[PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of WQ_HIGHPRI queue

Uladzislau Rezki (Sony)
2020-09-18T19:48:47Z

Recently the separate worker thread has been introduced to
maintain the local page cache from the regular kernel context,
instead of kvfree_rcu() contexts. That was done because a caller
of the k[v]free_rcu() can be any context type what is a problem
from the allocation point of view.

From the other hand, the lock-less way of obtaining a page has
been introduced and directly injected to the k[v]free_rcu() path.

Therefore it is not important anymore to use a high priority "wq"
for the external job that used to fill a page cache ASAP when it
was empty.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d51209343029..f2b4215631f7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3100,7 +3100,7 @@ struct kfree_rcu_cpu {
* lockless an access has to be protected by the
* per-cpu lock.
*/
- struct work_struct page_cache_work;
+ struct delayed_work page_cache_work;
atomic_t work_in_progress;
struct llist_head bkvcache;
int nr_bkv_objs;
@@ -3354,7 +3354,7 @@ 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;
bool pushed;
int i;
@@ -3440,7 +3440,6 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
{
unsigned long flags;
struct kfree_rcu_cpu *krcp;
- bool irq_disabled = irqs_disabled();
bool success;
void *ptr;

@@ -3473,9 +3472,9 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)

success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
if (!success) {
- // TODO: schedule the work from the hrtimer.
- if (!irq_disabled && !atomic_xchg(&krcp->work_in_progress, 1))
- queue_work(system_highpri_wq, &krcp->page_cache_work);
+ // Use delayed work, so we do not deadlock with rq->lock.
+ if (!atomic_xchg(&krcp->work_in_progress, 1))
+ schedule_delayed_work(&krcp->page_cache_work, 1);

if (head == NULL)
// Inline if kvfree_rcu(one_arg) call.
@@ -4475,7 +4474,7 @@ 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