Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <e0f336e9-d167-18a8-0af8-0d5517bae9a5@redhat.com>
Date:   Wed, 20 Oct 2021 12:37:32 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.1.0
Subject: Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU
Content-Language: en-US
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <1634631160-67276-1-git-send-email-wanpengli@tencent.com>
 <1634631160-67276-3-git-send-email-wanpengli@tencent.com>
 <24e67e43-c50c-7e0f-305a-c7f6129f8d70@redhat.com>
 <YW8BmRJHVvFscWTo@google.com>
 <CANRm+CzuWnO8FZPTvvOtpxqc5h786o7THyebOFpVAp3BF1xQiw@mail.gmail.com>
 <45fabf5a-96b5-49dc-0cba-55714ae3a4b5@redhat.com>
 <CANRm+CyPznw0O2qwnhhc=YEq+zSD3C7dqqG8-8XE6sLdhL7aXQ@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <CANRm+CyPznw0O2qwnhhc=YEq+zSD3C7dqqG8-8XE6sLdhL7aXQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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