[PATCH v2 14/16] rcu: Support reclaim for head-less object

Uladzislau Rezki (Sony)
2020-05-25T21:48:38Z

Update the kvfree_call_rcu() function with head-less support.
This allows RCU to reclaim objects without an embedded rcu_head.

tree-RCU:
We introduce two chains of arrays to store SLAB-backed and vmalloc
pointers, each. Storage in either of these arrays does not require
embedding an rcu_head within the object.

Maintaining the arrays may become impossible due to high memory
pressure. For such cases there is an emergency path. Objects with
rcu_head inside are just queued on a backup rcu_head list. Later on
that list is drained. As for the head-less variant, as the current
context can sleep, the following emergency measures are applied:
a) Synchronously wait until a grace period has elapsed.
b) Call kvfree().

tiny-RCU:
For double argument calls, there are no new changes in behavior. For
single argument call, kvfree() is directly inlined on the current
stack after a synchronize_rcu() call. Note that for tiny-RCU, any
call to synchronize_rcu() is actually a quiescent state, therefore
it does nothing.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
include/linux/rcutiny.h | 18 ++++++++++++++++-
kernel/rcu/tree.c | 45 +++++++++++++++++++++++++++++++++++++++--
2 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 00bbd0e328c8..e7e4ec5938af 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -31,9 +31,25 @@ static inline void synchronize_rcu_expedited(void)
synchronize_rcu();
}

+/*
+ * Add one more declaration of kvfree() here. It is
+ * not so straight forward to just include <linux/mm.h>
+ * where it is defined due to getting many compile
+ * errors caused by that include.
+ */
+extern void kvfree(const void *addr);
+
static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
{
- call_rcu(head, func);
+ if (head) {
+ call_rcu(head, func);
+ return;
+ }
+
+ // kvfree_rcu(one_arg) call.
+ might_sleep();
+ synchronize_rcu();
+ kvfree((void *) func);
}

void rcu_qs(void);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index da29e6078392..2459830a3851 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3180,6 +3180,13 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
if (IS_ENABLED(CONFIG_PREEMPT_RT))
return false;

+ /*
+ * NOTE: For one argument of kvfree_rcu() we can
+ * drop the lock and get the page in sleepable
+ * context. That would allow to maintain an array
+ * for the CONFIG_PREEMPT_RT as well if no cached
+ * pages are available.
+ */
bnode = (struct kvfree_rcu_bulk_data *)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
}
@@ -3219,16 +3226,33 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
{
unsigned long flags;
struct kfree_rcu_cpu *krcp;
+ bool success;
void *ptr;

+ if (head) {
+ ptr = (void *) head - (unsigned long) func;
+ } else {
+ /*
+ * Please note there is a limitation for the head-less
+ * variant, that is why there is a clear rule for such
+ * objects: it can be used from might_sleep() context
+ * only. For other places please embed an rcu_head to
+ * your data.
+ */
+ might_sleep();
+ ptr = (unsigned long *) func;
+ }
+
krcp = krc_this_cpu_lock(&flags);
- ptr = (void *)head - (unsigned long)func;

// Queue the object but don't yet schedule the batch.
if (debug_rcu_head_queue(ptr)) {
// Probable double kfree_rcu(), just leak.
WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
__func__, head);
+
+ // Mark as success and leave.
+ success = true;
goto unlock_return;
}

@@ -3236,10 +3260,16 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
* Under high memory pressure GFP_NOWAIT can fail,
* in that case the emergency path is maintained.
*/
- if (unlikely(!kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr))) {
+ success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
+ if (!success) {
+ if (head == NULL)
+ // Inline if kvfree_rcu(one_arg) call.
+ goto unlock_return;
+
head->func = func;
head->next = krcp->head;
krcp->head = head;
+ success = true;
}

WRITE_ONCE(krcp->count, krcp->count + 1);
@@ -3253,6 +3283,17 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)

unlock_return:
krc_this_cpu_unlock(krcp, flags);
+
+ /*
+ * Inline kvfree() after synchronize_rcu(). We can do
+ * it from might_sleep() context only, so the current
+ * CPU can pass the QS state.
+ */
+ if (!success) {
+ debug_rcu_head_unqueue((struct rcu_head *) ptr);
+ synchronize_rcu();
+ kvfree(ptr);
+ }
}
EXPORT_SYMBOL_GPL(kvfree_call_rcu);

--
2.20.1