On 03/16/2017 03:18 AM, Joonsoo Kim wrote:
> On Tue, Mar 07, 2017 at 02:15:45PM +0100, Vlastimil Babka wrote:
>> The main goal of direct compaction is to form a high-order page for allocation,
>> but it should also help against long-term fragmentation when possible. Most
>> lower-than-pageblock-order compactions are for non-movable allocations, which
>> means that if we compact in a movable pageblock and terminate as soon as we
>> create the high-order page, it's unlikely that the fallback heuristics will
>> claim the whole block. Instead there might be a single unmovable page in a
>> pageblock full of movable pages, and the next unmovable allocation might pick
>> another pageblock and increase long-term fragmentation.
>>
>> To help against such scenarios, this patch changes the termination criteria for
>> compaction so that the current pageblock is finished even though the high-order
>> page already exists. Note that it might be possible that the high-order page
>> formed elsewhere in the zone due to parallel activity, but this patch doesn't
>> try to detect that.
>>
>> This is only done with sync compaction, because async compaction is limited to
>> pageblock of the same migratetype, where it cannot result in a migratetype
>> fallback. (Async compaction also eagerly skips order-aligned blocks where
>> isolation fails, which is against the goal of migrating away as much of the
>> pageblock as possible.)
>>
>> As a result of this patch, long-term memory fragmentation should be reduced.
>>
>> In testing based on 4.9 kernel with stress-highalloc from mmtests configured
>> for order-4 GFP_KERNEL allocations, this patch has reduced the number of
>> unmovable allocations falling back to movable pageblocks by 20%. The number
>>
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>> ---
>> mm/compaction.c | 36 ++++++++++++++++++++++++++++++++++--
>> mm/internal.h | 1 +
>> 2 files changed, 35 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 2c288e75840d..bc7903130501 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1318,6 +1318,17 @@ static enum compact_result __compact_finished(struct zone *zone,
>> if (is_via_compact_memory(cc->order))
>> return COMPACT_CONTINUE;
>>
>> + if (cc->finishing_block) {
>> + /*
>> + * We have finished the pageblock, but better check again that
>> + * we really succeeded.
>> + */
>> + if (IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
>> + cc->finishing_block = false;
>> + else
>> + return COMPACT_CONTINUE;
>> + }
>> +
>> /* Direct compactor: Is a suitable page free? */
>> for (order = cc->order; order < MAX_ORDER; order++) {
>> struct free_area *area = &zone->free_area[order];
>> @@ -1338,8 +1349,29 @@ static enum compact_result __compact_finished(struct zone *zone,
>> * other migratetype buddy lists.
>> */
>> if (find_suitable_fallback(area, order, migratetype,
>> - true, &can_steal) != -1)
>> - return COMPACT_SUCCESS;
>> + true, &can_steal) != -1) {
>> +
>> + /* movable pages are OK in any pageblock */
>> + if (migratetype == MIGRATE_MOVABLE)
>> + return COMPACT_SUCCESS;
>> +
>> + /*
>> + * We are stealing for a non-movable allocation. Make
>> + * sure we finish compacting the current pageblock
>> + * first so it is as free as possible and we won't
>> + * have to steal another one soon. This only applies
>> + * to sync compaction, as async compaction operates
>> + * on pageblocks of the same migratetype.
>> + */
>> + if (cc->mode == MIGRATE_ASYNC ||
>> + IS_ALIGNED(cc->migrate_pfn,
>> + pageblock_nr_pages)) {
>> + return COMPACT_SUCCESS;
>> + }
>
> If cc->migratetype and cc->migrate_pfn's migratetype is the same, stopping
> the compaction here doesn't cause any fragmentation. Do we need to
> compact full pageblock in this case?
Probably not, but if we make patch 7/8 less aggressive, then I'd rather keep
this chance of clearing a whole unmovable pageblock of movable pages.
I also realized that the finishing_block flag is just an unnecessary
complication. Just keep going until migrate_pfn hits the end of pageblock, and
only then check the termination criteria.
> Thanks.
>
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
