Re: [PATCH -v2] mm: do batched scans for mem_cgroup

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

On Thu, 20 Aug 2009 12:05:33 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> On Thu, Aug 20, 2009 at 11:13:47AM +0800, KAMEZAWA Hiroyuki wrote:
> > On Thu, 20 Aug 2009 10:49:29 +0800
> > Wu Fengguang <fengguang.wu@intel.com> wrote:
> > 
> > > For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
> > > in which case shrink_list() _still_ calls isolate_pages() with the much
> > > larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
> > > scan rate by up to 32 times.
> > > 
> > > For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
> > > So when shrink_zone() expects to scan 4 pages in the active/inactive
> > > list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect.
> > > 
> > > The accesses to nr_saved_scan are not lock protected and so not 100%
> > > accurate, however we can tolerate small errors and the resulted small
> > > imbalanced scan rates between zones.
> > > 
> > > This batching won't blur up the cgroup limits, since it is driven by
> > > "pages reclaimed" rather than "pages scanned". When shrink_zone()
> > > decides to cancel (and save) one smallish scan, it may well be called
> > > again to accumulate up nr_saved_scan.
> > > 
> > > It could possibly be a problem for some tiny mem_cgroup (which may be
> > > _full_ scanned too much times in order to accumulate up nr_saved_scan).
> > > 
> > > CC: Rik van Riel <riel@redhat.com>
> > > CC: Minchan Kim <minchan.kim@gmail.com>
> > > CC: Balbir Singh <balbir@linux.vnet.ibm.com>
> > > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > 
> > Hmm, how about this ? 
> > ==
> > Now, nr_saved_scan is tied to zone's LRU.
> > But, considering how vmscan works, it should be tied to reclaim_stat.
> > 
> > By this, memcg can make use of nr_saved_scan information seamlessly.
> 
> Good idea, full patch updated with your signed-off-by :)
> 
looks nice :)
thanks,
-Kame


> Thanks,
> Fengguang
> ---
> mm: do batched scans for mem_cgroup
> 
> For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
> in which case shrink_list() _still_ calls isolate_pages() with the much
> larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
> scan rate by up to 32 times.
> 
> For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
> So when shrink_zone() expects to scan 4 pages in the active/inactive
> list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect.
> 
> The accesses to nr_saved_scan are not lock protected and so not 100%
> accurate, however we can tolerate small errors and the resulted small
> imbalanced scan rates between zones.
> 
> This batching won't blur up the cgroup limits, since it is driven by
> "pages reclaimed" rather than "pages scanned". When shrink_zone()
> decides to cancel (and save) one smallish scan, it may well be called
> again to accumulate up nr_saved_scan.
> 
> It could possibly be a problem for some tiny mem_cgroup (which may be
> _full_ scanned too much times in order to accumulate up nr_saved_scan).
> 
> CC: Rik van Riel <riel@redhat.com>
> CC: Minchan Kim <minchan.kim@gmail.com>
> CC: Balbir Singh <balbir@linux.vnet.ibm.com>
> CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  include/linux/mmzone.h |    6 +++++-
>  mm/page_alloc.c        |    2 +-
>  mm/vmscan.c            |   20 +++++++++++---------
>  3 files changed, 17 insertions(+), 11 deletions(-)
> 
> --- linux.orig/include/linux/mmzone.h	2009-07-30 10:45:15.000000000 +0800
> +++ linux/include/linux/mmzone.h	2009-08-20 11:51:08.000000000 +0800
> @@ -269,6 +269,11 @@ struct zone_reclaim_stat {
>  	 */
>  	unsigned long		recent_rotated[2];
>  	unsigned long		recent_scanned[2];
> +
> +	/*
> +	 * accumulated for batching
> +	 */
> +	unsigned long		nr_saved_scan[NR_LRU_LISTS];
>  };
>  
>  struct zone {
> @@ -323,7 +328,6 @@ struct zone {
>  	spinlock_t		lru_lock;	
>  	struct zone_lru {
>  		struct list_head list;
> -		unsigned long nr_saved_scan;	/* accumulated for batching */
>  	} lru[NR_LRU_LISTS];
>  
>  	struct zone_reclaim_stat reclaim_stat;
> --- linux.orig/mm/vmscan.c	2009-08-20 11:48:46.000000000 +0800
> +++ linux/mm/vmscan.c	2009-08-20 12:00:55.000000000 +0800
> @@ -1521,6 +1521,7 @@ static void shrink_zone(int priority, st
>  	enum lru_list l;
>  	unsigned long nr_reclaimed = sc->nr_reclaimed;
>  	unsigned long swap_cluster_max = sc->swap_cluster_max;
> +	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>  	int noswap = 0;
>  
>  	/* If we have no swap space, do not bother scanning anon pages. */
> @@ -1540,12 +1541,9 @@ static void shrink_zone(int priority, st
>  			scan >>= priority;
>  			scan = (scan * percent[file]) / 100;
>  		}
> -		if (scanning_global_lru(sc))
> -			nr[l] = nr_scan_try_batch(scan,
> -						  &zone->lru[l].nr_saved_scan,
> -						  swap_cluster_max);
> -		else
> -			nr[l] = scan;
> +		nr[l] = nr_scan_try_batch(scan,
> +					  &reclaim_stat->nr_saved_scan[l],
> +					  swap_cluster_max);
>  	}
>  
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> @@ -2128,6 +2126,7 @@ static void shrink_all_zones(unsigned lo
>  {
>  	struct zone *zone;
>  	unsigned long nr_reclaimed = 0;
> +	struct zone_reclaim_stat *reclaim_stat;
>  
>  	for_each_populated_zone(zone) {
>  		enum lru_list l;
> @@ -2144,11 +2143,14 @@ static void shrink_all_zones(unsigned lo
>  						l == LRU_ACTIVE_FILE))
>  				continue;
>  
> -			zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1;
> -			if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) {
> +			reclaim_stat = get_reclaim_stat(zone, sc);
> +			reclaim_stat->nr_saved_scan[l] +=
> +						(lru_pages >> prio) + 1;
> +			if (reclaim_stat->nr_saved_scan[l]
> +						>= nr_pages || pass > 3) {
>  				unsigned long nr_to_scan;
>  
> -				zone->lru[l].nr_saved_scan = 0;
> +				reclaim_stat->nr_saved_scan[l] = 0;
>  				nr_to_scan = min(nr_pages, lru_pages);
>  				nr_reclaimed += shrink_list(l, nr_to_scan, zone,
>  								sc, prio);
> --- linux.orig/mm/page_alloc.c	2009-08-20 11:57:54.000000000 +0800
> +++ linux/mm/page_alloc.c	2009-08-20 11:58:39.000000000 +0800
> @@ -3716,7 +3716,7 @@ static void __paginginit free_area_init_
>  		zone_pcp_init(zone);
>  		for_each_lru(l) {
>  			INIT_LIST_HEAD(&zone->lru[l].list);
> -			zone->lru[l].nr_saved_scan = 0;
> +			zone->reclaim_stat.nr_saved_scan[l] = 0;
>  		}
>  		zone->reclaim_stat.recent_rotated[0] = 0;
>  		zone->reclaim_stat.recent_rotated[1] = 0;
> 

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