Re: [PATCH v2 -mm 3/3] mm: only defer compaction for failed orderand higher

[Mel Gorman]

On Tue, Jan 24, 2012 at 01:23:32PM -0500, Rik van Riel wrote:
> Currently a failed order-9 (transparent hugepage) compaction can
> lead to memory compaction being temporarily disabled for a memory
> zone.  Even if we only need compaction for an order 2 allocation,
> eg. for jumbo frames networking.
> 

This is a throwback to when compaction was introduced just for
THP and hugetlbfs and compaction did not take place for order <=
PAGE_ALLOC_COSTLY_ORDER.

> The fix is relatively straightforward: keep track of the order at
> which compaction failed to create a free memory area. Only defer
> compaction at that order and higher, while letting compaction go
> through for lower orders.
> 
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  include/linux/compaction.h |   14 ++++++++++----
>  include/linux/mmzone.h     |    1 +
>  mm/compaction.c            |   11 ++++++++++-
>  mm/page_alloc.c            |    6 ++++--
>  mm/vmscan.c                |    2 +-
>  5 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 7a9323a..51a90b7 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -34,20 +34,26 @@ extern unsigned long compaction_suitable(struct zone *zone, int order);
>   * allocation success. 1 << compact_defer_limit compactions are skipped up
>   * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
>   */
> -static inline void defer_compaction(struct zone *zone)
> +static inline void defer_compaction(struct zone *zone, int order)
>  {
>  	zone->compact_considered = 0;
>  	zone->compact_defer_shift++;
>  
> +	if (order < zone->compact_order_failed)
> +		zone->compact_order_failed = order;
> +
>  	if (zone->compact_defer_shift > COMPACT_MAX_DEFER_SHIFT)
>  		zone->compact_defer_shift = COMPACT_MAX_DEFER_SHIFT;
>  }
>  
>  /* Returns true if compaction should be skipped this time */
> -static inline bool compaction_deferred(struct zone *zone)
> +static inline bool compaction_deferred(struct zone *zone, int order)
>  {
>  	unsigned long defer_limit = 1UL << zone->compact_defer_shift;
>  
> +	if (order < zone->compact_order_failed)
> +		return false;
> +
>  	/* Avoid possible overflow */
>  	if (++zone->compact_considered > defer_limit)
>  		zone->compact_considered = defer_limit;
> @@ -73,11 +79,11 @@ static inline unsigned long compaction_suitable(struct zone *zone, int order)
>  	return COMPACT_SKIPPED;
>  }
>  
> -static inline void defer_compaction(struct zone *zone)
> +static inline void defer_compaction(struct zone *zone, int order)
>  {
>  }
>  
> -static inline bool compaction_deferred(struct zone *zone)
> +static inline bool compaction_deferred(struct zone *zone, int order)
>  {
>  	return 1;
>  }
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 650ba2f..dff7115 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -365,6 +365,7 @@ struct zone {
>  	 */
>  	unsigned int		compact_considered;
>  	unsigned int		compact_defer_shift;
> +	int			compact_order_failed;
>  #endif
>  
>  	ZONE_PADDING(_pad1_)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 51ece75..e8cff81 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -673,9 +673,18 @@ static int __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
>  		INIT_LIST_HEAD(&cc->freepages);
>  		INIT_LIST_HEAD(&cc->migratepages);
>  
> -		if (cc->order < 0 || !compaction_deferred(zone))
> +		if (cc->order < 0 || !compaction_deferred(zone, cc->order))
>  			compact_zone(zone, cc);
>  
> +		if (cc->order > 0) {
> +			int ok = zone_watermark_ok(zone, cc->order,
> +						low_wmark_pages(zone), 0, 0);
> +			if (ok && cc->order > zone->compact_order_failed)
> +				zone->compact_order_failed = cc->order + 1;
> +			else if (!ok && cc->sync)
> +				defer_compaction(zone, cc->order);
> +		}
> +

That needs a comment. I think what you're trying to do is reset
compat_order_failed once compaction is successful.

The "!ok && cc->sync" check may be broken. __compact_pgdat() is
called from kswapd, not direct compaction so cc->sync will not be true.

>  		VM_BUG_ON(!list_empty(&cc->freepages));
>  		VM_BUG_ON(!list_empty(&cc->migratepages));
>  	}
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0027d8f..cd617d9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1990,7 +1990,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  	if (!order)
>  		return NULL;
>  
> -	if (compaction_deferred(preferred_zone)) {
> +	if (compaction_deferred(preferred_zone, order)) {
>  		*deferred_compaction = true;
>  		return NULL;
>  	}
> @@ -2012,6 +2012,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  		if (page) {
>  			preferred_zone->compact_considered = 0;
>  			preferred_zone->compact_defer_shift = 0;
> +			if (order >= preferred_zone->compact_order_failed)
> +				preferred_zone->compact_order_failed = order + 1;
>  			count_vm_event(COMPACTSUCCESS);
>  			return page;
>  		}
> @@ -2028,7 +2030,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  		 * defer if the failure was a sync compaction failure.
>  		 */
>  		if (sync_migration)
> -			defer_compaction(preferred_zone);
> +			defer_compaction(preferred_zone, order);
>  
>  		cond_resched();
>  	}
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fa17794..5d65991 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2199,7 +2199,7 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
>  	 * If compaction is deferred, reclaim up to a point where
>  	 * compaction will have a chance of success when re-enabled
>  	 */
> -	if (compaction_deferred(zone))
> +	if (compaction_deferred(zone, sc->order))
>  		return watermark_ok;
>  
>  	/* If compaction is not ready to start, keep reclaiming */
> 

-- 
Mel Gorman
SUSE Labs
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