Re: [PATCH v3 04/13] mm, compaction: move pageblock checks up from isolate_migratepages_range()

[Joonsoo Kim]

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@xxxxxxx>
> >>Cc: Minchan Kim <minchan@xxxxxxxxxx>
> >>Cc: Mel Gorman <mgorman@xxxxxxx>
> >>Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >>Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> >>Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> >>Cc: Christoph Lameter <cl@xxxxxxxxx>
> >>Cc: Rik van Riel <riel@xxxxxxxxxx>
> >>Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >>---
> >>  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.

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

Thanks.
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