linux-kernel - Re: [RFC][Patch v1 1/3] sched/isolation: API to get num of hosekeeping CPUs

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Date:   Tue, 22 Sep 2020 22:58:06 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
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 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.

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.


> > 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.