On 03/10/2017 10:53 AM, Yisheng Xie wrote:
> Hi Vlastimil,
>
> Thanks for comment.
> On 2017/3/10 15:30, Vlastimil Babka wrote:
>> On 03/10/2017 05:20 AM, Yisheng Xie wrote:
>>> If the migrate target is a large free page and we ignore suitable,
>>> it may not good for defrag. So move the ignore block suitable after
>>> check large free page.
>>
>> Right. But in practice I expect close to no impact, because direct
>> compaction shouldn't have to be called if there's a >=pageblock_order
>> page already available.
>>
> Maybe you are right and this change is just based on logical analyses.
I'm not opposing the change, it might be better for future-proofing the
function, just pointing out that it most likely won't have any visible
effect right now.
> Presently, only in direct compaction, we increase the compaction priority,
> and ignore suitable at MIN_COMPACT_PRIORITY. I have a silly question, can
> we do the similar thing in kcompactd? maybe by doing most work in kcompactd,
> we can get better perf of slow path.
That would need a very good evaluation at the very least. Migrating
pages into pageblocks other than movable ones brings the danger of later
unmovable/reclaimable allocations having to fallback to movable
pageblocks and causing permanent fragmentation. For direct compaction we
decided that it's better to risk permanent fragmentation than a
premature OOM, but for kcompactd there doesn't seem to be such
compelling reason.
> Thanks
> Yisheng Xie
>
>>> Signed-off-by: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
>>> ---
>>> mm/compaction.c | 6 +++---
>>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 0fdfde0..4bf2a5d 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -991,9 +991,6 @@ static bool too_many_isolated(struct zone *zone)
>>> static bool suitable_migration_target(struct compact_control *cc,
>>> struct page *page)
>>> {
>>> - if (cc->ignore_block_suitable)
>>> - return true;
>>> -
>>> /* If the page is a large free page, then disallow migration */
>>> if (PageBuddy(page)) {
>>> /*
>>> @@ -1005,6 +1002,9 @@ static bool suitable_migration_target(struct compact_control *cc,
>>> return false;
>>> }
>>>
>>> + if (cc->ignore_block_suitable)
>>> + return true;
>>> +
>>> /* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
>>> if (migrate_async_suitable(get_pageblock_migratetype(page)))
>>> return true;
>>>
>>
>>
>> .
>>
>
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