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

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Message-ID: <20200310135006.GA6397@fuller.cnet>
Date:   Tue, 10 Mar 2020 10:50:06 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Joao Martins <joao.m.martins@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [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@...cle.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@...cle.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!
> 
>