rcu/tree: Count number of batched kfree_rcu() locklessly

We can relax the correctness of counting of number of queued objects in
favor of not hurting performance, by locklessly sampling per-cpu
counters. This should be Ok since under high memory pressure, it should not
matter if we are off by a few objects while counting. The shrinker will
still do the reclaim.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Remove unused "flags" variable. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e299cd0ddd97c7e83beb1ac13d15b4ff32d3a270..3f1f57411fe15c39a9cfb8f110662c982758f0be 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2939,7 +2939,7 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
                                krcp->head = NULL;
                        }
 
-                       krcp->count = 0;
+                       WRITE_ONCE(krcp->count, 0);
 
                        /*
                         * One work is per one batch, so there are two "free channels",
@@ -3077,7 +3077,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
                krcp->head = head;
        }
 
-       krcp->count++;
+       WRITE_ONCE(krcp->count, krcp->count + 1);
 
        // Set timer to drain after KFREE_DRAIN_JIFFIES.
        if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
@@ -3097,15 +3097,13 @@ static unsigned long
 kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
        int cpu;
-       unsigned long flags, count = 0;
+       unsigned long count = 0;
 
        /* Snapshot count of all CPUs */
        for_each_online_cpu(cpu) {
                struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
-               spin_lock_irqsave(&krcp->lock, flags);
-               count += krcp->count;
-               spin_unlock_irqrestore(&krcp->lock, flags);
+               count += READ_ONCE(krcp->count);
        }
 
        return count;