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Date:	Fri, 27 Jun 2014 14:57:08 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
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 Wed, Jun 25, 2014 at 10:59:19AM +0200, Vlastimil Babka wrote:
> 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.
> 

Hello,

I have no idea here. :)
I saw the discussion between you and Naoya, and I also wonder when
we should check page_zone == cc->zone. Maybe we need help from more
experienced developers such as Andrew and others.

Thanks.
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