Re: [PATCH 09/10] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

[Michal Hocko]

On Mon 21-09-15 11:52:41, Mel Gorman wrote:
> High-order watermark checking exists for two reasons --  kswapd high-order
> awareness and protection for high-order atomic requests. Historically the
> kernel depended on MIGRATE_RESERVE to preserve min_free_kbytes as high-order
> free pages for as long as possible. This patch introduces MIGRATE_HIGHATOMIC
> that reserves pageblocks for high-order atomic allocations on demand and
> avoids using those blocks for order-0 allocations. This is more flexible
> and reliable than MIGRATE_RESERVE was.
> 
> A MIGRATE_HIGHORDER pageblock is created when an atomic high-order allocation
> request steals a pageblock but limits the total number to 1% of the zone.
> Callers that speculatively abuse atomic allocations for long-lived
> high-order allocations to access the reserve will quickly fail. Note that
> SLUB is currently not such an abuser as it reclaims at least once.  It is
> possible that the pageblock stolen has few suitable high-order pages and
> will need to steal again in the near future but there would need to be
> strong justification to search all pageblocks for an ideal candidate.
> 
> The pageblocks are unreserved if an allocation fails after a direct
> reclaim attempt.
> 
> The watermark checks account for the reserved pageblocks when the allocation
> request is not a high-order atomic allocation.
> 
> The reserved pageblocks can not be used for order-0 allocations. This may
> allow temporary wastage until a failed reclaim reassigns the pageblock. This
> is deliberate as the intent of the reservation is to satisfy a limited
> number of atomic high-order short-lived requests if the system requires them.
> 
> The stutter benchmark was used to evaluate this but while it was running
> there was a systemtap script that randomly allocated between 1 high-order
> page and 12.5% of memory's worth of order-3 pages using GFP_ATOMIC. This
> is much larger than the potential reserve and it does not attempt to be
> realistic.  It is intended to stress random high-order allocations from
> an unknown source, show that there is a reduction in failures without
> introducing an anomaly where atomic allocations are more reliable than
> regular allocations.  The amount of memory reserved varied throughout the
> workload as reserves were created and reclaimed under memory pressure. The
> allocation failures once the workload warmed up were as follows;
> 
> 4.2-rc5-vanilla		70%
> 4.2-rc5-atomic-reserve	56%
> 
> The failure rate was also measured while building multiple kernels. The
> failure rate was 14% but is 6% with this patch applied.
> 
> Overall, this is a small reduction but the reserves are small relative
> to the number of allocation requests. In early versions of the patch,
> the failure rate reduced by a much larger amount but that required much
> larger reserves and perversely made atomic allocations seem more reliable
> than regular allocations.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

I believe I've acked this one previously. Anyway
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  include/linux/mmzone.h |   6 ++-
>  mm/page_alloc.c        | 125 +++++++++++++++++++++++++++++++++++++++++++++----
>  mm/vmstat.c            |   1 +
>  3 files changed, 122 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 40a856d28764..ad8cf52de55b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -39,6 +39,8 @@ enum {
>  	MIGRATE_UNMOVABLE,
>  	MIGRATE_MOVABLE,
>  	MIGRATE_RECLAIMABLE,
> +	MIGRATE_PCPTYPES,	/* the number of types on the pcp lists */
> +	MIGRATE_HIGHATOMIC = MIGRATE_PCPTYPES,
>  #ifdef CONFIG_CMA
>  	/*
>  	 * MIGRATE_CMA migration type is designed to mimic the way
> @@ -61,8 +63,6 @@ enum {
>  	MIGRATE_TYPES
>  };
>  
> -#define MIGRATE_PCPTYPES (MIGRATE_RECLAIMABLE+1)
> -
>  #ifdef CONFIG_CMA
>  #  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
>  #else
> @@ -334,6 +334,8 @@ struct zone {
>  	/* zone watermarks, access with *_wmark_pages(zone) macros */
>  	unsigned long watermark[NR_WMARK];
>  
> +	unsigned long nr_reserved_highatomic;
> +
>  	/*
>  	 * We don't know if the memory that we're going to allocate will be freeable
>  	 * or/and it will be released eventually, so to avoid totally wasting several
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ae01a2c1e863..811d6fc4ad5d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1615,6 +1615,88 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
>  	return -1;
>  }
>  
> +/*
> + * Reserve a pageblock for exclusive use of high-order atomic allocations if
> + * there are no empty page blocks that contain a page with a suitable order
> + */
> +static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
> +				unsigned int alloc_order)
> +{
> +	int mt;
> +	unsigned long max_managed, flags;
> +
> +	/*
> +	 * Limit the number reserved to 1 pageblock or roughly 1% of a zone.
> +	 * Check is race-prone but harmless.
> +	 */
> +	max_managed = (zone->managed_pages / 100) + pageblock_nr_pages;
> +	if (zone->nr_reserved_highatomic >= max_managed)
> +		return;
> +
> +	/* Yoink! */
> +	spin_lock_irqsave(&zone->lock, flags);
> +
> +	mt = get_pageblock_migratetype(page);
> +	if (mt != MIGRATE_HIGHATOMIC &&
> +			!is_migrate_isolate(mt) && !is_migrate_cma(mt)) {
> +		zone->nr_reserved_highatomic += pageblock_nr_pages;
> +		set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
> +		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC);
> +	}
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +}
> +
> +/*
> + * Used when an allocation is about to fail under memory pressure. This
> + * potentially hurts the reliability of high-order allocations when under
> + * intense memory pressure but failed atomic allocations should be easier
> + * to recover from than an OOM.
> + */
> +static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
> +{
> +	struct zonelist *zonelist = ac->zonelist;
> +	unsigned long flags;
> +	struct zoneref *z;
> +	struct zone *zone;
> +	struct page *page;
> +	int order;
> +
> +	for_each_zone_zonelist_nodemask(zone, z, zonelist, ac->high_zoneidx,
> +								ac->nodemask) {
> +		/* Preserve at least one pageblock */
> +		if (zone->nr_reserved_highatomic <= pageblock_nr_pages)
> +			continue;
> +
> +		spin_lock_irqsave(&zone->lock, flags);
> +		for (order = 0; order < MAX_ORDER; order++) {
> +			struct free_area *area = &(zone->free_area[order]);
> +
> +			if (list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
> +				continue;
> +
> +			page = list_entry(area->free_list[MIGRATE_HIGHATOMIC].next,
> +						struct page, lru);
> +
> +			zone->nr_reserved_highatomic -= pageblock_nr_pages;
> +
> +			/*
> +			 * Convert to ac->migratetype and avoid the normal
> +			 * pageblock stealing heuristics. Minimally, the caller
> +			 * is doing the work and needs the pages. More
> +			 * importantly, if the block was always converted to
> +			 * MIGRATE_UNMOVABLE or another type then the number
> +			 * of pageblocks that cannot be completely freed
> +			 * may increase.
> +			 */
> +			set_pageblock_migratetype(page, ac->migratetype);
> +			move_freepages_block(zone, page, ac->migratetype);
> +			spin_unlock_irqrestore(&zone->lock, flags);
> +			return;
> +		}
> +		spin_unlock_irqrestore(&zone->lock, flags);
> +	}
> +}
> +
>  /* Remove an element from the buddy allocator from the fallback list */
>  static inline struct page *
>  __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> @@ -1670,7 +1752,7 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
>   * Call me with the zone->lock already held.
>   */
>  static struct page *__rmqueue(struct zone *zone, unsigned int order,
> -						int migratetype)
> +				int migratetype, gfp_t gfp_flags)
>  {
>  	struct page *page;
>  
> @@ -1700,7 +1782,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>  
>  	spin_lock(&zone->lock);
>  	for (i = 0; i < count; ++i) {
> -		struct page *page = __rmqueue(zone, order, migratetype);
> +		struct page *page = __rmqueue(zone, order, migratetype, 0);
>  		if (unlikely(page == NULL))
>  			break;
>  
> @@ -2072,7 +2154,7 @@ int split_free_page(struct page *page)
>  static inline
>  struct page *buffered_rmqueue(struct zone *preferred_zone,
>  			struct zone *zone, unsigned int order,
> -			gfp_t gfp_flags, int migratetype)
> +			gfp_t gfp_flags, int alloc_flags, int migratetype)
>  {
>  	unsigned long flags;
>  	struct page *page;
> @@ -2115,7 +2197,15 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
>  			WARN_ON_ONCE(order > 1);
>  		}
>  		spin_lock_irqsave(&zone->lock, flags);
> -		page = __rmqueue(zone, order, migratetype);
> +
> +		page = NULL;
> +		if (unlikely(order) && (alloc_flags & ALLOC_HARDER)) {
> +			page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> +			if (page)
> +				trace_mm_page_alloc_zone_locked(page, order, migratetype);
> +		}
> +
> +		page = __rmqueue(zone, order, migratetype, gfp_flags);
>  		spin_unlock(&zone->lock);
>  		if (!page)
>  			goto failed;
> @@ -2225,15 +2315,24 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
>  			unsigned long mark, int classzone_idx, int alloc_flags,
>  			long free_pages)
>  {
> -	/* free_pages may go negative - that's OK */
>  	long min = mark;
>  	int o;
>  	long free_cma = 0;
>  
> +	/* free_pages may go negative - that's OK */
>  	free_pages -= (1 << order) - 1;
> +
>  	if (alloc_flags & ALLOC_HIGH)
>  		min -= min / 2;
> -	if (alloc_flags & ALLOC_HARDER)
> +
> +	/*
> +	 * If the caller does not have rights to ALLOC_HARDER then subtract
> +	 * the high-atomic reserves. This will over-estimate the size of the
> +	 * atomic reserve but it avoids a search.
> +	 */
> +	if (likely(!(alloc_flags & ALLOC_HARDER)))
> +		free_pages -= z->nr_reserved_highatomic;
> +	else
>  		min -= min / 4;
>  
>  #ifdef CONFIG_CMA
> @@ -2418,10 +2517,18 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  
>  try_this_zone:
>  		page = buffered_rmqueue(ac->preferred_zone, zone, order,
> -						gfp_mask, ac->migratetype);
> +				gfp_mask, alloc_flags, ac->migratetype);
>  		if (page) {
>  			if (prep_new_page(page, order, gfp_mask, alloc_flags))
>  				goto try_this_zone;
> +
> +			/*
> +			 * If this is a high-order atomic allocation then check
> +			 * if the pageblock should be reserved for the future
> +			 */
> +			if (unlikely(order && (alloc_flags & ALLOC_HARDER)))
> +				reserve_highatomic_pageblock(page, zone, order);
> +
>  			return page;
>  		}
>  	}
> @@ -2694,9 +2801,11 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  
>  	/*
>  	 * If an allocation failed after direct reclaim, it could be because
> -	 * pages are pinned on the per-cpu lists. Drain them and try again
> +	 * pages are pinned on the per-cpu lists or in high alloc reserves.
> +	 * Shrink them them and try again
>  	 */
>  	if (!page && !drained) {
> +		unreserve_highatomic_pageblock(ac);
>  		drain_all_pages(NULL);
>  		drained = true;
>  		goto retry;
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 49963aa2dff3..3427a155f85e 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -901,6 +901,7 @@ static char * const migratetype_names[MIGRATE_TYPES] = {
>  	"Unmovable",
>  	"Reclaimable",
>  	"Movable",
> +	"HighAtomic",
>  #ifdef CONFIG_CMA
>  	"CMA",
>  #endif
> -- 
> 2.4.6

-- 
Michal Hocko
SUSE Labs
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