Re: [RFC] respect the referenced bit of KVM guest pages?

[KOSAKI Motohiro]

> > This one of the reasons why we unconditionally deactivate
> > the active anon pages, and do background scanning of the
> > active anon list when reclaiming page cache pages.
> > 
> > We want to always move some pages to the inactive anon
> > list, so it does not get too small.
> 
> Right, the current code tries to pull inactive list out of
> smallish-size state as long as there are vmscan activities.
> 
> However there is a possible (and tricky) hole: mem cgroups
> don't do batched vmscan. 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 10 times when
> it is still small, and may thus prevent it from growing up for ever.
> 
> In that case, LRU becomes FIFO.
> 
> Jeff, can you confirm if the mem cgroup's inactive list is small?
> If so, this patch should help.

This patch does right thing.
However, I would explain why I and memcg folks didn't do that in past days.

Strangely, some memcg struct declaration is hide in *.c. Thus we can't
make inline function and we hesitated to introduce many function calling
overhead.

So, Can we move some memcg structure declaration to *.h and make 
mem_cgroup_get_saved_scan() inlined function?


> 
> Thanks,
> Fengguang
> ---
> 
> mm: do batched scans for mem_cgroup
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
>  include/linux/memcontrol.h |    3 +++
>  mm/memcontrol.c            |   12 ++++++++++++
>  mm/vmscan.c                |    9 +++++----
>  3 files changed, 20 insertions(+), 4 deletions(-)
> 
> --- linux.orig/include/linux/memcontrol.h	2009-08-15 13:12:49.000000000 +0800
> +++ linux/include/linux/memcontrol.h	2009-08-15 13:18:13.000000000 +0800
> @@ -98,6 +98,9 @@ int mem_cgroup_inactive_file_is_low(stru
>  unsigned long mem_cgroup_zone_nr_pages(struct mem_cgroup *memcg,
>  				       struct zone *zone,
>  				       enum lru_list lru);
> +unsigned long *mem_cgroup_get_saved_scan(struct mem_cgroup *memcg,
> +					 struct zone *zone,
> +					 enum lru_list lru);
>  struct zone_reclaim_stat *mem_cgroup_get_reclaim_stat(struct mem_cgroup *memcg,
>  						      struct zone *zone);
>  struct zone_reclaim_stat*
> --- linux.orig/mm/memcontrol.c	2009-08-15 13:07:34.000000000 +0800
> +++ linux/mm/memcontrol.c	2009-08-15 13:17:56.000000000 +0800
> @@ -115,6 +115,7 @@ struct mem_cgroup_per_zone {
>  	 */
>  	struct list_head	lists[NR_LRU_LISTS];
>  	unsigned long		count[NR_LRU_LISTS];
> +	unsigned long		nr_saved_scan[NR_LRU_LISTS];
>  
>  	struct zone_reclaim_stat reclaim_stat;
>  };
> @@ -597,6 +598,17 @@ unsigned long mem_cgroup_zone_nr_pages(s
>  	return MEM_CGROUP_ZSTAT(mz, lru);
>  }
>  
> +unsigned long *mem_cgroup_get_saved_scan(struct mem_cgroup *memcg,
> +					 struct zone *zone,
> +					 enum lru_list lru)
> +{
> +	int nid = zone->zone_pgdat->node_id;
> +	int zid = zone_idx(zone);
> +	struct mem_cgroup_per_zone *mz = mem_cgroup_zoneinfo(memcg, nid, zid);
> +
> +	return &mz->nr_saved_scan[lru];
> +}

I think this fuction is a bit strange.
shrink_zone don't hold any lock. so, shouldn't we case memcg removing race?



> +
>  struct zone_reclaim_stat *mem_cgroup_get_reclaim_stat(struct mem_cgroup *memcg,
>  						      struct zone *zone)
>  {
> --- linux.orig/mm/vmscan.c	2009-08-15 13:04:54.000000000 +0800
> +++ linux/mm/vmscan.c	2009-08-15 13:19:03.000000000 +0800
> @@ -1534,6 +1534,7 @@ static void shrink_zone(int priority, st
>  	for_each_evictable_lru(l) {
>  		int file = is_file_lru(l);
>  		unsigned long scan;
> +		unsigned long *saved_scan;
>  
>  		scan = zone_nr_pages(zone, sc, l);
>  		if (priority || noswap) {
> @@ -1541,11 +1542,11 @@ static void shrink_zone(int priority, st
>  			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);
> +			saved_scan = &zone->lru[l].nr_saved_scan;
>  		else
> -			nr[l] = scan;
> +			saved_scan = mem_cgroup_get_saved_scan(sc->mem_cgroup,
> +							       zone, l);
> +		nr[l] = nr_scan_try_batch(scan, saved_scan, swap_cluster_max);
>  	}
>  
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||



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