linux-kernel - Re: [PATCH v2] cpuidle-haltpoll: Enable kvm guest polling when dedicated physical CPUs are available

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Date:   Tue, 10 Sep 2019 19:15:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] cpuidle-haltpoll: Enable kvm guest polling when
 dedicated physical CPUs are available

On 04/09/19 11:48, Rafael J. Wysocki wrote:
> On 8/29/2019 10:49 AM, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> The downside of guest side polling is that polling is performed even
>> with other runnable tasks in the host. However, even if poll in kvm
>> can aware whether or not other runnable tasks in the same pCPU, it
>> can still incur extra overhead in over-subscribe scenario. Now we can
>> just enable guest polling when dedicated pCPUs are available.
>>
>> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> 
> As stated before, I'm going to queue up this change for 5.4, with the
> Paolo's ACK.
> 
> BTW, in the future please CC power management changes to
> linux-pm@...r.kernel.org for easier handling.

Thanks.  This patch makes sense to me and I don't know what
"limitations" are there in KVM_HINTS_REALTIME that Marcelo mentioned.

Any improvements that Marcelo can discuss can be made on top of this
during the 5.4 merge window, via the KVM tree.

Thanks Rafael for handling the reviewing and merging of this series.

Paolo

> 
>> -- 
>> v1 -> v2:
>>   * export kvm_arch_para_hints to fix haltpoll driver build as module
>> error
>>   * just disable haltpoll driver instead of both driver and governor
>>     since KVM_HINTS_REALTIME is not defined in other arches, and governor
>>     doesn't depend on x86, to fix the warning on powerpc
>>
>>   arch/x86/kernel/kvm.c              | 1 +
>>   drivers/cpuidle/cpuidle-haltpoll.c | 3 ++-
>>   2 files changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index f48401b..68463c1 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -711,6 +711,7 @@ unsigned int kvm_arch_para_hints(void)
>>   {
>>       return cpuid_edx(kvm_cpuid_base() | KVM_CPUID_FEATURES);
>>   }
>> +EXPORT_SYMBOL_GPL(kvm_arch_para_hints);
>>     static uint32_t __init kvm_detect(void)
>>   {
>> diff --git a/drivers/cpuidle/cpuidle-haltpoll.c
>> b/drivers/cpuidle/cpuidle-haltpoll.c
>> index 9ac093d..7aee38a 100644
>> --- a/drivers/cpuidle/cpuidle-haltpoll.c
>> +++ b/drivers/cpuidle/cpuidle-haltpoll.c
>> @@ -53,7 +53,8 @@ static int __init haltpoll_init(void)
>>         cpuidle_poll_state_init(drv);
>>   -    if (!kvm_para_available())
>> +    if (!kvm_para_available() ||
>> +        !kvm_para_has_hint(KVM_HINTS_REALTIME))
>>           return 0;
>>         ret = cpuidle_register(&haltpoll_driver, NULL);
> 
>