kvfree_rcu: Release a page cache under memory pressure

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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cb89dbdedc46381114cf94594d0fff68bf86e0a4..4405fd32e8ab07004abedf45c74d45c9f5b0ae67 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4290,6 +4290,11 @@
                        whole algorithm to behave better in low memory
                        condition.
 
+       rcutree.rcu_delay_page_cache_fill_msec= [KNL]
+                       Set the page-cache refill delay (in milliseconds)
+                       in response to low-memory conditions.  The range
+                       of permitted values is in the range 0:100000.
+
        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 8e78b2430c168f29054be180424dc9ea573019f4..74d840aa877bec5d512a009cd8afdcf098e150e1 100644 (file)
--- 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 pages to be freed to make them
+// available for other parts of the system. This usually happens
+// under low memory conditions, and in that case we should also
+// defer page-cache filling for a short time period.
+//
+// The default value is 5 seconds, which is long enough to reduce
+// interference with the shrinker while it asks other systems to
+// drain their caches.
+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)
 {
@@ -3171,6 +3182,7 @@ 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 cache refills
  * @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.
@@ -3190,7 +3202,8 @@ 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;
 
@@ -3256,6 +3269,26 @@ 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.
@@ -3446,7 +3479,7 @@ 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;
 }
 
@@ -3455,12 +3488,16 @@ 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);
 
@@ -3477,6 +3514,7 @@ 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
@@ -3484,10 +3522,15 @@ 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);
+               }
        }
 }
 
@@ -3639,12 +3682,19 @@ 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;
@@ -3661,6 +3711,8 @@ 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);
@@ -4687,6 +4739,18 @@ 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 rcutree.rcu_delay_page_cache_fill_msec 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);
 
@@ -4696,7 +4760,7 @@ 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))