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Date:	Thu, 19 Mar 2015 17:52:01 +0900
From:	Gioh Kim <gioh.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
	rientjes@...gle.com, iamjoonsoo.kim@....com, mgorman@...e.de
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, gunho.lee@....com
Subject: Re: [PATCH] [RFC] mm/compaction: initialize compaction information



2015-03-19 오후 5:41에 Vlastimil Babka 이(가) 쓴 글:
> On 03/19/2015 06:30 AM, Gioh Kim wrote:
>> I tried to start compaction via /proc/sys/vm/compact_memory
>> as soon as I turned on my ARM-based platform.
>> But the compaction didn't start.
>> I found some variables in struct zone are not initalized.
>>
>> I think zone->compact_cached_free_pfn and some cache values for compaction
>> are initalized when the kernel starts compaction, not via
>> /proc/sys/vm/compact_memory.
>> If my guess is correct, an initialization are needed for that case.
>>
>>
>> Signed-off-by: Gioh Kim <gioh.kim@....com>
>> ---
>>   mm/compaction.c |    8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 8c0d945..944a9cc 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1299,6 +1299,14 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>>   		__reset_isolation_suitable(zone);
>>
>>   	/*
>> +	 * If this is activated by /proc/sys/vm/compact_memory
>> +	 * and the first try, cached information for compaction is not
>> +	 * initialized.
>> +	 */
>> +	if (cc->order == -1 && zone->compact_cached_free_pfn == 0)
>> +		__reset_isolation_suitable(zone);
>> +
>> +	/*
>>   	 * Setup to move all movable pages to the end of the zone. Used cached
>>   	 * information on where the scanners should start but check that it
>>   	 * is initialised by ensuring the values are within zone boundaries.
>
> The code below this comment already does the initialization if the cached values
> are outside zone boundaries (e.g. due to not being initialized). So if I go
> through what your __reset_isolation_suitable(zone) call possibly fixes:
>
> - the code below comment should take care of zone->compact_cached_migrate_pfn
> and zone->compact_cached_free_pfn.
> - the value of zone->compact_blockskip_flush shouldn't affect whether compaction
> is done.
> - the state of pageblock_skip bits shouldn't matter for compaction via
> /proc/sys... as that sets ignore_skip_hint = true
>
> It might be perhaps possible that the cached scanner positions are close to
> meeting and compaction occurs but doesn't process much. That would be also true
> if both were zero, but at least on my x86 system, lowest zone's start_pfn is 1
> so that would be detected and corrected. Maybe it is zero on yours though? (ARM?).

YES, it is. As comment above, my platform is based on ARM.
zone's start_pfn is 0.

>
> So in any case, the problem should be identified in more detail so we know the
> fix is not accidental. It could be also worthwile to always reset scanner
> positions when doing a /proc triggered compaction, so it's not depending on what
> happened before.
>

Excuse my poor english.
I cannot catch exactly what you want.
Is this what you want? This resets the position if compaction is started via /proc.


diff --git a/mm/compaction.c b/mm/compaction.c
index 8c0d945..827ec06 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1587,8 +1587,10 @@ static void __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
                 INIT_LIST_HEAD(&cc->freepages);
                 INIT_LIST_HEAD(&cc->migratepages);

-               if (cc->order == -1 || !compaction_deferred(zone, cc->order))
+               if (cc->order == -1 || !compaction_deferred(zone, cc->order)) {
+                       __reset_isolation_suitable(zone);
                         compact_zone(zone, cc);
+               }

                 if (cc->order > 0) {
                         if (zone_watermark_ok(zone, cc->order,

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