Implement an access to the page cache as lock-free variant. This
is done because there are extra places where an access is required,
therefore making it lock-less will remove any lock contention.
For example we have a shrinker path as well as a reclaim kthread.
In both cases a current CPU can access to a remote per-cpu page
cache that would require taking a lock to protect it.
An "rcuscale" performance test suite can detect it and shows some
slight improvements:
../kvm.sh --memory 16G --torture rcuscale --allcpus --duration 10 \
--kconfig CONFIG_NR_CPUS=64 --bootargs "rcuscale.kfree_rcu_test=1 \
rcuscale.kfree_nthreads=16 rcuscale.holdoff=20 rcuscale.kfree_loops=10000 \
rcuscale.kfree_rcu_test_double=1 torture.disable_onoff_at_boot" --trust-make
100 iterations, checking total time taken by all kfree'ers in ns.:
default: AVG: 10968415107.5 MIN: 10668412500 MAX: 11312145160
patch: AVG: 10787596486.1 MIN: 10397559880 MAX: 11214901050
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
kernel/rcu/tree.c | 91 +++++++++++++++++++++++++++++------------------
1 file changed, 56 insertions(+), 35 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9c8cfb01e9a6..4f04664d5ac0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3167,8 +3167,9 @@ struct kfree_rcu_cpu {
atomic_t work_in_progress;
struct hrtimer hrtimer;
+ // lock-free cache.
struct llist_head bkvcache;
- int nr_bkv_objs;
+ atomic_t nr_bkv_objs;
};
static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
@@ -3215,49 +3216,79 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
raw_spin_unlock_irqrestore(&krcp->lock, flags);
}
+/*
+ * Increment 'v', if 'v' is below 'thresh'. Returns true if we
+ * succeeded, false if 'v' + 1 would be bigger than 'thresh'.
+ *
+ * Decrement 'v' if 'v' is upper 'thresh'. Returns true if we
+ * succeeded, false if 'v' - 1 would be smaller than 'thresh'.
+ */
+static inline bool
+atomic_test_inc_dec(atomic_t *v, unsigned int thresh, bool inc)
+{
+ unsigned int cur = atomic_read(v);
+ unsigned int old;
+
+ for (;;) {
+ if (inc) {
+ if (cur >= thresh)
+ return false;
+ } else {
+ if (cur <= thresh)
+ return false;
+ }
+
+ old = atomic_cmpxchg(v, cur, inc ? (cur + 1):(cur - 1));
+ if (old == cur)
+ break;
+
+ cur = old;
+ }
+
+ return true;
+}
+
static inline struct kvfree_rcu_bulk_data *
get_cached_bnode(struct kfree_rcu_cpu *krcp)
{
- if (!krcp->nr_bkv_objs)
- return NULL;
+ struct kvfree_rcu_bulk_data *bnode = NULL;
- krcp->nr_bkv_objs--;
- return (struct kvfree_rcu_bulk_data *)
- llist_del_first(&krcp->bkvcache);
+ if (atomic_test_inc_dec(&krcp->nr_bkv_objs, 0, false))
+ bnode = (struct kvfree_rcu_bulk_data *)
+ llist_del_first(&krcp->bkvcache);
+
+ return bnode;
}
static inline bool
put_cached_bnode(struct kfree_rcu_cpu *krcp,
struct kvfree_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;
+ if (atomic_test_inc_dec(&krcp->nr_bkv_objs, rcu_min_cached_objs, true)) {
+ llist_add((struct llist_node *) bnode, &krcp->bkvcache);
+ return true;
+ }
+ return false;
}
static int
drain_page_cache(struct kfree_rcu_cpu *krcp)
{
- unsigned long flags;
- struct llist_node *page_list, *pos, *n;
- int freed = 0;
+ struct kvfree_rcu_bulk_data *bnode;
+ int num_pages, i;
- raw_spin_lock_irqsave(&krcp->lock, flags);
- page_list = llist_del_all(&krcp->bkvcache);
- krcp->nr_bkv_objs = 0;
- raw_spin_unlock_irqrestore(&krcp->lock, flags);
+ num_pages = atomic_read(&krcp->nr_bkv_objs);
+
+ for (i = 0; i < num_pages; i++) {
+ bnode = get_cached_bnode(krcp);
+ if (!bnode)
+ break;
- llist_for_each_safe(pos, n, page_list) {
- free_page((unsigned long)pos);
- freed++;
+ free_page((unsigned long) bnode);
}
- return freed;
+ return i;
}
/*
@@ -3314,10 +3345,8 @@ static void kfree_rcu_work(struct work_struct *work)
}
rcu_lock_release(&rcu_callback_map);
- raw_spin_lock_irqsave(&krcp->lock, flags);
if (put_cached_bnode(krcp, bkvhead[i]))
bkvhead[i] = NULL;
- raw_spin_unlock_irqrestore(&krcp->lock, flags);
if (bkvhead[i])
free_page((unsigned long) bkvhead[i]);
@@ -3460,7 +3489,6 @@ static void fill_page_cache_func(struct work_struct *work)
struct kfree_rcu_cpu *krcp =
container_of(work, struct kfree_rcu_cpu,
page_cache_work.work);
- unsigned long flags;
bool pushed;
int i;
@@ -3469,10 +3497,7 @@ static void fill_page_cache_func(struct work_struct *work)
__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
if (bnode) {
- raw_spin_lock_irqsave(&krcp->lock, flags);
pushed = put_cached_bnode(krcp, bnode);
- raw_spin_unlock_irqrestore(&krcp->lock, flags);
-
if (!pushed) {
free_page((unsigned long) bnode);
break;
@@ -3647,17 +3672,13 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
{
int cpu;
unsigned long count = 0;
- unsigned long flags;
/* Snapshot count of all CPUs */
for_each_possible_cpu(cpu) {
struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
count += READ_ONCE(krcp->count);
-
- raw_spin_lock_irqsave(&krcp->lock, flags);
- count += krcp->nr_bkv_objs;
- raw_spin_unlock_irqrestore(&krcp->lock, flags);
+ count += atomic_read(&krcp->nr_bkv_objs);
}
WRITE_ONCE(backoff_page_cache_fill, true);
--
2.20.1
