[PATCH v2 1/5] kvfree_rcu: Release a page cache under memory pressure

Uladzislau Rezki (Sony)
2021-04-15T17:20:21Z

From: Zhang Qiang <qiang.zhang@windriver.com>

Add a drain_page_cache() function to drain a per-cpu page cache.
The reason behind of it is a system can run into a low memory
condition, in that case a page shrinker can ask for its users
to free their caches in order to get extra memory available for
other needs in a system.

When a system hits such condition, a page cache is drained for
all CPUs in a system. By default a page cache work is delayed
with 5 seconds interval until a memory pressure disappears, if
needed it can be changed. See a rcu_delay_page_cache_fill_msec
module parameter.

Co-developed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Zqiang <qiang.zhang@windriver.com>
---
.../admin-guide/kernel-parameters.txt | 5 ++
kernel/rcu/tree.c | 82 +++++++++++++++++--
2 files changed, 78 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 78dc87435ca7..6b769f5cf14c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4154,6 +4154,11 @@
whole algorithm to behave better in low memory
condition.

+ rcutree.rcu_delay_page_cache_fill_msec= [KNL]
+ Set delay for a page-cache refill when a low memory
+ condition occurs. That is in milliseconds. Allowed
+ value is within a 0:100000 range.
+
rcutree.jiffies_till_first_fqs= [KNL]
Set delay from grace-period initialization to
first attempt to force quiescent states.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2c9cf4df942c..742152d6b952 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -186,6 +186,17 @@ module_param(rcu_unlock_delay, int, 0444);
static int rcu_min_cached_objs = 5;
module_param(rcu_min_cached_objs, int, 0444);

+// A page shrinker can ask for freeing extra pages to get them
+// available for other needs in a system. Usually it happens
+// under low memory condition, in that case we should hold on
+// a bit with page cache filling.
+//
+// Default value is 5 seconds. That is long enough to reduce
+// an interfering and racing with a shrinker where the cache
+// is drained.
+static int rcu_delay_page_cache_fill_msec = 5000;
+module_param(rcu_delay_page_cache_fill_msec, int, 0444);
+
/* Retrieve RCU kthreads priority for rcutorture */
int rcu_get_gp_kthreads_prio(void)
{
@@ -3144,6 +3155,7 @@ struct kfree_rcu_cpu_work {
* Even though it is lockless an access has to be protected by the
* per-cpu lock.
* @page_cache_work: A work to refill the cache when it is empty
+ * @backoff_page_cache_fill: Delay a cache filling
* @work_in_progress: Indicates that page_cache_work is running
* @hrtimer: A hrtimer for scheduling a page_cache_work
* @nr_bkv_objs: number of allocated objects at @bkvcache.
@@ -3163,7 +3175,8 @@ struct kfree_rcu_cpu {
bool initialized;
int count;

- struct work_struct page_cache_work;
+ struct delayed_work page_cache_work;
+ atomic_t backoff_page_cache_fill;
atomic_t work_in_progress;
struct hrtimer hrtimer;

@@ -3229,6 +3242,26 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,

}

+static int
+drain_page_cache(struct kfree_rcu_cpu *krcp)
+{
+ unsigned long flags;
+ struct llist_node *page_list, *pos, *n;
+ int freed = 0;
+
+ 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);
+
+ llist_for_each_safe(pos, n, page_list) {
+ free_page((unsigned long)pos);
+ freed++;
+ }
+
+ return freed;
+}
+
/*
* This function is invoked in workqueue context after a grace period.
* It frees all the objects queued on ->bhead_free or ->head_free.
@@ -3419,7 +3452,7 @@ schedule_page_work_fn(struct hrtimer *t)
struct kfree_rcu_cpu *krcp =
container_of(t, struct kfree_rcu_cpu, hrtimer);

- queue_work(system_highpri_wq, &krcp->page_cache_work);
+ queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, 0);
return HRTIMER_NORESTART;
}

@@ -3428,12 +3461,16 @@ static void fill_page_cache_func(struct work_struct *work)
struct kvfree_rcu_bulk_data *bnode;
struct kfree_rcu_cpu *krcp =
container_of(work, struct kfree_rcu_cpu,
- page_cache_work);
+ page_cache_work.work);
unsigned long flags;
+ int nr_pages;
bool pushed;
int i;

- for (i = 0; i < rcu_min_cached_objs; i++) {
+ nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
+ 1 : rcu_min_cached_objs;
+
+ for (i = 0; i < nr_pages; i++) {
bnode = (struct kvfree_rcu_bulk_data *)
__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);

@@ -3450,6 +3487,7 @@ static void fill_page_cache_func(struct work_struct *work)
}

atomic_set(&krcp->work_in_progress, 0);
+ atomic_set(&krcp->backoff_page_cache_fill, 0);
}

static void
@@ -3457,10 +3495,15 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
{
if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
!atomic_xchg(&krcp->work_in_progress, 1)) {
- hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
- HRTIMER_MODE_REL);
- krcp->hrtimer.function = schedule_page_work_fn;
- hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
+ if (atomic_read(&krcp->backoff_page_cache_fill)) {
+ queue_delayed_work(system_wq,
+ &krcp->page_cache_work,
+ msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
+ } else {
+ hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ krcp->hrtimer.function = schedule_page_work_fn;
+ hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
+ }
}
}

@@ -3612,12 +3655,19 @@ 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);
+
+ atomic_set(&krcp->backoff_page_cache_fill, 1);
}

return count;
@@ -3634,6 +3684,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);

count = krcp->count;
+ count += drain_page_cache(krcp);
+
raw_spin_lock_irqsave(&krcp->lock, flags);
if (krcp->monitor_todo)
kfree_rcu_drain_unlock(krcp, flags);
@@ -4599,6 +4651,18 @@ static void __init kfree_rcu_batch_init(void)
int cpu;
int i;

+ /* Clamp it to [0:100] seconds interval. */
+ if (rcu_delay_page_cache_fill_msec < 0 ||
+ rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) {
+
+ rcu_delay_page_cache_fill_msec =
+ clamp(rcu_delay_page_cache_fill_msec, 0,
+ (int) (100 * MSEC_PER_SEC));
+
+ pr_info("Adjusting a cache fill delay interval to %d ms.\n",
+ rcu_delay_page_cache_fill_msec);
+ }
+
for_each_possible_cpu(cpu) {
struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);

@@ -4608,7 +4672,7 @@ static void __init kfree_rcu_batch_init(void)
}

INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
- INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
+ INIT_DELAYED_WORK(&krcp->page_cache_work, fill_page_cache_func);
krcp->initialized = true;
}
if (register_shrinker(&kfree_rcu_shrinker))
--
2.20.1