Re: [PATCH 1/5] mm: kswapd: Stop high-order balancing when any suitable zone is balanced

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri,  3 Dec 2010 11:45:30 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> When the allocator enters its slow path, kswapd is woken up to balance the
> node. It continues working until all zones within the node are balanced. For
> order-0 allocations, this makes perfect sense but for higher orders it can
> have unintended side-effects. If the zone sizes are imbalanced, kswapd may
> reclaim heavily within a smaller zone discarding an excessive number of
> pages. The user-visible behaviour is that kswapd is awake and reclaiming
> even though plenty of pages are free from a suitable zone.
> 
> This patch alters the "balance" logic for high-order reclaim allowing kswapd
> to stop if any suitable zone becomes balanced to reduce the number of pages
> it reclaims from other zones. kswapd still tries to ensure that order-0
> watermarks for all zones are met before sleeping.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>

a nitpick.

> ---
>  include/linux/mmzone.h |    3 +-
>  mm/page_alloc.c        |    8 ++++--
>  mm/vmscan.c            |   55 +++++++++++++++++++++++++++++++++++++++++-------
>  3 files changed, 54 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 39c24eb..7177f51 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -645,6 +645,7 @@ typedef struct pglist_data {
>  	wait_queue_head_t kswapd_wait;
>  	struct task_struct *kswapd;
>  	int kswapd_max_order;
> +	enum zone_type classzone_idx;
>  } pg_data_t;
>  
>  #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
> @@ -660,7 +661,7 @@ typedef struct pglist_data {
>  
>  extern struct mutex zonelists_mutex;
>  void build_all_zonelists(void *data);
> -void wakeup_kswapd(struct zone *zone, int order);
> +void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx);
>  int zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>  		int classzone_idx, int alloc_flags);
>  enum memmap_context {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e409270..82e3499 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1915,13 +1915,14 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
>  
>  static inline
>  void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> -						enum zone_type high_zoneidx)
> +						enum zone_type high_zoneidx,
> +						enum zone_type classzone_idx)
>  {
>  	struct zoneref *z;
>  	struct zone *zone;
>  
>  	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> -		wakeup_kswapd(zone, order);
> +		wakeup_kswapd(zone, order, classzone_idx);
>  }
>  
>  static inline int
> @@ -1998,7 +1999,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		goto nopage;
>  
>  restart:
> -	wake_all_kswapd(order, zonelist, high_zoneidx);
> +	wake_all_kswapd(order, zonelist, high_zoneidx,
> +						zone_idx(preferred_zone));
>  
>  	/*
>  	 * OK, we're below the kswapd watermark and have kicked background
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d31d7ce..d070d19 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2165,11 +2165,14 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>   * interoperates with the page allocator fallback scheme to ensure that aging
>   * of pages is balanced across the zones.
>   */
> -static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
> +static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
> +							int classzone_idx)
>  {
>  	int all_zones_ok;
> +	int any_zone_ok;
>  	int priority;
>  	int i;
> +	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
>  	unsigned long total_scanned;
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
>  	struct scan_control sc = {
> @@ -2192,7 +2195,6 @@ loop_again:
>  	count_vm_event(PAGEOUTRUN);
>  
>  	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
> -		int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
>  		unsigned long lru_pages = 0;
>  		int has_under_min_watermark_zone = 0;
>  
> @@ -2201,6 +2203,7 @@ loop_again:
>  			disable_swap_token();
>  
>  		all_zones_ok = 1;
> +		any_zone_ok = 0;
>  
>  		/*
>  		 * Scan in the highmem->dma direction for the highest
> @@ -2310,10 +2313,12 @@ loop_again:
>  				 * spectulatively avoid congestion waits
>  				 */
>  				zone_clear_flag(zone, ZONE_CONGESTED);
> +				if (i <= classzone_idx)
> +					any_zone_ok = 1;
>  			}
>  
>  		}
> -		if (all_zones_ok)
> +		if (all_zones_ok || (order && any_zone_ok))
>  			break;		/* kswapd: all done */
>  		/*
>  		 * OK, kswapd is getting into trouble.  Take a nap, then take
> @@ -2336,7 +2341,7 @@ loop_again:
>  			break;
>  	}
>  out:
> -	if (!all_zones_ok) {
> +	if (!(all_zones_ok || (order && any_zone_ok))) {

Could you add a comment ?

And this means...

	all_zones_ok .... all_zone_balanced
	any_zones_ok .... fallback_allocation_ok
?

Thanks,
-Kame

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