linux-kernel - Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz

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Message-ID: <ZteAfUXZd1TgIwiL@pavilion.home>
Date: Tue, 3 Sep 2024 23:32:45 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz_full

Le Tue, Sep 03, 2024 at 09:24:08AM -0400, Waiman Long a écrit :
> 
> On 9/3/24 09:10, Frederic Weisbecker wrote:
> > Le Sun, Aug 18, 2024 at 07:45:18PM -0400, Waiman Long a écrit :
> > > The HK_FLAG_SCHED/HK_TYPE_SCHED flag is defined and is also used
> > > in kernel/sched/fair.c since commit de201559df87 ("sched/isolation:
> > > Introduce housekeeping flags"). However, the corresponding cpumask isn't
> > > currently updated anywhere. So the mask is always cpu_possible_mask.
> > > 
> > > Add it in nohz_full setup so that nohz_full CPUs will now be removed
> > > from HK_TYPE_SCHED cpumask.
> > > 
> > > Signed-off-by: Waiman Long <longman@...hat.com>
> > > ---
> > >   kernel/sched/isolation.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > > index 5891e715f00d..a514994af319 100644
> > > --- a/kernel/sched/isolation.c
> > > +++ b/kernel/sched/isolation.c
> > > @@ -196,7 +196,7 @@ static int __init housekeeping_nohz_full_setup(char *str)
> > >   	unsigned long 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_SCHED;
> > >   	return housekeeping_setup(str, flags);
> > >   }
> > find_new_ilb() already has HK_FLAG_MISC to prevent an isolated CPU
> > from being elected as an ilb. So I think we should simply remove HK_FLAG_SCHED.
> 
> There is a check for HK_TYPE_SCHED in nohz_balance_enter_idle() and
> nohz_newidle_balance(), though it is essentially a no-op as the cpumask has
> all the CPUs. If we remove HK_TYPE_SCHED, the question now will be whether
> we should remove the checks at these 2 functions or change them to
> HK_TYPE_MISC.

Just remove those two. They are dead code and the nohz_full handling
of load balancing needs a rethink anyway.

After discussing with Peter lately, the rules should be:

1) If a nohz_full CPU is part of a multi-CPU domain, then it should
   be part of load balancing. Peter even says that nohz_full should be
   forbidden in this case, because the tick plays a role in the
   load balancing.

2) Otherwise, if CPU is not part of a domain or it is the only CPU of all its
   domains, then it can be out of the load balancing machinery.

I'm a bit scared about rule 1) because I know there are existing users of
nohz_full on multi-CPU domains... So I feel a bit trapped.

Thanks.