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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 13 Mar 2020 18:49:45 +0100
Message-ID: <CAJZ5v0jLOKj5LN5Kmredixomer4BKdBPNwP7gOf7A0tS_WMbDQ@mail.gmail.com>
Subject: Re: [PATCH v2] cpuidle-haltpoll: allow force loading on hosts without
 the REALTIME hint
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Joao Martins <joao.m.martins@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        kvm-devel <kvm@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Mar 12, 2020 at 5:36 PM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Wed, Mar 04, 2020 at 12:32:48PM +0100, 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, 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@redhat.com>

I'm taking this as a Reviewed-by, thanks!