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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

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@kernel.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@suse.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@kernel.org>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Dongjun Shin <d.j.shin@samsung.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Sunghwan Yun <sunghwan.yun@samsung.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.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) {
 		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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