Re: [PATCH 2/2] mm/compaction: cleanup isolate_freepages()

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On Wed, Apr 23, 2014 at 04:31:14PM +0200, Vlastimil Babka wrote:
> >>>
> >>> 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.
> >>
> > 
> > Okay. I think a way to fix it.
> > By assigning pfn(start of scanning window) to
> > end_pfn(end of scanning window) for the next loop, we can solve the problem
> > you mentioned. How about below?
> > 
> > -             pfn -= pageblock_nr_pages, end_pfn -= pageblock_nr_pages) {
> > +            end_pfn = pfn, pfn -= pageblock_nr_pages) {
> 
> Hm that's perhaps a bit subtle but it would work.
> Maybe better names for pfn and end_pfn would be block_start_pfn and
> block_end_pfn. And in those comments, s/scanning window/current pageblock/.
> And please don't move the low_pfn assignment like you did. The comment
> above the original location explains it, the comment above the new
> location doesn't. It's use in the loop is also related to 'pfn', not
> 'end_pfn'.

Okay.
Following patch solves all your concerns.
End result looks so nice to me. :)

Thanks.

--------->8----------------
>From ae653cf8b9f8c7423cad73af38cde94eec564b50 Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Date: Fri, 25 Apr 2014 17:12:58 +0900
Subject: [PATCH] mm-compaction-cleanup-isolate_freepages-fix3

What I did here is taking end_pfn out of the loop and considering zone
boundary once. After then, we can just set previous pfn to end_pfn on
every iteration to move scanning window. 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.

In addition, I change both the variable names, from pfn and
end_pfn to block_start_pfn and block_end_pfn, respectively.
They represent their meaning perfectly.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/compaction.c b/mm/compaction.c
index 1c992dc..ba80bea 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 block_start_pfn;	/* start of current pageblock */
+	unsigned long block_end_pfn;	/* end of current pageblock */
 	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;
 
@@ -682,31 +682,33 @@ static void isolate_freepages(struct zone *zone,
 	 * Initialise the free scanner. The starting point is where we last
 	 * successfully isolated from, zone-cached value, or the end of the
 	 * zone when isolating for the first time. We need this aligned to
-	 * the pageblock boundary, because we do pfn -= pageblock_nr_pages
-	 * in the for loop.
+	 * the pageblock boundary, because we do
+	 * block_start_pfn -= pageblock_nr_pages in the for loop.
+	 * For ending point, take care when isolating in last pageblock of a
+	 * a zone which ends in the middle of a pageblock.
 	 * The low boundary is the end of the pageblock the migration scanner
 	 * is using.
 	 */
-	pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+	block_start_pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+	block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
+						zone_end_pfn(zone));
 	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.
+	 * If no pages are isolated, the block_start_pfn < low_pfn check
+	 * will kick in.
 	 */
 	next_free_pfn = 0;
 
-	z_end_pfn = zone_end_pfn(zone);
-
 	/*
 	 * Isolate free pages until enough are available to migrate the
 	 * pages on cc->migratepages. We stop searching if the migrate
 	 * and free page scanners meet or enough free pages are isolated.
 	 */
-	for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
-					pfn -= pageblock_nr_pages) {
+	for (;block_start_pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
+				block_end_pfn = block_start_pfn,
+				block_start_pfn -= pageblock_nr_pages) {
 		unsigned long isolated;
-		unsigned long end_pfn;
 
 		/*
 		 * This can iterate a massively long zone without finding any
@@ -715,7 +717,7 @@ static void isolate_freepages(struct zone *zone,
 		 */
 		cond_resched();
 
-		if (!pfn_valid(pfn))
+		if (!pfn_valid(block_start_pfn))
 			continue;
 
 		/*
@@ -725,7 +727,7 @@ static void isolate_freepages(struct zone *zone,
 		 * i.e. it's possible that all pages within a zones range of
 		 * pages do not belong to a single zone.
 		 */
-		page = pfn_to_page(pfn);
+		page = pfn_to_page(block_start_pfn);
 		if (page_zone(page) != zone)
 			continue;
 
@@ -738,15 +740,8 @@ 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);
+		isolated = isolate_freepages_block(cc, block_start_pfn,
+					block_end_pfn, freelist, false);
 		nr_freepages += isolated;
 
 		/*
@@ -754,9 +749,9 @@ 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 = block_start_pfn;
 		}
 	}
 
@@ -767,7 +762,7 @@ static void isolate_freepages(struct zone *zone,
 	 * If we crossed the migrate scanner, we want to keep it that way
 	 * so that compact_finished() may detect this
 	 */
-	if (pfn < low_pfn)
+	if (block_start_pfn < low_pfn)
 		next_free_pfn = cc->migrate_pfn;
 
 	cc->free_pfn = next_free_pfn;
-- 
1.7.9.5

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