Date:	Wed, 23 Apr 2014 09:30:16 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Heesub Shin <heesub.shin@...sung.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Dongjun Shin <d.j.shin@...sung.com>,
	Sunghwan Yun <sunghwan.yun@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	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 2/2] mm/compaction: cleanup isolate_freepages()

On 04/23/2014 04:58 AM, Joonsoo Kim wrote:
> On Tue, Apr 22, 2014 at 03:17:30PM +0200, Vlastimil Babka wrote:
>> On 04/22/2014 08:52 AM, Minchan Kim wrote:
>>> On Tue, Apr 22, 2014 at 08:33:35AM +0200, Vlastimil Babka wrote:
>>>> On 22.4.2014 1:53, Minchan Kim wrote:
>>>>> On Mon, Apr 21, 2014 at 11:43:24PM +0200, Vlastimil Babka wrote:
>>>>>> On 21.4.2014 21:41, Andrew Morton wrote:
>>>>>>> On Thu, 17 Apr 2014 09:07:45 +0900 Minchan Kim <minchan@...nel.org> wrote:
>>>>>>>
>>>>>>>> Hi Vlastimil,
>>>>>>>>
>>>>>>>> Below just nitpicks.
>>>>>>> It seems you were ignored ;)
>>>>>> Oops, I managed to miss your e-mail, sorry.
>>>>>>
>>>>>>>>>  {
>>>>>>>>>  	struct page *page;
>>>>>>>>> -	unsigned long high_pfn, low_pfn, pfn, z_end_pfn;
>>>>>>>>> +	unsigned long pfn, low_pfn, next_free_pfn, z_end_pfn;
>>>>>>>> Could you add comment for each variable?
>>>>>>>>
>>>>>>>> unsigned long pfn; /* scanning cursor */
>>>>>>>> unsigned long low_pfn; /* lowest pfn free scanner is able to scan */
>>>>>>>> unsigned long next_free_pfn; /* start pfn for scaning at next truen */
>>>>>>>> unsigned long z_end_pfn; /* zone's end pfn */
>>>>>>>>
>>>>>>>>
>>>>>>>>> @@ -688,11 +688,10 @@ static void isolate_freepages(struct zone *zone,
>>>>>>>>>  	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
>>>>>>>>>  	/*
>>>>>>>>> -	 * Take care that if the migration scanner is at the end of the zone
>>>>>>>>> -	 * that the free scanner does not accidentally move to the next zone
>>>>>>>>> -	 * in the next isolation cycle.
>>>>>>>>> +	 * Seed the value for max(next_free_pfn, pfn) updates. If there are
>>>>>>>>> +	 * none, the pfn < low_pfn check will kick in.
>>>>>>>>        "none" what? I'd like to clear more.
>>>>>> If there are no updates to next_free_pfn within the for cycle. Which
>>>>>> matches Andrew's formulation below.
>>>>>>
>>>>>>> I did this:
>>>>>> Thanks!
>>>>>>
>>>>>>> --- a/mm/compaction.c~mm-compaction-cleanup-isolate_freepages-fix
>>>>>>> +++ a/mm/compaction.c
>>>>>>> @@ -662,7 +662,10 @@ static void isolate_freepages(struct zon
>>>>>>>  				struct compact_control *cc)
>>>>>>>  {
>>>>>>>  	struct page *page;
>>>>>>> -	unsigned long pfn, low_pfn, next_free_pfn, z_end_pfn;
>>>>>>> +	unsigned long pfn;	     /* scanning cursor */
>>>>>>> +	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
>>>>>>> +	unsigned long next_free_pfn; /* start pfn for scaning at next round */
>>>>>>> +	unsigned long z_end_pfn;     /* zone's end pfn */
>>>>>> Yes that works.
>>>>>>
>>>>>>>  	int nr_freepages = cc->nr_freepages;
>>>>>>>  	struct list_head *freelist = &cc->freepages;
>>>>>>> @@ -679,8 +682,8 @@ static void isolate_freepages(struct zon
>>>>>>>  	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
>>>>>>>  	/*
>>>>>>> -	 * Seed the value for max(next_free_pfn, pfn) updates. If there are
>>>>>>> -	 * none, the pfn < low_pfn check will kick in.
>>>>>>> +	 * Seed the value for max(next_free_pfn, pfn) updates. If no pages are
>>>>>>> +	 * isolated, the pfn < low_pfn check will kick in.
>>>>>> OK.
>>>>>>
>>>>>>>  	 */
>>>>>>>  	next_free_pfn = 0;
>>>>>>>>> @@ -766,9 +765,9 @@ static void isolate_freepages(struct zone *zone,
>>>>>>>>>  	 * so that compact_finished() may detect this
>>>>>>>>>  	 */
>>>>>>>>>  	if (pfn < low_pfn)
>>>>>>>>> -		cc->free_pfn = max(pfn, zone->zone_start_pfn);
>>>>>>>>> -	else
>>>>>>>>> -		cc->free_pfn = high_pfn;
>>>>>>>>> +		next_free_pfn = max(pfn, zone->zone_start_pfn);
>>>>>>>> Why we need max operation?
>>>>>>>> IOW, what's the problem if we do (next_free_pfn = pfn)?
>>>>>>> An answer to this would be useful, thanks.
>>>>>> The idea (originally, not new here) is that the free scanner wants
>>>>>> to remember the highest-pfn
>>>>>> block where it managed to isolate some pages. If the following page
>>>>>> migration fails, these isolated
>>>>>> pages might be put back and would be skipped in further compaction
>>>>>> attempt if we used just
>>>>>> "next_free_pfn = pfn", until the scanners get reset.
>>>>>>
>>>>>> The question of course is if such situations are frequent and makes
>>>>>> any difference to compaction
>>>>>> outcome. And the downsides are potentially useless rescans and code
>>>>>> complexity. Maybe Mel
>>>>>> remembers how important this is? It should probably be profiled
>>>>>> before changes are made.
>>>>> I didn't mean it. What I mean is code snippet you introduced in 7ed695e069c3c.
>>>>> At that time, I didn't Cced so I missed that code so let's ask this time.
>>>>> In that patch, you added this.
>>>>>
>>>>> if (pfn < low_pfn)
>>>>>   cc->free_pfn = max(pfn, zone->zone_start_pfn);
>>>>> else
>>>>>   cc->free_pfn = high_pfn;
>>>>
>>>> Oh, right, this max(), not the one in the for loop. Sorry, I should
>>>> have read more closely.
>>>> But still maybe it's a good opportunity to kill the other max() as
>>>> well. I'll try some testing.
>>>>
>>>> Anyway, this is what I answered to Mel when he asked the same thing
>>>> when I sent
>>>> that 7ed695069c3c patch:
>>>>
>>>> If a zone starts in a middle of a pageblock and migrate scanner isolates
>>>> enough pages early to stay within that pageblock, low_pfn will be at the
>>>> end of that pageblock and after the for cycle in this function ends, pfn
>>>> might be at the beginning of that pageblock. It might not be an actual
>>>> problem (this compaction will finish at this point, and if someone else
>>>> is racing, he will probably check the boundaries himself), but I played
>>>> it safe.
>>>>
>>>>
>>>>> So the purpose of max(pfn, zone->zone_start_pfn) is to be detected by
>>>>> compact_finished to stop compaction. And your [1/2] patch in this patchset
>>>>> always makes free page scanner start on pageblock boundary so when the
>>>>> loop in isolate_freepages is finished and pfn is lower low_pfn, the pfn
>>>>> would be lower than migration scanner so compact_finished will always detect
>>>>> it so I think you could just do
>>>>>
>>>>> if (pfn < low_pfn)
>>>>>   next_free_pfn = pfn;
>>>>>
>>>>> cc->free_pfn = next_free_pfn;
>>>>
>>>> That could work. I was probably wrong about danger of racing in the
>>>> reply to Mel,
>>>> because free_pfn is stored in cc (private), not zone (shared).
>>>>
>>>>>
>>>>> Or, if you want to clear *reset*,
>>>>> if (pfn < lown_pfn)
>>>>>   next_free_pfn = zone->zone_start_pfn;
>>>>>
>>>>> cc->free_pfn = next_free_pfn;
>>>>
>>>> That would work as well but is less straightforward I think. Might
>>>> be misleading if
>>>> someone added tracepoints to track the free scanner progress with
>>>> pfn's (which
>>>> might happen soon...)
>>>
>>> My preference is to add following with pair of compact_finished
>>>
>>> static inline void finish_compact(struct compact_control *cc)
>>> {
>>>   cc->free_pfn = cc->migrate_pfn;
>>> }
>>
>> Yes setting free_pfn to migrate_pfn is probably the best way, as these
>> are the values compared in compact_finished. But I wouldn't introduce a
>> new function just for one instance of this. Also compact_finished()
>> doesn't test just the scanners to decide whether compaction should
>> continue, so the pairing would be imperfect anyway.
>> So Andrew, if you agree can you please fold in the patch below.
>>
>>> But I don't care.
>>> If you didn't send this patch as clean up, I would never interrupt
>>> on the way but you said it's cleanup patch and the one made me spend a
>>> few minutes to understand the code so it's not a clean up patch. ;-).
>>> So, IMO, it's worth to tidy it up.
>>
>> Yes, I understand and agree.
>>
>> ------8<------
>> From: Vlastimil Babka <vbabka@...e.cz>
>> Date: Tue, 22 Apr 2014 13:55:36 +0200
>> Subject: mm-compaction-cleanup-isolate_freepages-fix2
>>
>> Cleanup detection of compaction scanners crossing in isolate_freepages().
>> To make sure compact_finished() observes scanners crossing, we can just set
>> free_pfn to migrate_pfn instead of confusing max() construct.
>>
>> Suggested-by: Minchan Kim <minchan@...nel.org>
>> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
>> Cc: Christoph Lameter <cl@...ux.com>
>> Cc: Dongjun Shin <d.j.shin@...sung.com>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Michal Nazarewicz <mina86@...a86.com>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: Sunghwan Yun <sunghwan.yun@...sung.com>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>>
>> ---
>>  mm/compaction.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 37c15fe..1c992dc 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -768,7 +768,7 @@ static void isolate_freepages(struct zone *zone,
>>  	 * so that compact_finished() may detect this
>>  	 */
>>  	if (pfn < low_pfn)
>> -		next_free_pfn = max(pfn, zone->zone_start_pfn);
>> +		next_free_pfn = cc->migrate_pfn;
>>  
>>  	cc->free_pfn = next_free_pfn;
>>  	cc->nr_freepages = nr_freepages;
>> -- 
>> 1.8.4.5
>>
> 
> Hello,
> 
> How about doing more clean-up at this time?
> 
> What I did is that taking end_pfn out of the loop and consider zone
> boundary once. After then, we just subtract pageblock_nr_pages on
> every iteration. With this change, we can remove local variable, z_end_pfn.
> Another things I did are removing max() operation and un-needed
> assignment to isolate variable.
> 
> Thanks.
> 
> --------->8------------
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 1c992dc..95a506d 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -671,10 +671,10 @@ static void isolate_freepages(struct zone *zone,
>  				struct compact_control *cc)
>  {
>  	struct page *page;
> -	unsigned long pfn;	     /* scanning cursor */
> +	unsigned long pfn;	     /* start of scanning window */
> +	unsigned long end_pfn;	     /* end of scanning window */
>  	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
>  	unsigned long next_free_pfn; /* start pfn for scaning at next round */
> -	unsigned long z_end_pfn;     /* zone's end pfn */
>  	int nr_freepages = cc->nr_freepages;
>  	struct list_head *freelist = &cc->freepages;
>  
> @@ -688,15 +688,16 @@ static void isolate_freepages(struct zone *zone,
>  	 * is using.
>  	 */
>  	pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
> -	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
>  
>  	/*
> -	 * Seed the value for max(next_free_pfn, pfn) updates. If no pages are
> -	 * isolated, the pfn < low_pfn check will kick in.
> +	 * Take care when isolating in last pageblock of a zone which
> +	 * ends in the middle of a pageblock.
>  	 */
> -	next_free_pfn = 0;
> +	end_pfn = min(pfn + pageblock_nr_pages, zone_end_pfn(zone));
> +	low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
>  
> -	z_end_pfn = zone_end_pfn(zone);
> +	/* If no pages are isolated, the pfn < low_pfn check will kick in. */
> +	next_free_pfn = 0;
>  
>  	/*
>  	 * Isolate free pages until enough are available to migrate the
> @@ -704,9 +705,8 @@ static void isolate_freepages(struct zone *zone,
>  	 * and free page scanners meet or enough free pages are isolated.
>  	 */
>  	for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
> -					pfn -= pageblock_nr_pages) {
> +		pfn -= pageblock_nr_pages, end_pfn -= pageblock_nr_pages) {

If zone_end_pfn was in the middle of a pageblock, then your end_pfn will
always be in the middle of a pageblock and you will not scan half of all
pageblocks.

>  		unsigned long isolated;
> -		unsigned long end_pfn;
>  
>  		/*
>  		 * This can iterate a massively long zone without finding any
> @@ -738,13 +738,6 @@ static void isolate_freepages(struct zone *zone,
>  			continue;
>  
>  		/* Found a block suitable for isolating free pages from */
> -		isolated = 0;
> -
> -		/*
> -		 * Take care when isolating in last pageblock of a zone which
> -		 * ends in the middle of a pageblock.
> -		 */
> -		end_pfn = min(pfn + pageblock_nr_pages, z_end_pfn);
>  		isolated = isolate_freepages_block(cc, pfn, end_pfn,
>  						   freelist, false);
>  		nr_freepages += isolated;
> @@ -754,9 +747,9 @@ static void isolate_freepages(struct zone *zone,
>  		 * looking for free pages, the search will restart here as
>  		 * page migration may have returned some pages to the allocator
>  		 */
> -		if (isolated) {
> +		if (isolated && next_free_pfn == 0) {
>  			cc->finished_update_free = true;
> -			next_free_pfn = max(next_free_pfn, pfn);
> +			next_free_pfn = pfn;
>  		}
>  	}
>  
> 

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