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SubjectRe: [PATCH] cpuidle-haltpoll: allow force loading on hosts without the REALTIME hint
On Wed, Mar 04, 2020 at 11:31:31AM +0100, Rafael J. Wysocki wrote:
> On Friday, February 28, 2020 6:10:18 PM CET Maciej S. Szmigiero wrote:
> > A friendly ping here.
> >
> > Maciej
> >
> > 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");
>
> Why not to say "Load unconditionally" here?

Makes sense to me.

> As is, one needs to know what "the REALTIME hint" is to understand it.
>
> > > +
> > > 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())
>
> And you don't need to break this line.
>
> > > return -ENODEV;
> > >
> > > ret = cpuidle_register_driver(drv);
> > >
> >
> >
>
> Thanks!
>
>

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