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

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Date:   Wed, 4 Sep 2019 11:48:13 +0200
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 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.

> --
> 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);