(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@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, 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);
>
