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Date: Tue, 26 May 2020 12:31:29 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Roman Gushchin <guro@...com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 13/19] mm: memcg/slab: simplify memcg cache creation
On 4/22/20 10:47 PM, Roman Gushchin wrote:
> Because the number of non-root kmem_caches doesn't depend on the
> number of memory cgroups anymore and is generally not very big,
> there is no more need for a dedicated workqueue.
>
> Also, as there is no more need to pass any arguments to the
> memcg_create_kmem_cache() except the root kmem_cache, it's
> possible to just embed the work structure into the kmem_cache
> and avoid the dynamic allocation of the work structure.
>
> This will also simplify the synchronization: for each root kmem_cache
> there is only one work. So there will be no more concurrent attempts
> to create a non-root kmem_cache for a root kmem_cache: the second and
> all following attempts to queue the work will fail.
>
> On the kmem_cache destruction path there is no more need to call the
> expensive flush_workqueue() and wait for all pending works to be
> finished. Instead, cancel_work_sync() can be used to cancel/wait for
> only one work.
>
> Signed-off-by: Roman Gushchin <guro@...com>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> include/linux/memcontrol.h | 1 -
> mm/memcontrol.c | 48 +-------------------------------------
> mm/slab.h | 2 ++
> mm/slab_common.c | 22 +++++++++--------
> 4 files changed, 15 insertions(+), 58 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 698b92d60da5..87e6da5015b3 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1440,7 +1440,6 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size);
> void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size);
>
> extern struct static_key_false memcg_kmem_enabled_key;
> -extern struct workqueue_struct *memcg_kmem_cache_wq;
>
> extern int memcg_nr_cache_ids;
> void memcg_get_cache_ids(void);
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9fe2433fbe67..55fd42155a37 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -379,8 +379,6 @@ void memcg_put_cache_ids(void)
> */
> DEFINE_STATIC_KEY_FALSE(memcg_kmem_enabled_key);
> EXPORT_SYMBOL(memcg_kmem_enabled_key);
> -
> -struct workqueue_struct *memcg_kmem_cache_wq;
> #endif
>
> static int memcg_shrinker_map_size;
> @@ -2900,39 +2898,6 @@ static void memcg_free_cache_id(int id)
> ida_simple_remove(&memcg_cache_ida, id);
> }
>
> -struct memcg_kmem_cache_create_work {
> - struct kmem_cache *cachep;
> - struct work_struct work;
> -};
> -
> -static void memcg_kmem_cache_create_func(struct work_struct *w)
> -{
> - struct memcg_kmem_cache_create_work *cw =
> - container_of(w, struct memcg_kmem_cache_create_work, work);
> - struct kmem_cache *cachep = cw->cachep;
> -
> - memcg_create_kmem_cache(cachep);
> -
> - kfree(cw);
> -}
> -
> -/*
> - * Enqueue the creation of a per-memcg kmem_cache.
> - */
> -static void memcg_schedule_kmem_cache_create(struct kmem_cache *cachep)
> -{
> - struct memcg_kmem_cache_create_work *cw;
> -
> - cw = kmalloc(sizeof(*cw), GFP_NOWAIT | __GFP_NOWARN);
> - if (!cw)
> - return;
> -
> - cw->cachep = cachep;
> - INIT_WORK(&cw->work, memcg_kmem_cache_create_func);
> -
> - queue_work(memcg_kmem_cache_wq, &cw->work);
> -}
> -
> /**
> * memcg_kmem_get_cache: select memcg or root cache for allocation
> * @cachep: the original global kmem cache
> @@ -2949,7 +2914,7 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>
> memcg_cachep = READ_ONCE(cachep->memcg_params.memcg_cache);
> if (unlikely(!memcg_cachep)) {
> - memcg_schedule_kmem_cache_create(cachep);
> + queue_work(system_wq, &cachep->memcg_params.work);
> return cachep;
> }
>
> @@ -7122,17 +7087,6 @@ static int __init mem_cgroup_init(void)
> {
> int cpu, node;
>
> -#ifdef CONFIG_MEMCG_KMEM
> - /*
> - * Kmem cache creation is mostly done with the slab_mutex held,
> - * so use a workqueue with limited concurrency to avoid stalling
> - * all worker threads in case lots of cgroups are created and
> - * destroyed simultaneously.
> - */
> - memcg_kmem_cache_wq = alloc_workqueue("memcg_kmem_cache", 0, 1);
> - BUG_ON(!memcg_kmem_cache_wq);
> -#endif
> -
> cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
> memcg_hotplug_cpu_dead);
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 28c582ec997a..a4e115cb8bdc 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -45,12 +45,14 @@ struct kmem_cache {
> * @memcg_cache: pointer to memcg kmem cache, used by all non-root memory
> * cgroups.
> * @root_caches_node: list node for slab_root_caches list.
> + * @work: work struct used to create the non-root cache.
> */
> struct memcg_cache_params {
> struct kmem_cache *root_cache;
>
> struct kmem_cache *memcg_cache;
> struct list_head __root_caches_node;
> + struct work_struct work;
> };
> #endif /* CONFIG_SLOB */
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e9deaafddbb6..10aa2acb84ca 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -132,10 +132,18 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
>
> LIST_HEAD(slab_root_caches);
>
> +static void memcg_kmem_cache_create_func(struct work_struct *work)
> +{
> + struct kmem_cache *cachep = container_of(work, struct kmem_cache,
> + memcg_params.work);
> + memcg_create_kmem_cache(cachep);
> +}
> +
> void slab_init_memcg_params(struct kmem_cache *s)
> {
> s->memcg_params.root_cache = NULL;
> s->memcg_params.memcg_cache = NULL;
> + INIT_WORK(&s->memcg_params.work, memcg_kmem_cache_create_func);
> }
>
> static void init_memcg_params(struct kmem_cache *s,
> @@ -584,15 +592,9 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
> return 0;
> }
>
> -static void flush_memcg_workqueue(struct kmem_cache *s)
> +static void cancel_memcg_cache_creation(struct kmem_cache *s)
> {
> - /*
> - * SLAB and SLUB create memcg kmem_caches through workqueue and SLUB
> - * deactivates the memcg kmem_caches through workqueue. Make sure all
> - * previous workitems on workqueue are processed.
> - */
> - if (likely(memcg_kmem_cache_wq))
> - flush_workqueue(memcg_kmem_cache_wq);
> + cancel_work_sync(&s->memcg_params.work);
> }
> #else
> static inline int shutdown_memcg_caches(struct kmem_cache *s)
> @@ -600,7 +602,7 @@ static inline int shutdown_memcg_caches(struct kmem_cache *s)
> return 0;
> }
>
> -static inline void flush_memcg_workqueue(struct kmem_cache *s)
> +static inline void cancel_memcg_cache_creation(struct kmem_cache *s)
> {
> }
> #endif /* CONFIG_MEMCG_KMEM */
> @@ -619,7 +621,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
> if (unlikely(!s))
> return;
>
> - flush_memcg_workqueue(s);
> + cancel_memcg_cache_creation(s);
>
> get_online_cpus();
> get_online_mems();
>
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