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rcu/tree: cache specified number of objects

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In order to reduce the dynamic need for pages in kfree_rcu(),
pre-allocate a configurable number of pages per CPU and link
them in a list. When kfree_rcu() reclaims objects, the object's
container page is cached into a list instead of being released
to the low-level page allocator.

Such an approach provides O(1) access to free pages while also
reducing the number of requests to the page allocator. It also
makes the kfree_rcu() code to have free pages available during
a low memory condition.

A read-only sysfs parameter (rcu_min_cached_objs) reflects the
minimum number of allowed cached pages per CPU.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Uladzislau Rezki (Sony) 2020-05-25 23:47:52 +02:00 committed by Paul E. McKenney
parent 69f08d3999
commit 53c72b590b
2 changed files with 70 additions and 4 deletions
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8
Documentation/admin-guide/kernel-parameters.txt
View File

@ -4038,6 +4038,14 @@
latencies, which will choose a value aligned
with the appropriate hardware boundaries.
rcutree.rcu_min_cached_objs= [KNL]
Minimum number of objects which are cached and
maintained per one CPU. Object size is equal
to PAGE_SIZE. The cache allows to reduce the
pressure to page allocator, also it makes the
whole algorithm to behave better in low memory
condition.
rcutree.jiffies_till_first_fqs= [KNL]
Set delay from grace-period initialization to
first attempt to force quiescent states.

66
kernel/rcu/tree.c
View File

@ -175,6 +175,15 @@ module_param(gp_init_delay, int, 0444);
static int gp_cleanup_delay;
module_param(gp_cleanup_delay, int, 0444);
/*
* This rcu parameter is runtime-read-only. It reflects
* a minimum allowed number of objects which can be cached
* per-CPU. Object size is equal to one page. This value
* can be changed at boot time.
*/
static int rcu_min_cached_objs = 2;
module_param(rcu_min_cached_objs, int, 0444);
/* Retrieve RCU kthreads priority for rcutorture */
int rcu_get_gp_kthreads_prio(void)
{
@ -2997,7 +3006,6 @@ struct kfree_rcu_cpu_work {
* 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
@ -3013,13 +3021,22 @@ 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];
raw_spinlock_t lock;
struct delayed_work monitor_work;
bool monitor_todo;
bool initialized;
int count;
/*
* A simple cache list that contains objects for
* reuse purpose. In order to save some per-cpu
* space the list is singular. Even though it is
* lockless an access has to be protected by the
* per-cpu lock.
*/
struct llist_head bkvcache;
int nr_bkv_objs;
};
static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
@ -3056,6 +3073,31 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
local_irq_restore(flags);
}
static inline struct kfree_rcu_bulk_data *
get_cached_bnode(struct kfree_rcu_cpu *krcp)
{
if (!krcp->nr_bkv_objs)
return NULL;
krcp->nr_bkv_objs--;
return (struct kfree_rcu_bulk_data *)
llist_del_first(&krcp->bkvcache);
}
static inline bool
put_cached_bnode(struct kfree_rcu_cpu *krcp,
struct kfree_rcu_bulk_data *bnode)
{
// Check the limit.
if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
return false;
llist_add((struct llist_node *) bnode, &krcp->bkvcache);
krcp->nr_bkv_objs++;
return true;
}
/*
* This function is invoked in workqueue context after a grace period.
* It frees all the objects queued on ->bhead_free or ->head_free.
@ -3091,7 +3133,12 @@ static void kfree_rcu_work(struct work_struct *work)
kfree_bulk(bhead->nr_records, bhead->records);
rcu_lock_release(&rcu_callback_map);
if (cmpxchg(&krcp->bcached, NULL, bhead))
krcp = krc_this_cpu_lock(&flags);
if (put_cached_bnode(krcp, bhead))
bhead = NULL;
krc_this_cpu_unlock(krcp, flags);
if (bhead)
free_page((unsigned long) bhead);
cond_resched_tasks_rcu_qs();
@ -3224,7 +3271,7 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
/* Check if a new block is required. */
if (!krcp->bhead ||
krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
bnode = xchg(&krcp->bcached, NULL);
bnode = get_cached_bnode(krcp);
if (!bnode) {
WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
@ -4277,12 +4324,23 @@ static void __init kfree_rcu_batch_init(void)
for_each_possible_cpu(cpu) {
struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
struct kfree_rcu_bulk_data *bnode;
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;
}
for (i = 0; i < rcu_min_cached_objs; i++) {
bnode = (struct kfree_rcu_bulk_data *)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
if (bnode)
put_cached_bnode(krcp, bnode);
else
pr_err("Failed to preallocate for %d CPU!\n", cpu);
}
INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
krcp->initialized = true;
}