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

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Message-ID: <e0f336e9-d167-18a8-0af8-0d5517bae9a5@redhat.com>
Date:   Wed, 20 Oct 2021 12:37:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

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