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Date: Tue, 3 Dec 2013 16:34:10 +0400
From: Vladimir Davydov <vdavydov@...allels.com>
To: Dave Chinner <david@...morbit.com>
CC: <hannes@...xchg.org>, <mhocko@...e.cz>, <dchinner@...hat.com>,
<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
<linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
<devel@...nvz.org>, <glommer@...nvz.org>,
Al Viro <viro@...iv.linux.org.uk>,
Balbir Singh <bsingharora@...il.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH v12 12/18] fs: make icache, dcache shrinkers memcg-aware
On 12/03/2013 03:45 PM, Dave Chinner wrote:
> On Mon, Dec 02, 2013 at 03:19:47PM +0400, Vladimir Davydov wrote:
>> Using the per-memcg LRU infrastructure introduced by previous patches,
>> this patch makes dcache and icache shrinkers memcg-aware. To achieve
>> that, it converts s_dentry_lru and s_inode_lru from list_lru to
>> memcg_list_lru and restricts the reclaim to per-memcg parts of the lists
>> in case of memcg pressure.
>>
>> Other FS objects are currently ignored and only reclaimed on global
>> pressure, because their shrinkers are heavily FS-specific and can't be
>> converted to be memcg-aware so easily. However, we can pass on target
>> memcg to the FS layer and let it decide if per-memcg objects should be
>> reclaimed.
> And now you have a big problem, because that means filesystems like
> XFS won't reclaim inodes during memcg reclaim.
>
> That is, for XFS, prune_icache_lru() does not free any memory. All
> it does is remove all the VFS references to the struct xfs_inode,
> which is then reclaimed via the sb->s_op->free_cached_objects()
> method.
>
> IOWs, what you've done is broken.
Missed that, thanks for pointing out.
>
>> Note that with this patch applied we lose global LRU order, but it does
> We don't have global LRU order today for the filesystem caches.
> We have per superblock, per-node LRU reclaim order.
>
>> --- a/fs/dcache.c
>> +++ b/fs/dcache.c
>> @@ -343,18 +343,24 @@ static void dentry_unlink_inode(struct dentry * dentry)
>> #define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
>> static void d_lru_add(struct dentry *dentry)
>> {
>> + struct list_lru *lru =
>> + mem_cgroup_kmem_list_lru(&dentry->d_sb->s_dentry_lru, dentry);
>> +
>> D_FLAG_VERIFY(dentry, 0);
>> dentry->d_flags |= DCACHE_LRU_LIST;
>> this_cpu_inc(nr_dentry_unused);
>> - WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
>> + WARN_ON_ONCE(!list_lru_add(lru, &dentry->d_lru));
>> }
> This is what I mean about pushing memcg cruft into places where it
> is not necessary. This can be done entirely behind list_lru_add(),
> without the caller having to care.
>
>> @@ -970,9 +976,9 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
>> }
>>
>> /**
>> - * prune_dcache_sb - shrink the dcache
>> - * @sb: superblock
>> - * @nr_to_scan : number of entries to try to free
>> + * prune_dcache_lru - shrink the dcache
>> + * @lru: dentry lru list
>> + * @nr_to_scan: number of entries to try to free
>> * @nid: which node to scan for freeable entities
>> *
>> * Attempt to shrink the superblock dcache LRU by @nr_to_scan entries. This is
>> @@ -982,14 +988,13 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
>> * This function may fail to free any resources if all the dentries are in
>> * use.
>> */
>> -long prune_dcache_sb(struct super_block *sb, unsigned long nr_to_scan,
>> - int nid)
>> +long prune_dcache_lru(struct list_lru *lru, unsigned long nr_to_scan, int nid)
>> {
>> LIST_HEAD(dispose);
>> long freed;
>>
>> - freed = list_lru_walk_node(&sb->s_dentry_lru, nid, dentry_lru_isolate,
>> - &dispose, &nr_to_scan);
>> + freed = list_lru_walk_node(lru, nid, dentry_lru_isolate,
>> + &dispose, &nr_to_scan);
>> shrink_dentry_list(&dispose);
>> return freed;
>> }
> And here, you pass an LRU when what we really need to pass is the
> struct shrink_control that contains nr_to_scan, nid, and the memcg
> that pruning is targetting.
>
> Because of the tight integration of the LRUs and shrinkers, it makes
> sense to pass the shrink control all the way into the list. i.e:
>
> freed = list_lru_scan(&sb->s_dentry_lru, sc, dentry_lru_isolate,
> &dispose);
>
> And again, that hides everything to do with memcg based LRUs and
> reclaim from the callers. It's clean, simple and hard to get wrong.
>
>> @@ -1029,7 +1034,7 @@ void shrink_dcache_sb(struct super_block *sb)
>> do {
>> LIST_HEAD(dispose);
>>
>> - freed = list_lru_walk(&sb->s_dentry_lru,
>> + freed = memcg_list_lru_walk_all(&sb->s_dentry_lru,
>> dentry_lru_isolate_shrink, &dispose, UINT_MAX);
>>
> list_lru_walk() is, by definition, supposed to walk every single
> object on the LRU. With memcg awareness, it should be walking all
> the memcg lists, too.
>
>> diff --git a/fs/super.c b/fs/super.c
>> index cece164..b198da4 100644
>> --- a/fs/super.c
>> +++ b/fs/super.c
>> @@ -57,6 +57,8 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
>> struct shrink_control *sc)
>> {
>> struct super_block *sb;
>> + struct list_lru *inode_lru;
>> + struct list_lru *dentry_lru;
>> long fs_objects = 0;
>> long total_objects;
>> long freed = 0;
>> @@ -75,11 +77,14 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
>> if (!grab_super_passive(sb))
>> return SHRINK_STOP;
>>
>> - if (sb->s_op->nr_cached_objects)
>> + if (sb->s_op->nr_cached_objects && !sc->memcg)
>> fs_objects = sb->s_op->nr_cached_objects(sb, sc->nid);
>>
>> - inodes = list_lru_count_node(&sb->s_inode_lru, sc->nid);
>> - dentries = list_lru_count_node(&sb->s_dentry_lru, sc->nid);
>> + inode_lru = mem_cgroup_list_lru(&sb->s_inode_lru, sc->memcg);
>> + dentry_lru = mem_cgroup_list_lru(&sb->s_dentry_lru, sc->memcg);
>> +
>> + inodes = list_lru_count_node(inode_lru, sc->nid);
>> + dentries = list_lru_count_node(dentry_lru, sc->nid);
>> total_objects = dentries + inodes + fs_objects + 1;
> Again: list_lru_count_sc(&sb->s_inode_lru, sc).
>
> And push the scan control down into ->nr_cached_objects, too.
Thanks for the tip! Now I see your point, it would really look better if
we used shrink_control here.
Thanks.
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