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Date:   Tue, 22 Sep 2020 17:15:28 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, mtosatti@...hat.com,
        sassmann@...hat.com, jeffrey.t.kirsher@...el.com,
        jacob.e.keller@...el.com, jlelli@...hat.com, hch@...radead.org,
        bhelgaas@...gle.com, mike.marciniszyn@...el.com,
        dennis.dalessandro@...el.com, thomas.lendacky@....com,
        jerinj@...vell.com, mathias.nyman@...el.com, jiri@...dia.com
Subject: Re: [RFC][Patch v1 1/3] sched/isolation: API to get num of
 hosekeeping CPUs


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 and any
> of the isolation features but I'd rather stay cautious to separate HK_FLAG_TICK
> from the rest, just in case running in nohz_full mode ever becomes interesting
> 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)
>  	unsigned int flags;
>  
>  	flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
> -		HK_FLAG_MISC | HK_FLAG_KTHREAD;
> +		HK_FLAG_MISC | HK_FLAG_KTHREAD | HK_FLAG_MANAGED_IRQ;
>  
>  	return housekeeping_setup(str, flags);
>  }
>
>
> "nohz_full=" 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.
>
-- 
Nitesh



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