_isolation_suitable(zone);
> -
> if (is_via_compact_memory(cc->order) ||
> !compaction_deferred(zone, cc->order))
> compact_zone(zone, cc);
> @@ -1736,6 +1730,7 @@ static void compact_node(int nid)
> .order = -1,
> .mode = MIGRATE_SYNC,
> .ignore_skip_hint = true,
> + .whole_zone = true,
> };
>
> __compact_pgdat(NODE_DATA(nid), &cc);
> diff --git a/mm/internal.h b/mm/internal.h
> index 556bc9d0a817..2acdee8ab0e6 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -178,7 +178,7 @@ struct compact_control {
> enum migrate_mode mode; /* Async or sync migration mode */
> bool ignore_skip_hint; /* Scan blocks even if marked skip */
> bool direct_compaction; /* False from kcompactd or /proc/... */
> - bool whole_zone; /* Whole zone has been scanned */
> + bool whole_zone; /* Whole zone should/has been scanned */
> int order; /* order a direct compactor needs */
> const gfp_t gfp_mask; /* gfp mask of a direct compactor */
> const unsigned int alloc_flags; /* alloc flags of a direct compactor */
> --
> 2.8.2
>
> --
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--
Michal Hocko
SUSE Labs