Re: [PATCH 3/6] memcg, slab: cleanup barrier usage when accessingmemcg_caches

[Michal Hocko]

On Thu 19-12-13 10:37:27, Vladimir Davydov wrote:
> On 12/18/2013 09:14 PM, Michal Hocko wrote:
> > On Wed 18-12-13 17:16:54, Vladimir Davydov wrote:
> >> First, in memcg_create_kmem_cache() we should issue the write barrier
> >> after the kmem_cache is initialized, but before storing the pointer to
> >> it in its parent's memcg_params.
> >>
> >> Second, we should always issue the read barrier after
> >> cache_from_memcg_idx() to conform with the write barrier.
> >>
> >> Third, its better to use smp_* versions of barriers, because we don't
> >> need them on UP systems.
> > Please be (much) more verbose on Why. Barriers are tricky and should be
> > documented accordingly. So if you say that we should issue a barrier
> > always be specific why we should do it.
> 
> In short, we have kmem_cache::memcg_params::memcg_caches is an array of
> pointers to per-memcg caches. We access it lock-free so we should use
> memory barriers during initialization. Obviously we should place a write
> barrier just before we set the pointer in order to make sure nobody will
> see a partially initialized structure. Besides there must be a read
> barrier between reading the pointer and accessing the structure, to
> conform with the write barrier. It's all that similar to rcu_assign and
> rcu_deref. Currently the barrier usage looks rather strange:
> 
> memcg_create_kmem_cache:
>     initialize kmem
>     set the pointer in memcg_caches
>     wmb() // ???
> 
> __memcg_kmem_get_cache:
>     <...>
>     read_barrier_depends() // ???
>     cachep = root_cache->memcg_params->memcg_caches[memcg_id]
>     <...>

Why do we need explicit memory barriers when we can use RCU?
__memcg_kmem_get_cache already dereferences within rcu_read_lock.

Btw. cache_from_memcg_idx is desperately asking for a comment about
required locking.

> Nothing prevents some archs from moving initialization after setting the
> pointer, or reading data before reading the pointer to it.
> 
> Of course, I will include a detailed description in the next version of
> this patch.
> 
> Thanks.
> 
> >> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxx>
> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> >> Cc: Glauber Costa <glommer@xxxxxxxxx>
> >> Cc: Christoph Lameter <cl@xxxxxxxxx>
> >> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> ---
> >>  mm/memcontrol.c |   24 ++++++++++--------------
> >>  mm/slab.h       |    6 +++++-
> >>  2 files changed, 15 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index e6ad6ff..e37fdb5 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -3429,12 +3429,14 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
> >>  
> >>  	atomic_set(&new_cachep->memcg_params->nr_pages , 0);
> >>  
> >> -	cachep->memcg_params->memcg_caches[idx] = new_cachep;
> >>  	/*
> >> -	 * the readers won't lock, make sure everybody sees the updated value,
> >> -	 * so they won't put stuff in the queue again for no reason
> >> +	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
> >> +	 * barrier here to ensure nobody will see the kmem_cache partially
> >> +	 * initialized.
> >>  	 */
> >> -	wmb();
> >> +	smp_wmb();
> >> +
> >> +	cachep->memcg_params->memcg_caches[idx] = new_cachep;
> >>  out:
> >>  	mutex_unlock(&memcg_cache_mutex);
> >>  	return new_cachep;
> >> @@ -3573,7 +3575,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
> >>  					  gfp_t gfp)
> >>  {
> >>  	struct mem_cgroup *memcg;
> >> -	int idx;
> >> +	struct kmem_cache *memcg_cachep;
> >>  
> >>  	VM_BUG_ON(!cachep->memcg_params);
> >>  	VM_BUG_ON(!cachep->memcg_params->is_root_cache);
> >> @@ -3587,15 +3589,9 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
> >>  	if (!memcg_can_account_kmem(memcg))
> >>  		goto out;
> >>  
> >> -	idx = memcg_cache_id(memcg);
> >> -
> >> -	/*
> >> -	 * barrier to mare sure we're always seeing the up to date value.  The
> >> -	 * code updating memcg_caches will issue a write barrier to match this.
> >> -	 */
> >> -	read_barrier_depends();
> >> -	if (likely(cache_from_memcg_idx(cachep, idx))) {
> >> -		cachep = cache_from_memcg_idx(cachep, idx);
> >> +	memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
> >> +	if (likely(memcg_cachep)) {
> >> +		cachep = memcg_cachep;
> >>  		goto out;
> >>  	}
> >>  
> >> diff --git a/mm/slab.h b/mm/slab.h
> >> index 0859c42..1d8b53f 100644
> >> --- a/mm/slab.h
> >> +++ b/mm/slab.h
> >> @@ -163,9 +163,13 @@ static inline const char *cache_name(struct kmem_cache *s)
> >>  static inline struct kmem_cache *
> >>  cache_from_memcg_idx(struct kmem_cache *s, int idx)
> >>  {
> >> +	struct kmem_cache *cachep;
> >> +
> >>  	if (!s->memcg_params)
> >>  		return NULL;
> >> -	return s->memcg_params->memcg_caches[idx];
> >> +	cachep = s->memcg_params->memcg_caches[idx];
> >> +	smp_read_barrier_depends();	/* see memcg_register_cache() */
> >> +	return cachep;
> >>  }
> >>  
> >>  static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
> >> -- 
> >> 1.7.10.4
> >>
> 

-- 
Michal Hocko
SUSE Labs
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