From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> Subject: mm: make counting of list_lru_one::nr_items lockless During the reclaiming slab of a memcg, shrink_slab iterates over all registered shrinkers in the system, and tries to count and consume objects related to the cgroup. In case of memory pressure, this behaves bad: I observe high system time and time spent in list_lru_count_one() for many processes on RHEL7 kernel. This patch makes list_lru_node::memcg_lrus rcu protected, that allows to skip taking spinlock in list_lru_count_one(). Shakeel Butt with the patch observes significant perf graph change. He says: ======================================================================== Setup: running a fork-bomb in a memcg of 200MiB on a 8GiB and 4 vcpu VM and recording the trace with 'perf record -g -a'. The trace without the patch: + 34.19% fb.sh [kernel.kallsyms] [k] queued_spin_lock_slowpath + 30.77% fb.sh [kernel.kallsyms] [k] _raw_spin_lock + 3.53% fb.sh [kernel.kallsyms] [k] list_lru_count_one + 2.26% fb.sh [kernel.kallsyms] [k] super_cache_count + 1.68% fb.sh [kernel.kallsyms] [k] shrink_slab + 0.59% fb.sh [kernel.kallsyms] [k] down_read_trylock + 0.48% fb.sh [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore + 0.38% fb.sh [kernel.kallsyms] [k] shrink_node_memcg + 0.32% fb.sh [kernel.kallsyms] [k] queue_work_on + 0.26% fb.sh [kernel.kallsyms] [k] count_shadow_nodes With the patch: + 0.16% swapper [kernel.kallsyms] [k] default_idle + 0.13% oom_reaper [kernel.kallsyms] [k] mutex_spin_on_owner + 0.05% perf [kernel.kallsyms] [k] copy_user_generic_string + 0.05% init.real [kernel.kallsyms] [k] wait_consider_task + 0.05% kworker/0:0 [kernel.kallsyms] [k] finish_task_switch + 0.04% kworker/2:1 [kernel.kallsyms] [k] finish_task_switch + 0.04% kworker/3:1 [kernel.kallsyms] [k] finish_task_switch + 0.04% kworker/1:0 [kernel.kallsyms] [k] finish_task_switch + 0.03% binary [kernel.kallsyms] [k] copy_page ======================================================================== Thanks Shakeel for the testing. [ktkhai@xxxxxxxxxxxxx: v2] Link: http://lkml.kernel.org/r/151203869520.3915.2587549826865799173.stgit@localhost.localdomain Link: http://lkml.kernel.org/r/150583358557.26700.8490036563698102569.stgit@localhost.localdomain Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Acked-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/list_lru.h | 3 + mm/list_lru.c | 67 ++++++++++++++++++++++++------------- 2 files changed, 47 insertions(+), 23 deletions(-) diff -puN include/linux/list_lru.h~mm-make-count-list_lru_one-nr_items-lockless include/linux/list_lru.h --- a/include/linux/list_lru.h~mm-make-count-list_lru_one-nr_items-lockless +++ a/include/linux/list_lru.h @@ -32,6 +32,7 @@ struct list_lru_one { }; struct list_lru_memcg { + struct rcu_head rcu; /* array of per cgroup lists, indexed by memcg_cache_id */ struct list_lru_one *lru[0]; }; @@ -43,7 +44,7 @@ struct list_lru_node { struct list_lru_one lru; #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB) /* for cgroup aware lrus points to per cgroup lists, otherwise NULL */ - struct list_lru_memcg *memcg_lrus; + struct list_lru_memcg __rcu *memcg_lrus; #endif long nr_items; } ____cacheline_aligned_in_smp; diff -puN mm/list_lru.c~mm-make-count-list_lru_one-nr_items-lockless mm/list_lru.c --- a/mm/list_lru.c~mm-make-count-list_lru_one-nr_items-lockless +++ a/mm/list_lru.c @@ -52,14 +52,15 @@ static inline bool list_lru_memcg_aware( static inline struct list_lru_one * list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx) { + struct list_lru_memcg *memcg_lrus; /* - * The lock protects the array of per cgroup lists from relocation - * (see memcg_update_list_lru_node). + * Either lock or RCU protects the array of per cgroup lists + * from relocation (see memcg_update_list_lru_node). */ - lockdep_assert_held(&nlru->lock); - if (nlru->memcg_lrus && idx >= 0) - return nlru->memcg_lrus->lru[idx]; - + memcg_lrus = rcu_dereference_check(nlru->memcg_lrus, + lockdep_is_held(&nlru->lock)); + if (memcg_lrus && idx >= 0) + return memcg_lrus->lru[idx]; return &nlru->lru; } @@ -168,10 +169,10 @@ static unsigned long __list_lru_count_on struct list_lru_one *l; unsigned long count; - spin_lock(&nlru->lock); + rcu_read_lock(); l = list_lru_from_memcg_idx(nlru, memcg_idx); count = l->nr_items; - spin_unlock(&nlru->lock); + rcu_read_unlock(); return count; } @@ -324,24 +325,41 @@ fail: static int memcg_init_list_lru_node(struct list_lru_node *nlru) { + struct list_lru_memcg *memcg_lrus; int size = memcg_nr_cache_ids; - nlru->memcg_lrus = kvmalloc(size * sizeof(void *), GFP_KERNEL); - if (!nlru->memcg_lrus) + memcg_lrus = kvmalloc(sizeof(*memcg_lrus) + + size * sizeof(void *), GFP_KERNEL); + if (!memcg_lrus) return -ENOMEM; - if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) { - kvfree(nlru->memcg_lrus); + if (__memcg_init_list_lru_node(memcg_lrus, 0, size)) { + kvfree(memcg_lrus); return -ENOMEM; } + RCU_INIT_POINTER(nlru->memcg_lrus, memcg_lrus); return 0; } static void memcg_destroy_list_lru_node(struct list_lru_node *nlru) { - __memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids); - kvfree(nlru->memcg_lrus); + struct list_lru_memcg *memcg_lrus; + /* + * This is called when shrinker has already been unregistered, + * and nobody can use it. So, there is no need to use kvfree_rcu(). + */ + memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus, true); + __memcg_destroy_list_lru_node(memcg_lrus, 0, memcg_nr_cache_ids); + kvfree(memcg_lrus); +} + +static void kvfree_rcu(struct rcu_head *head) +{ + struct list_lru_memcg *mlru; + + mlru = container_of(head, struct list_lru_memcg, rcu); + kvfree(mlru); } static int memcg_update_list_lru_node(struct list_lru_node *nlru, @@ -351,8 +369,9 @@ static int memcg_update_list_lru_node(st BUG_ON(old_size > new_size); - old = nlru->memcg_lrus; - new = kvmalloc(new_size * sizeof(void *), GFP_KERNEL); + old = rcu_dereference_protected(nlru->memcg_lrus, + lockdep_is_held(&list_lrus_mutex)); + new = kvmalloc(sizeof(*new) + new_size * sizeof(void *), GFP_KERNEL); if (!new) return -ENOMEM; @@ -361,29 +380,33 @@ static int memcg_update_list_lru_node(st return -ENOMEM; } - memcpy(new, old, old_size * sizeof(void *)); + memcpy(&new->lru, &old->lru, old_size * sizeof(void *)); /* - * The lock guarantees that we won't race with a reader - * (see list_lru_from_memcg_idx). + * The locking below allows readers that hold nlru->lock avoid taking + * rcu_read_lock (see list_lru_from_memcg_idx). * * Since list_lru_{add,del} may be called under an IRQ-safe lock, * we have to use IRQ-safe primitives here to avoid deadlock. */ spin_lock_irq(&nlru->lock); - nlru->memcg_lrus = new; + rcu_assign_pointer(nlru->memcg_lrus, new); spin_unlock_irq(&nlru->lock); - kvfree(old); + call_rcu(&old->rcu, kvfree_rcu); return 0; } static void memcg_cancel_update_list_lru_node(struct list_lru_node *nlru, int old_size, int new_size) { + struct list_lru_memcg *memcg_lrus; + + memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus, + lockdep_is_held(&list_lrus_mutex)); /* do not bother shrinking the array back to the old size, because we * cannot handle allocation failures here */ - __memcg_destroy_list_lru_node(nlru->memcg_lrus, old_size, new_size); + __memcg_destroy_list_lru_node(memcg_lrus, old_size, new_size); } static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware) _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
