Messages in this thread Patch in this message | | | Date | Mon, 24 Feb 2020 19:45:51 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH tip/core/rcu 1/2] rcu: Support kfree_bulk() interface in kfree_rcu() |
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Hi,
On Fri, Feb 14, 2020 at 04:00:52PM -0800, paulmck@kernel.org wrote: > From: "Uladzislau Rezki (Sony)" <urezki@gmail.com> > > 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) <urezki@gmail.com> > Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > --- > 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 d91c915..51a3aa8 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) > +
Why use the "magic" number "3" here? Could we just define struct kfree_rcu_bulk_data as:
struct kfree_rcu_bulk_data { struct kfree_rcu_bulk_data *next; struct rcu_head *head_free_debug; unsigned long nr_records; void *records[]; }
?
And the the above macro becomes:
#define KFREE_BULK_MAX_ENTR ((PAGE_SIZE - sizeof(struct kfree_rcu_bulk_data)) / sizeof(void *))
Regards, Boqun
> +/** > + * 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; > - } > > - // 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; > + for (i = 0; i < KFREE_N_BATCHES; i++) { > + krwp = &(krcp->krw_arr[i]); > > - 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; > + /* > + * 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; > + } > + } > + > + 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; > } > -- > 2.9.5 >
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