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

Mel Gorman
2010-11-18T16:16:42Z

On Wed, Nov 03, 2010 at 04:28:26PM +0100, Andrea Arcangeli wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> If transparent hugepage is enabled initialize min_free_kbytes to an optimal
> value by default. This moves the hugeadm algorithm in kernel.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -85,6 +85,47 @@ struct khugepaged_scan {
> .mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
> };
>
> +
> +static int set_recommended_min_free_kbytes(void)
> +{
> + struct zone *zone;
> + int nr_zones = 0;
> + unsigned long recommended_min;
> + extern int min_free_kbytes;
> +
> + if (!test_bit(TRANSPARENT_HUGEPAGE_FLAG,
> + &transparent_hugepage_flags) &&
> + !test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> + &transparent_hugepage_flags))
> + return 0;
> +
> + for_each_populated_zone(zone)
> + nr_zones++;
> +
> + /* 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.

> + return 0;
> +}
> +late_initcall(set_recommended_min_free_kbytes);
> +
> static int start_khugepaged(void)
> {
> int err = 0;
> @@ -108,6 +149,8 @@ static int start_khugepaged(void)
> mutex_unlock(&khugepaged_mutex);
> if (wakeup)
> wake_up_interruptible(&khugepaged_wait);
> +
> + set_recommended_min_free_kbytes();
> } else
> /* wakeup to exit */
> wake_up_interruptible(&khugepaged_wait);
> @@ -177,6 +220,13 @@ static ssize_t enabled_store(struct kobj
> ret = err;
> }
>
> + if (ret > 0 &&
> + (test_bit(TRANSPARENT_HUGEPAGE_FLAG,
> + &transparent_hugepage_flags) ||
> + test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> + &transparent_hugepage_flags)))
> + set_recommended_min_free_kbytes();
> +
> return ret;
> }
> static struct kobj_attribute enabled_attr =
> @@ -464,6 +514,8 @@ static int __init hugepage_init(void)
>
> start_khugepaged();
>
> + set_recommended_min_free_kbytes();
> +
> out:
> return err;
> }
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab