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

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Date:   Wed, 11 Mar 2020 22:30:17 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] cpuidle-haltpoll: allow force loading on hosts without
 the REALTIME hint

Hi Marcelo,

On 10.03.2020 14:50, Marcelo Tosatti wrote:
> 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.

Can you ack the v2 patch that has all the review comments resolved then?

It is available here:
https://lore.kernel.org/kvm/20200304113248.1143057-1-mail@maciej.szmigiero.name/

Thanks,
Maciej