Re: [PATCH v2 2/9] mm/compaction: introduce compaction depleted state on zone

Vlastimil Babka
2015-09-25T08:05:00Z

On 08/24/2015 04:19 AM, Joonsoo Kim wrote:
> Further compaction attempt is deferred when some of compaction attempts
> already fails. But, after some number of trial are skipped, compaction
> restarts work to check whether compaction is now possible or not. It
> scans whole range of zone to determine this possibility and if compaction
> possibility doesn't recover, this whole range scan is quite big overhead.
> As a first step to reduce this overhead, this patch implement compaction
> depleted state on zone.
>
> The way to determine depletion of compaction possility is checking number
> of success on previous compaction attempt. If number of successful
> compaction is below than specified threshold, we guess that compaction
> will not successful next time so mark the zone as compaction depleted.
> In this patch, threshold is choosed by 1 to imitate current compaction
> deferring algorithm. In the following patch, compaction algorithm will be
> changed and this threshold is also adjusted to that change.
>
> In this patch, only state definition is implemented. There is no action
> for this new state so no functional change. But, following patch will
> add some handling for this new state.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> ---
> include/linux/mmzone.h | 3 +++
> mm/compaction.c | 44 +++++++++++++++++++++++++++++++++++++++++---
> 2 files changed, 44 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 754c259..700e9b5 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -517,6 +517,8 @@ struct zone {
> unsigned int compact_considered;
> unsigned int compact_defer_shift;
> int compact_order_failed;
> + unsigned long compact_success;
> + unsigned long compact_depletion_depth;
> #endif
>
> #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> @@ -543,6 +545,7 @@ enum zone_flags {
> * many pages under writeback
> */
> ZONE_FAIR_DEPLETED, /* fair zone policy batch depleted */
> + ZONE_COMPACTION_DEPLETED, /* compaction possiblity depleted */
> };
>
> static inline unsigned long zone_end_pfn(const struct zone *zone)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c2d3d6a..de96e9d 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -129,6 +129,23 @@ static struct page *pageblock_pfn_to_page(unsigned long start_pfn,
>
> /* Do not skip compaction more than 64 times */
> #define COMPACT_MAX_DEFER_SHIFT 6
> +#define COMPACT_MIN_DEPLETE_THRESHOLD 1UL
> +
> +static bool compaction_depleted(struct zone *zone)
> +{
> + unsigned long threshold;
> + unsigned long success = zone->compact_success;
> +
> + /*
> + * Now, to imitate current compaction deferring approach,
> + * choose threshold to 1. It will be changed in the future.
> + */
> + threshold = COMPACT_MIN_DEPLETE_THRESHOLD;
> + if (success >= threshold)
> + return false;
> +
> + return true;
> +}
>
> /*
> * Compaction is deferred when compaction fails to result in a page
> @@ -223,6 +240,16 @@ static void __reset_isolation_suitable(struct zone *zone)
> zone->compact_cached_free_pfn = end_pfn;
> zone->compact_blockskip_flush = false;
>
> + if (compaction_depleted(zone)) {
> + if (test_bit(ZONE_COMPACTION_DEPLETED, &zone->flags))
> + zone->compact_depletion_depth++;
> + else {
> + set_bit(ZONE_COMPACTION_DEPLETED, &zone->flags);
> + zone->compact_depletion_depth = 0;
> + }
> + }
> + zone->compact_success = 0;

It's possible that the following comment is made moot by further patches, but:

I assume doing this in __reset_isolation_suitable() is to react on the
compaction_restarting() state. But __reset_isolation_suitable() is called also
from manually invoked compaction, and from kswapd. What if compaction has
succeeded S times, but threshold is T, S < T and T is larger than 1 (by a later
patch). Then kswapd or manual compaction will reset S to zero, without giving it
chance to reach T, even when compaction would succeed?