From 34c881745549e78f31ec65f319457c82aacc53bd Mon Sep 17 00:00:00 2001 From: "Uladzislau Rezki (Sony)" Date: Mon, 20 Jan 2020 15:42:25 +0100 Subject: [PATCH] rcu: Support kfree_bulk() interface in kfree_rcu() The kfree_rcu() logic can be improved further by using kfree_bulk() interface along with "basic batching support" introduced earlier. The are at least two advantages of using "bulk" interface: - in case of large number of kfree_rcu() requests kfree_bulk() reduces the per-object overhead caused by calling kfree() per-object. - reduces the number of cache-misses due to "pointer chasing" between objects which can be far spread between each other. This approach defines a new kfree_rcu_bulk_data structure that stores pointers in an array with a specific size. Number of entries in that array depends on PAGE_SIZE making kfree_rcu_bulk_data structure to be exactly one page. Since it deals with "block-chain" technique there is an extra need in dynamic allocation when a new block is required. Memory is allocated with GFP_NOWAIT | __GFP_NOWARN flags, i.e. that allows to skip direct reclaim under low memory condition to prevent stalling and fails silently under high memory pressure. The "emergency path" gets maintained when a system is run out of memory. In that case objects are linked into regular list. The "rcuperf" was run to analyze this change in terms of memory consumption and kfree_bulk() throughput. 1) Testing on the Intel(R) Xeon(R) W-2135 CPU @ 3.70GHz, 12xCPUs with following parameters: kfree_loops=200000 kfree_alloc_num=1000 kfree_rcu_test=1 kfree_vary_obj_size=1 dev.2020.01.10a branch Default / CONFIG_SLAB 53607352517 ns, loops: 200000, batches: 1885, memory footprint: 1248MB 53529637912 ns, loops: 200000, batches: 1921, memory footprint: 1193MB 53570175705 ns, loops: 200000, batches: 1929, memory footprint: 1250MB Patch / CONFIG_SLAB 23981587315 ns, loops: 200000, batches: 810, memory footprint: 1219MB 23879375281 ns, loops: 200000, batches: 822, memory footprint: 1190MB 24086841707 ns, loops: 200000, batches: 794, memory footprint: 1380MB Default / CONFIG_SLUB 51291025022 ns, loops: 200000, batches: 1713, memory footprint: 741MB 51278911477 ns, loops: 200000, batches: 1671, memory footprint: 719MB 51256183045 ns, loops: 200000, batches: 1719, memory footprint: 647MB Patch / CONFIG_SLUB 50709919132 ns, loops: 200000, batches: 1618, memory footprint: 456MB 50736297452 ns, loops: 200000, batches: 1633, memory footprint: 507MB 50660403893 ns, loops: 200000, batches: 1628, memory footprint: 429MB in case of CONFIG_SLAB there is double increase in performance and slightly higher memory usage. As for CONFIG_SLUB, the performance figures are better together with lower memory usage. 2) Testing on the HiKey-960, arm64, 8xCPUs with below parameters: CONFIG_SLAB=y kfree_loops=200000 kfree_alloc_num=1000 kfree_rcu_test=1 102898760401 ns, loops: 200000, batches: 5822, memory footprint: 158MB 89947009882 ns, loops: 200000, batches: 6715, memory footprint: 115MB rcuperf shows approximately ~12% better throughput in case of using "bulk" interface. The "drain logic" or its RCU callback does the work faster that leads to better throughput. Signed-off-by: Uladzislau Rezki (Sony) Tested-by: Joel Fernandes (Google) Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 204 ++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 169 insertions(+), 35 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index d91c9156fab2..51a3aa884a7c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2689,22 +2689,47 @@ EXPORT_SYMBOL_GPL(call_rcu); #define KFREE_DRAIN_JIFFIES (HZ / 50) #define KFREE_N_BATCHES 2 +/* + * This macro defines how many entries the "records" array + * will contain. It is based on the fact that the size of + * kfree_rcu_bulk_data structure becomes exactly one page. + */ +#define KFREE_BULK_MAX_ENTR ((PAGE_SIZE / sizeof(void *)) - 3) + +/** + * struct kfree_rcu_bulk_data - single block to store kfree_rcu() pointers + * @nr_records: Number of active pointers in the array + * @records: Array of the kfree_rcu() pointers + * @next: Next bulk object in the block chain + * @head_free_debug: For debug, when CONFIG_DEBUG_OBJECTS_RCU_HEAD is set + */ +struct kfree_rcu_bulk_data { + unsigned long nr_records; + void *records[KFREE_BULK_MAX_ENTR]; + struct kfree_rcu_bulk_data *next; + struct rcu_head *head_free_debug; +}; + /** * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period * @head_free: List of kfree_rcu() objects waiting for a grace period + * @bhead_free: Bulk-List of kfree_rcu() objects waiting for a grace period * @krcp: Pointer to @kfree_rcu_cpu structure */ struct kfree_rcu_cpu_work { struct rcu_work rcu_work; struct rcu_head *head_free; + struct kfree_rcu_bulk_data *bhead_free; struct kfree_rcu_cpu *krcp; }; /** * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period * @head: List of kfree_rcu() objects not yet waiting for a grace period + * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period + * @bcached: Keeps at most one object for later reuse when build chain blocks * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period * @lock: Synchronize access to this structure * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES @@ -2718,6 +2743,8 @@ struct kfree_rcu_cpu_work { */ struct kfree_rcu_cpu { struct rcu_head *head; + struct kfree_rcu_bulk_data *bhead; + struct kfree_rcu_bulk_data *bcached; struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES]; spinlock_t lock; struct delayed_work monitor_work; @@ -2727,14 +2754,24 @@ struct kfree_rcu_cpu { static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc); +static __always_inline void +debug_rcu_head_unqueue_bulk(struct rcu_head *head) +{ +#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD + for (; head; head = head->next) + debug_rcu_head_unqueue(head); +#endif +} + /* * This function is invoked in workqueue context after a grace period. - * It frees all the objects queued on ->head_free. + * It frees all the objects queued on ->bhead_free or ->head_free. */ static void kfree_rcu_work(struct work_struct *work) { unsigned long flags; struct rcu_head *head, *next; + struct kfree_rcu_bulk_data *bhead, *bnext; struct kfree_rcu_cpu *krcp; struct kfree_rcu_cpu_work *krwp; @@ -2744,22 +2781,41 @@ static void kfree_rcu_work(struct work_struct *work) spin_lock_irqsave(&krcp->lock, flags); head = krwp->head_free; krwp->head_free = NULL; + bhead = krwp->bhead_free; + krwp->bhead_free = NULL; spin_unlock_irqrestore(&krcp->lock, flags); - // List "head" is now private, so traverse locklessly. + /* "bhead" is now private, so traverse locklessly. */ + for (; bhead; bhead = bnext) { + bnext = bhead->next; + + debug_rcu_head_unqueue_bulk(bhead->head_free_debug); + + rcu_lock_acquire(&rcu_callback_map); + kfree_bulk(bhead->nr_records, bhead->records); + rcu_lock_release(&rcu_callback_map); + + if (cmpxchg(&krcp->bcached, NULL, bhead)) + free_page((unsigned long) bhead); + + cond_resched_tasks_rcu_qs(); + } + + /* + * Emergency case only. It can happen under low memory + * condition when an allocation gets failed, so the "bulk" + * path can not be temporary maintained. + */ for (; head; head = next) { unsigned long offset = (unsigned long)head->func; next = head->next; - // Potentially optimize with kfree_bulk in future. debug_rcu_head_unqueue(head); rcu_lock_acquire(&rcu_callback_map); trace_rcu_invoke_kfree_callback(rcu_state.name, head, offset); - if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset))) { - /* Could be optimized with kfree_bulk() in future. */ + if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset))) kfree((void *)head - offset); - } rcu_lock_release(&rcu_callback_map); cond_resched_tasks_rcu_qs(); @@ -2774,26 +2830,48 @@ static void kfree_rcu_work(struct work_struct *work) */ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp) { + struct kfree_rcu_cpu_work *krwp; + bool queued = false; int i; - struct kfree_rcu_cpu_work *krwp = NULL; lockdep_assert_held(&krcp->lock); - for (i = 0; i < KFREE_N_BATCHES; i++) - if (!krcp->krw_arr[i].head_free) { - krwp = &(krcp->krw_arr[i]); - break; + + for (i = 0; i < KFREE_N_BATCHES; i++) { + krwp = &(krcp->krw_arr[i]); + + /* + * Try to detach bhead or head and attach it over any + * available corresponding free channel. It can be that + * a previous RCU batch is in progress, it means that + * immediately to queue another one is not possible so + * return false to tell caller to retry. + */ + if ((krcp->bhead && !krwp->bhead_free) || + (krcp->head && !krwp->head_free)) { + /* Channel 1. */ + if (!krwp->bhead_free) { + krwp->bhead_free = krcp->bhead; + krcp->bhead = NULL; + } + + /* Channel 2. */ + if (!krwp->head_free) { + krwp->head_free = krcp->head; + krcp->head = NULL; + } + + /* + * One work is per one batch, so there are two "free channels", + * "bhead_free" and "head_free" the batch can handle. It can be + * that the work is in the pending state when two channels have + * been detached following each other, one by one. + */ + queue_rcu_work(system_wq, &krwp->rcu_work); + queued = true; } + } - // If a previous RCU batch is in progress, we cannot immediately - // queue another one, so return false to tell caller to retry. - if (!krwp) - return false; - - krwp->head_free = krcp->head; - krcp->head = NULL; - INIT_RCU_WORK(&krwp->rcu_work, kfree_rcu_work); - queue_rcu_work(system_wq, &krwp->rcu_work); - return true; + return queued; } static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp, @@ -2830,19 +2908,65 @@ static void kfree_rcu_monitor(struct work_struct *work) spin_unlock_irqrestore(&krcp->lock, flags); } +static inline bool +kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, + struct rcu_head *head, rcu_callback_t func) +{ + struct kfree_rcu_bulk_data *bnode; + + if (unlikely(!krcp->initialized)) + return false; + + lockdep_assert_held(&krcp->lock); + + /* Check if a new block is required. */ + if (!krcp->bhead || + krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) { + bnode = xchg(&krcp->bcached, NULL); + if (!bnode) { + WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); + + bnode = (struct kfree_rcu_bulk_data *) + __get_free_page(GFP_NOWAIT | __GFP_NOWARN); + } + + /* Switch to emergency path. */ + if (unlikely(!bnode)) + return false; + + /* Initialize the new block. */ + bnode->nr_records = 0; + bnode->next = krcp->bhead; + bnode->head_free_debug = NULL; + + /* Attach it to the head. */ + krcp->bhead = bnode; + } + +#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD + head->func = func; + head->next = krcp->bhead->head_free_debug; + krcp->bhead->head_free_debug = head; +#endif + + /* Finally insert. */ + krcp->bhead->records[krcp->bhead->nr_records++] = + (void *) head - (unsigned long) func; + + return true; +} + /* - * Queue a request for lazy invocation of kfree() after a grace period. + * Queue a request for lazy invocation of kfree_bulk()/kfree() after a grace + * period. Please note there are two paths are maintained, one is the main one + * that uses kfree_bulk() interface and second one is emergency one, that is + * used only when the main path can not be maintained temporary, due to memory + * pressure. * * Each kfree_call_rcu() request is added to a batch. The batch will be drained - * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch - * will be kfree'd in workqueue context. This allows us to: - * - * 1. Batch requests together to reduce the number of grace periods during - * heavy kfree_rcu() load. - * - * 2. It makes it possible to use kfree_bulk() on a large number of - * kfree_rcu() requests thus reducing cache misses and the per-object - * overhead of kfree(). + * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch will + * be free'd in workqueue context. This allows us to: batch requests together to + * reduce the number of grace periods during heavy kfree_rcu() load. */ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func) { @@ -2861,9 +2985,16 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func) __func__, head); goto unlock_return; } - head->func = func; - head->next = krcp->head; - krcp->head = head; + + /* + * Under high memory pressure GFP_NOWAIT can fail, + * in that case the emergency path is maintained. + */ + if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func))) { + head->func = func; + head->next = krcp->head; + krcp->head = head; + } // Set timer to drain after KFREE_DRAIN_JIFFIES. if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING && @@ -3769,8 +3900,11 @@ static void __init kfree_rcu_batch_init(void) struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu); spin_lock_init(&krcp->lock); - for (i = 0; i < KFREE_N_BATCHES; i++) + for (i = 0; i < KFREE_N_BATCHES; i++) { + INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work); krcp->krw_arr[i].krcp = krcp; + } + INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor); krcp->initialized = true; }