[PATCH 1/3] rcu/tree: use more permissive parameters when attaching a head

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

It is documneted that a headless object can be reclaimed from
might_sleep() context only. Because of that when a head is
dynamically attached it makes sense to drop the lock and do
an allocation with much more permissve flags comparing if it
is done from atomic context.

That is why use GFP_KERNEL flag plus some extra ones which
would make an allocation most likely to be succeed. The big
advantage of doing so is a direct reclaim process.

Tested such approach on my local tiny system with 145MB of
ram(the minimum amount the KVM system is capable of booting)
and 4xCPUs. For stressing the rcuperf module was used. During
tests with difference combinations i did not observe any hit
of our last emergency case, when synchronize_rcu() is involved.

Please note, the "dynamically attaching" path was enabled only,
apart of that all types of objects were considered as headless
variant during testing.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6172e6296dd7..24f620a06219 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3148,13 +3148,10 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
 {
 	unsigned long *ptr;
 
+	/* Try hard to get the memory. */
 	ptr = kmalloc(sizeof(unsigned long *) +
-			sizeof(struct rcu_head), GFP_NOWAIT | __GFP_NOWARN);
-
-	if (!ptr)
-		ptr = kmalloc(sizeof(unsigned long *) +
-				sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
-
+		sizeof(struct rcu_head), GFP_KERNEL |
+			__GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL);
 	if (!ptr)
 		return NULL;
 
@@ -3222,9 +3219,20 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 	if (!success) {
 		/* Is headless object? */
 		if (head == NULL) {
+			/* Drop the lock. */
+			if (krcp->initialized)
+				spin_unlock(&krcp->lock);
+			local_irq_restore(flags);
+
 			head = attach_rcu_head_to_object(ptr);
 			if (head == NULL)
-				goto unlock_return;
+				goto inline_return;
+
+			/* Take it back. */
+			local_irq_save(flags);
+			krcp = this_cpu_ptr(&krc);
+			if (krcp->initialized)
+				spin_lock(&krcp->lock);
 
 			/*
 			 * Tag the headless object. Such objects have a back-pointer
@@ -3263,6 +3271,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 		spin_unlock(&krcp->lock);
 	local_irq_restore(flags);
 
+inline_return:
 	/*
 	 * High memory pressure, so inline kvfree() after
 	 * synchronize_rcu(). We can do it from might_sleep()
-- 
2.20.1