Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

[Paolo Bonzini]

On 20/10/21 04:49, Wanpeng Li wrote:
> > The intent of the extra check was to avoid the locked instruction that comes with
> > disabling preemption via rcu_read_lock().  But thinking more, the extra op should
> > be little more than a basic arithmetic operation in the grand scheme on modern x86
> > since the cache line is going to be locked and written no matter what, either
> > immediately before or immediately after.
>
> I observe the main overhead of rcuwait_wake_up() is from rcu
> operations, especially rcu_read_lock/unlock().

Do you have CONFIG_PREEMPT_RCU set?  If so, maybe something like this would help:

diff --git a/kernel/exit.c b/kernel/exit.c
index fd1c04193e18..ca1e60a1234d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -235,8 +235,6 @@ int rcuwait_wake_up(struct rcuwait *w)
 	int ret = 0;
 	struct task_struct *task;
 
-	rcu_read_lock();
-
 	/*
 	 * Order condition vs @task, such that everything prior to the load
 	 * of @task is visible. This is the condition as to why the user called
@@ -250,6 +248,14 @@ int rcuwait_wake_up(struct rcuwait *w)
 	 */
 	smp_mb(); /* (B) */
 
+#ifdef CONFIG_PREEMPT_RCU
+	/* The cost of rcu_read_lock() is nontrivial for preemptable RCU.  */
+	if (!rcuwait_active(w))
+		return ret;
+#endif
+
+	rcu_read_lock();
+
 	task = rcu_dereference(w->task);
 	if (task)
 		ret = wake_up_process(task);

(If you don't, rcu_read_lock is essentially preempt_disable() and it
should not have a large overhead).  You still need the memory barrier
though, in order to avoid missed wakeups; shameless plug for my
article at https://lwn.net/Articles/847481/.

Paolo

> > So with Paolo's other comment, maybe just this?  And if this doesn't provide the
> > desired performance boost, changes to the rcuwait behavior should go in separate
> > patch.
> Ok.