linux-kernel - Re: [PATCH 05/10] mm, compaction: remember position within pageblock in free pages scanner

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Date:	Wed, 11 Jun 2014 13:41:40 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>
CC:	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 05/10] mm, compaction: remember position within pageblock
 in free pages scanner

On 06/11/2014 10:16 AM, Joonsoo Kim wrote:
> On Wed, Jun 11, 2014 at 11:12:13AM +0900, Minchan Kim wrote:
>> On Mon, Jun 09, 2014 at 11:26:17AM +0200, Vlastimil Babka wrote:
>>> Unlike the migration scanner, the free scanner remembers the beginning of the
>>> last scanned pageblock in cc->free_pfn. It might be therefore rescanning pages
>>> uselessly when called several times during single compaction. This might have
>>> been useful when pages were returned to the buddy allocator after a failed
>>> migration, but this is no longer the case.
>>>
>>> This patch changes the meaning of cc->free_pfn so that if it points to a
>>> middle of a pageblock, that pageblock is scanned only from cc->free_pfn to the
>>> end. isolate_freepages_block() will record the pfn of the last page it looked
>>> at, which is then used to update cc->free_pfn.
>>>
>>> In the mmtests stress-highalloc benchmark, this has resulted in lowering the
>>> ratio between pages scanned by both scanners, from 2.5 free pages per migrate
>>> page, to 2.25 free pages per migrate page, without affecting success rates.
>>>
>>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> Reviewed-by: Minchan Kim <minchan@...nel.org>
>>
>> Below is a nitpick.
>>
>>> Cc: Minchan Kim <minchan@...nel.org>
>>> Cc: Mel Gorman <mgorman@...e.de>
>>> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
>>> Cc: Michal Nazarewicz <mina86@...a86.com>
>>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>>> Cc: Christoph Lameter <cl@...ux.com>
>>> Cc: Rik van Riel <riel@...hat.com>
>>> Cc: David Rientjes <rientjes@...gle.com>
>>> ---
>>>   mm/compaction.c | 33 ++++++++++++++++++++++++++++-----
>>>   1 file changed, 28 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 83f72bd..58dfaaa 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -297,7 +297,7 @@ static bool suitable_migration_target(struct page *page)
>>>    * (even though it may still end up isolating some pages).
>>>    */
>>>   static unsigned long isolate_freepages_block(struct compact_control *cc,
>>> -				unsigned long blockpfn,
>>> +				unsigned long *start_pfn,
>>>   				unsigned long end_pfn,
>>>   				struct list_head *freelist,
>>>   				bool strict)
>>> @@ -306,6 +306,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>>   	struct page *cursor, *valid_page = NULL;
>>>   	unsigned long flags;
>>>   	bool locked = false;
>>> +	unsigned long blockpfn = *start_pfn;
>>>
>>>   	cursor = pfn_to_page(blockpfn);
>>>
>>> @@ -314,6 +315,9 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>>   		int isolated, i;
>>>   		struct page *page = cursor;
>>>
>>> +		/* Record how far we have got within the block */
>>> +		*start_pfn = blockpfn;
>>> +
>>
>> Couldn't we move this out of the loop for just one store?

Ah, I get it now. Ignore my previous reply.

> Hello, Vlastimil.
>
> Moreover, start_pfn can't be updated to end pfn with this approach.
> Is it okay?

That's intentional, as end_pfn means the scanner would restart at the 
beginning of next pageblock. So I want to record last pfn *inside* the 
pageblock that was fully scanned. Note that there's a high change that 
fully scanning pageblock means that I haven't isolated enough and 
isolate_freepages() will advance to the previous pageblock anyway, and 
the recorded value will be overwritten. But still it's better to prevent 
this corner case.

So outside the loop, I would need to do:

*start_pfn = max(blockpfn, end_pfn - 1);

It looks a bit tricky but probably better than multiple assignments.

Thanks.

> Thanks.
>

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