Date:	Wed, 25 Jun 2014 10:59:19 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>, 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>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/13] mm, compaction: move pageblock checks up from
 isolate_migratepages_range()

On 06/25/2014 02:53 AM, Joonsoo Kim wrote:
> On Tue, Jun 24, 2014 at 05:42:50PM +0200, Vlastimil Babka wrote:
>> On 06/24/2014 10:33 AM, Joonsoo Kim wrote:
>>> On Fri, Jun 20, 2014 at 05:49:34PM +0200, Vlastimil Babka wrote:
>>>> isolate_migratepages_range() is the main function of the compaction scanner,
>>>> called either on a single pageblock by isolate_migratepages() during regular
>>>> compaction, or on an arbitrary range by CMA's __alloc_contig_migrate_range().
>>>> It currently perfoms two pageblock-wide compaction suitability checks, and
>>>> because of the CMA callpath, it tracks if it crossed a pageblock boundary in
>>>> order to repeat those checks.
>>>>
>>>> However, closer inspection shows that those checks are always true for CMA:
>>>> - isolation_suitable() is true because CMA sets cc->ignore_skip_hint to true
>>>> - migrate_async_suitable() check is skipped because CMA uses sync compaction
>>>>
>>>> We can therefore move the checks to isolate_migratepages(), reducing variables
>>>> and simplifying isolate_migratepages_range(). The update_pageblock_skip()
>>>> function also no longer needs set_unsuitable parameter.
>>>>
>>>> Furthermore, going back to compact_zone() and compact_finished() when pageblock
>>>> is unsuitable is wasteful - the checks are meant to skip pageblocks quickly.
>>>> The patch therefore also introduces a simple loop into isolate_migratepages()
>>>> so that it does not return immediately on pageblock checks, but keeps going
>>>> until isolate_migratepages_range() gets called once. Similarily to
>>>> isolate_freepages(), the function periodically checks if it needs to reschedule
>>>> or abort async compaction.
>>>>
>>>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>>>> 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 | 112 +++++++++++++++++++++++++++++---------------------------
>>>>   1 file changed, 59 insertions(+), 53 deletions(-)
>>>>
>>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>>> index 3064a7f..ebe30c9 100644
>>>> --- a/mm/compaction.c
>>>> +++ b/mm/compaction.c
>>>> @@ -132,7 +132,7 @@ void reset_isolation_suitable(pg_data_t *pgdat)
>>>>    */
>>>>   static void update_pageblock_skip(struct compact_control *cc,
>>>>   			struct page *page, unsigned long nr_isolated,
>>>> -			bool set_unsuitable, bool migrate_scanner)
>>>> +			bool migrate_scanner)
>>>>   {
>>>>   	struct zone *zone = cc->zone;
>>>>   	unsigned long pfn;
>>>> @@ -146,12 +146,7 @@ static void update_pageblock_skip(struct compact_control *cc,
>>>>   	if (nr_isolated)
>>>>   		return;
>>>>
>>>> -	/*
>>>> -	 * Only skip pageblocks when all forms of compaction will be known to
>>>> -	 * fail in the near future.
>>>> -	 */
>>>> -	if (set_unsuitable)
>>>> -		set_pageblock_skip(page);
>>>> +	set_pageblock_skip(page);
>>>>
>>>>   	pfn = page_to_pfn(page);
>>>>
>>>> @@ -180,7 +175,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
>>>>
>>>>   static void update_pageblock_skip(struct compact_control *cc,
>>>>   			struct page *page, unsigned long nr_isolated,
>>>> -			bool set_unsuitable, bool migrate_scanner)
>>>> +			bool migrate_scanner)
>>>>   {
>>>>   }
>>>>   #endif /* CONFIG_COMPACTION */
>>>> @@ -345,8 +340,7 @@ isolate_fail:
>>>>
>>>>   	/* Update the pageblock-skip if the whole pageblock was scanned */
>>>>   	if (blockpfn == end_pfn)
>>>> -		update_pageblock_skip(cc, valid_page, total_isolated, true,
>>>> -				      false);
>>>> +		update_pageblock_skip(cc, valid_page, total_isolated, false);
>>>>
>>>>   	count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
>>>>   	if (total_isolated)
>>>> @@ -474,14 +468,12 @@ unsigned long
>>>>   isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>>>>   		unsigned long low_pfn, unsigned long end_pfn, bool unevictable)
>>>>   {
>>>> -	unsigned long last_pageblock_nr = 0, pageblock_nr;
>>>>   	unsigned long nr_scanned = 0, nr_isolated = 0;
>>>>   	struct list_head *migratelist = &cc->migratepages;
>>>>   	struct lruvec *lruvec;
>>>>   	unsigned long flags;
>>>>   	bool locked = false;
>>>>   	struct page *page = NULL, *valid_page = NULL;
>>>> -	bool set_unsuitable = true;
>>>>   	const isolate_mode_t mode = (cc->mode == MIGRATE_ASYNC ?
>>>>   					ISOLATE_ASYNC_MIGRATE : 0) |
>>>>   				    (unevictable ? ISOLATE_UNEVICTABLE : 0);
>>>> @@ -545,28 +537,6 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>>>>   		if (!valid_page)
>>>>   			valid_page = page;
>>>>
>>>> -		/* If isolation recently failed, do not retry */
>>>> -		pageblock_nr = low_pfn >> pageblock_order;
>>>> -		if (last_pageblock_nr != pageblock_nr) {
>>>> -			int mt;
>>>> -
>>>> -			last_pageblock_nr = pageblock_nr;
>>>> -			if (!isolation_suitable(cc, page))
>>>> -				goto next_pageblock;
>>>> -
>>>> -			/*
>>>> -			 * For async migration, also only scan in MOVABLE
>>>> -			 * blocks. Async migration is optimistic to see if
>>>> -			 * the minimum amount of work satisfies the allocation
>>>> -			 */
>>>> -			mt = get_pageblock_migratetype(page);
>>>> -			if (cc->mode == MIGRATE_ASYNC &&
>>>> -			    !migrate_async_suitable(mt)) {
>>>> -				set_unsuitable = false;
>>>> -				goto next_pageblock;
>>>> -			}
>>>> -		}
>>>> -
>>>>   		/*
>>>>   		 * Skip if free. page_order cannot be used without zone->lock
>>>>   		 * as nothing prevents parallel allocations or buddy merging.
>>>> @@ -668,8 +638,7 @@ next_pageblock:
>>>>   	 * if the whole pageblock was scanned without isolating any page.
>>>>   	 */
>>>>   	if (low_pfn == end_pfn)
>>>> -		update_pageblock_skip(cc, valid_page, nr_isolated,
>>>> -				      set_unsuitable, true);
>>>> +		update_pageblock_skip(cc, valid_page, nr_isolated, true);
>>>>
>>>>   	trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
>>>>
>>>> @@ -840,34 +809,74 @@ typedef enum {
>>>>   } isolate_migrate_t;
>>>>
>>>>   /*
>>>> - * Isolate all pages that can be migrated from the block pointed to by
>>>> - * the migrate scanner within compact_control.
>>>> + * Isolate all pages that can be migrated from the first suitable block,
>>>> + * starting at the block pointed to by the migrate scanner pfn within
>>>> + * compact_control.
>>>>    */
>>>>   static isolate_migrate_t isolate_migratepages(struct zone *zone,
>>>>   					struct compact_control *cc)
>>>>   {
>>>>   	unsigned long low_pfn, end_pfn;
>>>> +	struct page *page;
>>>>
>>>> -	/* Do not scan outside zone boundaries */
>>>> -	low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);
>>>> +	/* Start at where we last stopped, or beginning of the zone */
>>>> +	low_pfn = cc->migrate_pfn;
>>>>
>>>>   	/* Only scan within a pageblock boundary */
>>>>   	end_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages);
>>>>
>>>> -	/* Do not cross the free scanner or scan within a memory hole */
>>>> -	if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
>>>> -		cc->migrate_pfn = end_pfn;
>>>> -		return ISOLATE_NONE;
>>>> -	}
>>>> +	/*
>>>> +	 * Iterate over whole pageblocks until we find the first suitable.
>>>> +	 * Do not cross the free scanner.
>>>> +	 */
>>>> +	for (; end_pfn <= cc->free_pfn;
>>>> +			low_pfn = end_pfn, end_pfn += pageblock_nr_pages) {
>>>> +
>>>> +		/*
>>>> +		 * This can potentially iterate a massively long zone with
>>>> +		 * many pageblocks unsuitable, so periodically check if we
>>>> +		 * need to schedule, or even abort async compaction.
>>>> +		 */
>>>> +		if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
>>>> +						&& compact_should_abort(cc))
>>>> +			break;
>>>>
>>>> -	/* Perform the isolation */
>>>> -	low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn, false);
>>>> -	if (!low_pfn || cc->contended)
>>>> -		return ISOLATE_ABORT;
>>>> +		/* Do not scan within a memory hole */
>>>> +		if (!pfn_valid(low_pfn))
>>>> +			continue;
>>>> +
>>>> +		page = pfn_to_page(low_pfn);
>>>> +		/* If isolation recently failed, do not retry */
>>>> +		if (!isolation_suitable(cc, page))
>>>> +			continue;
>>>>
>>>> +		/*
>>>> +		 * For async compaction, also only scan in MOVABLE blocks.
>>>> +		 * Async compaction is optimistic to see if the minimum amount
>>>> +		 * of work satisfies the allocation.
>>>> +		 */
>>>> +		if (cc->mode == MIGRATE_ASYNC &&
>>>> +		    !migrate_async_suitable(get_pageblock_migratetype(page)))
>>>> +			continue;
>>>> +
>>>> +		/* Perform the isolation */
>>>> +		low_pfn = isolate_migratepages_range(zone, cc, low_pfn,
>>>> +								end_pfn, false);
>>>> +		if (!low_pfn || cc->contended)
>>>> +			return ISOLATE_ABORT;
>>>> +
>>>> +		/*
>>>> +		 * Either we isolated something and proceed with migration. Or
>>>> +		 * we failed and compact_zone should decide if we should
>>>> +		 * continue or not.
>>>> +		 */
>>>> +		break;
>>>> +	}
>>>> +
>>>> +	/* Record where migration scanner will be restarted */
>>>
>>> If we make isolate_migratepages* interface like as isolate_freepages*,
>>> we can get more clean and micro optimized code. Because
>>> isolate_migratepages_range() can handle arbitrary range and this patch
>>> make isolate_migratepages() also handle arbitrary range, there would
>>> be some redundant codes. :)
>>
>> I'm not sure if it's worth already. Where is the arbitrary range
>> adding overhead? I can only imagine that next_pageblock: label could
>> do a 'break;' instead of setting up next_capture_pfn, but that's
>> about it AFAICS.
>
> In fact, there is just minor overhead, pfn_valid().
> And isolate_freepages variants seems to do this correctly. :)
>
> Someone could wonder why there are two isolate_migratepages variants
> with arbitrary range compaction ability. IMHO, one
> isolate_migratepage_xxx for pageblock range and two
> isolate_migratepage_yyy/zzz for compaction and CMA is better
> architecture.

OK, I will try. But we need to resolve the "where to test for 
page_zone() == cc->zone?" question.

> And, One additional note. You can move update_pageblock_skip() to
> isolate_migratepages() now.

Right, thanks.

> Thanks.
>

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