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

[Minchan Kim]

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@xxxxxxxxxx> 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;

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;

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's why I asked about max operation. What am I missing?
> 
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-- 
Kind regards,
Minchan Kim
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