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Subject: Re: [PATCH] cpuidle-haltpoll: allow force loading on hosts without
 the REALTIME hint
From:   "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     Joao Martins <joao.m.martins@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>, kvm@vger.kernel.org
References: <20200221174331.1480468-1-mail@maciej.szmigiero.name>
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Message-ID: <114f7b8d-6f88-222a-d1fa-abcfc0e6a1f2@maciej.szmigiero.name>
Date:   Mon, 24 Feb 2020 21:10:18 +0100
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(CC'ing also Marcelo as the cpuidle-haltpoll driver author and the KVM ML).

On 21.02.2020 18:43, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> Before commit 1328edca4a14 ("cpuidle-haltpoll: Enable kvm guest polling
> when dedicated physical CPUs are available") the cpuidle-haltpoll driver
> could also be used in scenarios when the host does not advertise the
> KVM_HINTS_REALTIME hint.
> 
> While the behavior introduced by the aforementioned commit makes sense as
> the default there are cases where the old behavior is desired, for example,
> when other kernel changes triggered by presence by this hint are unwanted,
> for some workloads where the latency benefit from polling overweights the
> loss from idle CPU capacity that otherwise would be available, or just when
> running under older Qemu versions that lack this hint.
> 
> Let's provide a typical "force" module parameter that allows restoring the
> old behavior.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>  drivers/cpuidle/cpuidle-haltpoll.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/cpuidle-haltpoll.c b/drivers/cpuidle/cpuidle-haltpoll.c
> index b0ce9bc78113..07e5b36076bb 100644
> --- a/drivers/cpuidle/cpuidle-haltpoll.c
> +++ b/drivers/cpuidle/cpuidle-haltpoll.c
> @@ -18,6 +18,11 @@
>  #include <linux/kvm_para.h>
>  #include <linux/cpuidle_haltpoll.h>
>  
> +static bool force __read_mostly;
> +module_param(force, bool, 0444);
> +MODULE_PARM_DESC(force,
> +		 "Load even if the host does not provide the REALTIME hint");
> +
>  static struct cpuidle_device __percpu *haltpoll_cpuidle_devices;
>  static enum cpuhp_state haltpoll_hp_state;
>  
> @@ -90,6 +95,11 @@ static void haltpoll_uninit(void)
>  	haltpoll_cpuidle_devices = NULL;
>  }
>  
> +static bool haltpool_want(void)
> +{
> +	return kvm_para_has_hint(KVM_HINTS_REALTIME) || force;
> +}
> +
>  static int __init haltpoll_init(void)
>  {
>  	int ret;
> @@ -102,7 +112,7 @@ static int __init haltpoll_init(void)
>  	cpuidle_poll_state_init(drv);
>  
>  	if (!kvm_para_available() ||
> -		!kvm_para_has_hint(KVM_HINTS_REALTIME))
> +	    !haltpool_want())
>  		return -ENODEV;
>  
>  	ret = cpuidle_register_driver(drv);
>