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

Andrea Arcangeli
2010-12-09T18:49:03Z

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