Re: [RFC PATCH 3/6] memcg: restructure shrink_slab to walk memcg hierarchy

[Glauber Costa <glommer@parallels.com>]

On 08/17/2012 12:53 AM, Ying Han wrote:
> This patch moves the main slab shrinking to do_shrink_slab() and restructures
> shrink_slab() to walk the memory cgroup hiearchy. The memcg context is embedded
> inside the shrink_control. The underling shrinker will be respecting the new
> field by only reclaiming slab objects charged to the memcg.
> 
> The hierarchy walk in shrink_slab() is slightly different than the walk in
> shrink_zone(), where the latter one walks each memcg once for each priority
> under concurrent reclaim threads. It makes less sense for slab since they are
> spread out the system instead of per-zone. So here each shrink_slab() will
> trigger a full walk of each memcg under the sub-tree.
> 
> One optimization is under global reclaim, where we skip walking the whole tree
> but instead pass into shrinker w/ mem_cgroup=NULL. Then it will end up scanning
> the full dentry lru list.
> 
> Signed-off-by: Ying Han <yinghan@google.com>
> ---
>  mm/vmscan.c |   43 +++++++++++++++++++++++++++++++++++--------
>  1 files changed, 35 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6ffdff6..7a3a1a4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -204,7 +204,7 @@ static inline int do_shrinker_shrink(struct shrinker *shrinker,
>   *
>   * Returns the number of slab objects which we shrunk.
>   */
> -unsigned long shrink_slab(struct shrink_control *shrink,
> +static unsigned long do_shrink_slab(struct shrink_control *shrink,
>  			  unsigned long nr_pages_scanned,
>  			  unsigned long lru_pages)
>  {
> @@ -214,12 +214,6 @@ unsigned long shrink_slab(struct shrink_control *shrink,
>  	if (nr_pages_scanned == 0)
>  		nr_pages_scanned = SWAP_CLUSTER_MAX;
>  
> -	if (!down_read_trylock(&shrinker_rwsem)) {
> -		/* Assume we'll be able to shrink next time */
> -		ret = 1;
> -		goto out;
> -	}
> -
>  	list_for_each_entry(shrinker, &shrinker_list, list) {
>  		unsigned long long delta;
>  		long total_scan;
> @@ -309,8 +303,41 @@ unsigned long shrink_slab(struct shrink_control *shrink,
>  
>  		trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
>  	}
> +
> +	return ret;
> +}
> +
It seems to me this will call all shrinkers, regardless of whether or
not they are memcg-aware. Can't we just skip the ones we know not to be
memcg-aware?  (basically all non-vfs for the moment...)

My fear is that if called, they will shrink. And that may not be what we
want.



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