Re: [patch 05/11] mm: move memcg hierarchy reclaim to genericreclaim code

[Michal Hocko]

On Mon 12-09-11 12:57:22, Johannes Weiner wrote:
> Memory cgroup limit reclaim and traditional global pressure reclaim
> will soon share the same code to reclaim from a hierarchical tree of
> memory cgroups.
> 
> In preparation of this, move the two right next to each other in
> shrink_zone().

I like the way how you've split mem_cgroup_hierarchical_reclaim into
mem_cgroup_reclaim and mem_cgroup_soft_reclaim and I guess this deserves
a note in the patch description. Especially that mem_cgroup_reclaim is
hierarchical even though it doesn't use mem_cgroup_iter directly but
rather via do_try_to_free_pages and shrink_zone.

I am not sure I see how shrink_mem_cgroup_zone works. See comments and
questions bellow:

> 
> Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> ---
>  include/linux/memcontrol.h |   25 ++++++-
>  mm/memcontrol.c            |  167 ++++++++++++++++++++++----------------------
>  mm/vmscan.c                |   43 ++++++++++-
>  3 files changed, 147 insertions(+), 88 deletions(-)
> 
[...]
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f4b404e..413e1f8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
[...]
> @@ -783,19 +781,33 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
>  	return memcg;
>  }
>  
> -struct mem_cgroup_iter {
> -	struct zone *zone;
> -	int priority;
> -	unsigned int generation;
> -};
> -
> -static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> -					  struct mem_cgroup *prev,
> -					  struct mem_cgroup_iter *iter)
> +/**
> + * mem_cgroup_iter - iterate over memory cgroup hierarchy
> + * @root: hierarchy root
> + * @prev: previously returned memcg, NULL on first invocation
> + * @iter: token for partial walks, NULL for full walks
> + *
> + * Returns references to children of the hierarchy starting at @root,

I guess you meant "starting at @prev"

> + * or @root itself, or %NULL after a full round-trip.
> + *
> + * Caller must pass the return value in @prev on subsequent
> + * invocations for reference counting, or use mem_cgroup_iter_break()
> + * to cancel a hierarchy walk before the round-trip is complete.
> + *
> + * Reclaimers can specify a zone and a priority level in @iter to
> + * divide up the memcgs in the hierarchy among all concurrent
> + * reclaimers operating on the same zone and priority.
> + */
> +struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> +				   struct mem_cgroup *prev,
> +				   struct mem_cgroup_iter *iter)
>  {
>  	struct mem_cgroup *mem = NULL;
>  	int id = 0;
>  
> +	if (mem_cgroup_disabled())
> +		return NULL;
> +
>  	if (!root)
>  		root = root_mem_cgroup;
>  
[...]
> @@ -1479,6 +1496,41 @@ u64 mem_cgroup_get_limit(struct mem_cgroup *memcg)
>  	return min(limit, memsw);
>  }
>  
> +static unsigned long mem_cgroup_reclaim(struct mem_cgroup *mem,
> +					gfp_t gfp_mask,
> +					unsigned long flags)
> +{
> +	unsigned long total = 0;
> +	bool noswap = false;
> +	int loop;
> +
> +	if (flags & MEM_CGROUP_RECLAIM_NOSWAP)
> +		noswap = true;
> +	else if (!(flags & MEM_CGROUP_RECLAIM_SHRINK) && mem->memsw_is_minimum)
> +		noswap = true;
> +
> +	for (loop = 0; loop < MEM_CGROUP_MAX_RECLAIM_LOOPS; loop++) {
> +		if (loop)
> +			drain_all_stock_async(mem);
> +		total += try_to_free_mem_cgroup_pages(mem, gfp_mask, noswap);
> +		/*
> +		 * Avoid freeing too much when shrinking to resize the
> +		 * limit.  XXX: Shouldn't the margin check be enough?

I guess the MEM_CGROUP_RECLAIM_SHRINK condition should help shrinkers to
die more easily on signal even if they make some progress.

> +		 */
> +		if (total && (flags & MEM_CGROUP_RECLAIM_SHRINK))
> +			break;
> +		if (mem_cgroup_margin(mem))
> +			break;
> +		/*
> +		 * If nothing was reclaimed after two attempts, there
> +		 * may be no reclaimable pages in this hierarchy.
> +		 */
> +		if (loop && !total)
> +			break;
> +	}
> +	return total;
> +}
> +
>  /**
>   * test_mem_cgroup_node_reclaimable
>   * @mem: the target memcg
[...]
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 92f4e22..8419e8f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2104,12 +2104,43 @@ restart:
>  static void shrink_zone(int priority, struct zone *zone,
>  			struct scan_control *sc)
>  {
> -	struct mem_cgroup_zone mz = {
> -		.mem_cgroup = sc->target_mem_cgroup,
> +	struct mem_cgroup *root = sc->target_mem_cgroup;
> +	struct mem_cgroup_iter iter = {
>  		.zone = zone,
> +		.priority = priority,
>  	};
> +	struct mem_cgroup *mem;
> +
> +	if (global_reclaim(sc)) {
> +		struct mem_cgroup_zone mz = {
> +			.mem_cgroup = NULL,
> +			.zone = zone,
> +		};
> +
> +		shrink_mem_cgroup_zone(priority, &mz, sc);
> +		return;
> +	}
> +
> +	mem = mem_cgroup_iter(root, NULL, &iter);
> +	do {
> +		struct mem_cgroup_zone mz = {
> +			.mem_cgroup = mem,
> +			.zone = zone,
> +		};
>  
> -	shrink_mem_cgroup_zone(priority, &mz, sc);
> +		shrink_mem_cgroup_zone(priority, &mz, sc);
> +		/*
> +		 * Limit reclaim has historically picked one memcg and
> +		 * scanned it with decreasing priority levels until
> +		 * nr_to_reclaim had been reclaimed.  This priority
> +		 * cycle is thus over after a single memcg.
> +		 */
> +		if (!global_reclaim(sc)) {

How can we have global_reclaim(sc) == true here?
Shouldn't we just check how much have we reclaimed from that group and
iterate only if it wasn't sufficient (at least SWAP_CLUSTER_MAX)?

> +			mem_cgroup_iter_break(root, mem);
> +			break;
> +		}
> +		mem = mem_cgroup_iter(root, mem, &iter);
> +	} while (mem);
>  }
>  
>  /*
[...]
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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