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Subject: Re: [RFC][Patch v1 1/3] sched/isolation: API to get num of
 hosekeeping CPUs
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, mtosatti@redhat.com,
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        dennis.dalessandro@intel.com, thomas.lendacky@amd.com,
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References: <20200909150818.313699-1-nitesh@redhat.com>
 <20200909150818.313699-2-nitesh@redhat.com> <20200921234044.GA31047@lenoir>
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <6e48babd-bcaf-dfc8-2126-2b6c146af0aa@redhat.com>
Date:   Tue, 22 Sep 2020 17:15:28 -0400
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On 9/22/20 4:58 PM, Frederic Weisbecker wrote:
> On Tue, Sep 22, 2020 at 09:50:55AM -0400, Nitesh Narayan Lal wrote:
>> On 9/22/20 6:08 AM, Frederic Weisbecker wrote:
>> TBH I don't have a very strong case here at the moment.
>> But still, IMHO, this will force the user to have both managed irqs and
>> nohz_full in their environments to avoid these kinds of issues. Is that =
how
>> we would like to proceed?
> Yep that sounds good to me. I never know how much we want to split each a=
nd any
> of the isolation features but I'd rather stay cautious to separate HK_FLA=
G_TICK
> from the rest, just in case running in nohz_full mode ever becomes intere=
sting
> alone for performance and not just latency/isolation.

Fair point.

>
> But look what you can do as well:
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 5a6ea03f9882..9df9598a9e39 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -141,7 +141,7 @@ static int __init housekeeping_nohz_full_setup(char *=
str)
>  =09unsigned int flags;
> =20
>  =09flags =3D HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
> -=09=09HK_FLAG_MISC | HK_FLAG_KTHREAD;
> +=09=09HK_FLAG_MISC | HK_FLAG_KTHREAD | HK_FLAG_MANAGED_IRQ;
> =20
>  =09return housekeeping_setup(str, flags);
>  }
>
>
> "nohz_full=3D" has historically gathered most wanted isolation features. =
It can
> as well isolate managed irqs.

Nice, yeap this will work.

>
>
>>> And then can we rename it to housekeeping_num_online()?
>> It could be just me, but does something like hk_num_online_cpus() makes =
more
>> sense here?
> Sure, that works as well.

Thanks a lot for all the help.

>
> Thanks.
>
--=20
Nitesh


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