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

[Wanpeng Li]

On Wed, 20 Oct 2021 at 14:47, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> 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:

Yes.

>
> 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/.

You are right, the cost of rcu_read_lock() for preemptable RCU
introduces too much overhead, do you want to send a separate patch?

    Wanpeng