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

[Paolo Bonzini]

On 20/10/21 12:02, Wanpeng Li wrote:
> > +#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 athttps://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?

Yes, I'll take care of this.  Thanks!

Paolo