Re: [PATCH v2] cpuidle-haltpoll: allow force loading on hosts without the REALTIME hint

[Marcelo Tosatti]

On Wed, Mar 04, 2020 at 12:32:48PM +0100, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> ---
>  drivers/cpuidle/cpuidle-haltpoll.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> Changes from v1:
> Make the module parameter description more general, don't unnecessarily
> break a line in haltpoll_init().
> 
> diff --git a/drivers/cpuidle/cpuidle-haltpoll.c b/drivers/cpuidle/cpuidle-haltpoll.c
> index b0ce9bc78113..db124bc1ca2c 100644
> --- a/drivers/cpuidle/cpuidle-haltpoll.c
> +++ b/drivers/cpuidle/cpuidle-haltpoll.c
> @@ -18,6 +18,10 @@
>  #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 unconditionally");
> +
>  static struct cpuidle_device __percpu *haltpoll_cpuidle_devices;
>  static enum cpuhp_state haltpoll_hp_state;
>  
> @@ -90,6 +94,11 @@ 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;
> @@ -101,8 +110,7 @@ static int __init haltpoll_init(void)
>  
>  	cpuidle_poll_state_init(drv);
>  
> -	if (!kvm_para_available() ||
> -		!kvm_para_has_hint(KVM_HINTS_REALTIME))
> +	if (!kvm_para_available() || !haltpool_want())
>  		return -ENODEV;
>  
>  	ret = cpuidle_register_driver(drv);

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>