Re: [PATCH 51 of 66] set recommended min free kbytes

[Andrea Arcangeli]

On Thu, Nov 18, 2010 at 04:16:24PM +0000, Mel Gorman wrote:
> > +	/* Make sure at least 2 hugepages are free for MIGRATE_RESERVE */
> > +	recommended_min = HPAGE_PMD_NR * nr_zones * 2;
> > +
> 
> The really important value is pageblock_nr_pages here. It'll just happen
> to work on x86 and x86-64 but anti-fragmentation is really about
> pageblocks, not PMDs.
> 
> > +	/*
> > +	 * Make sure that on average at least two pageblocks are almost free
> > +	 * of another type, one for a migratetype to fall back to and a
> > +	 * second to avoid subsequent fallbacks of other types There are 3
> > +	 * MIGRATE_TYPES we care about.
> > +	 */
> > +	recommended_min += HPAGE_PMD_NR * nr_zones * 3 * 3;
> > +
> 
> Same on the use of pageblock_nr_pages. Also, you can replace 3 with
> MIGRATE_PCPTYPES.
> 
> > +	/* don't ever allow to reserve more than 5% of the lowmem */
> > +	recommended_min = min(recommended_min,
> > +			      (unsigned long) nr_free_buffer_pages() / 20);
> > +	recommended_min <<= (PAGE_SHIFT-10);
> > +
> > +	if (recommended_min > min_free_kbytes) {
> > +		min_free_kbytes = recommended_min;
> > +		setup_per_zone_wmarks();
> > +	}
> 
> 
> The timing this is called is important. Would you mind doing a quick
> debugging check by adding a printk to setup_zone_migrate_reserve() to ensure
> MIGRATE_RESERVE is getting set on sensible pageblocks? (see where the comment
> Suitable for reserving if this block is movable is) If MIGRATE_RESERVE blocks
> are not being created in a sensible fashion, atomic high-order allocations
> will suffer in mysterious ways.
> 
> SEtting the higher min free kbytes from userspace happens to work because
> the system is initialised and MIGRATE_MOVABLE exists but that might not be
> the case when automatically set like this patch.

When min_free_kbytes doesn't need to be increased (like huge system
with lots of ram) setup_per_zone_wmarks wasn't called in the
late_initcall where this code runs, and the original
setup_per_zone_wmarks apparently wasn't calling
setup_zone_migrate_reserve (how can it be?).

Anyway now I patched it like this and it seems to work properly with
the unconditional setup_per_zone_wmarks in
late_initcall(set_recommended_min_free_kbytes).

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -103,7 +103,7 @@ static int set_recommended_min_free_kbyt
 		nr_zones++;
 
 	/* Make sure at least 2 hugepages are free for MIGRATE_RESERVE */
-	recommended_min = HPAGE_PMD_NR * nr_zones * 2;
+	recommended_min = pageblock_nr_pages * nr_zones * 2;
 
 	/*
 	 * Make sure that on average at least two pageblocks are almost free
@@ -111,17 +111,17 @@ static int set_recommended_min_free_kbyt
 	 * second to avoid subsequent fallbacks of other types There are 3
 	 * MIGRATE_TYPES we care about.
 	 */
-	recommended_min += HPAGE_PMD_NR * nr_zones * 3 * 3;
+	recommended_min += pageblock_nr_pages * nr_zones *
+			   MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;
 
 	/* don't ever allow to reserve more than 5% of the lowmem */
 	recommended_min = min(recommended_min,
 			      (unsigned long) nr_free_buffer_pages() / 20);
 	recommended_min <<= (PAGE_SHIFT-10);
 
-	if (recommended_min > min_free_kbytes) {
+	if (recommended_min > min_free_kbytes)
 		min_free_kbytes = recommended_min;
-		setup_per_zone_wmarks();
-	}
+	setup_per_zone_wmarks();
 	return 0;
 }
 late_initcall(set_recommended_min_free_kbytes);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3188,6 +3188,7 @@ static void setup_zone_migrate_reserve(s
 	 */
 	reserve = min(2, reserve);
 
+	printk("reserve start %d\n", reserve);
 	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
 		if (!pfn_valid(pfn))
 			continue;
@@ -3226,6 +3227,7 @@ static void setup_zone_migrate_reserve(s
 			move_freepages_block(zone, page, MIGRATE_MOVABLE);
 		}
 	}
+	printk("reserve end %d\n", reserve);
 }
 
 /*



hugeadm with CONFIG_TRANSPARENT_HUGEPAGE=n leads to:

reserve start 1
reserve end 0
reserve start 2
reserve end 0
reserve start 2
reserve end 0
reserve start 0
reserve end 0
reserve start 0
reserve end 0
reserve start 0
reserve end 0
reserve start 2
reserve end 0
reserve start 0
reserve end 0

With CONFIG_TRANSPARENT_HUGEPAGE=Y I boot I see:

reserve start 1
reserve end 0
reserve start 2
reserve end 0
reserve start 2
reserve end 0
reserve start 0
reserve end 0
reserve start 0
reserve end 0
reserve start 0
reserve end 0
reserve start 2
reserve end 0
reserve start 0
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