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Date: Thu, 4 Apr 2024 09:27:45 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Pierre Gondois <pierre.gondois@arm.com>
Cc: linux-kernel@vger.kernel.org, Aaron Lu <aaron.lu@intel.com>,
	Rui Zhang <rui.zhang@intel.com>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Waiman Long <longman@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 6/7] sched/fair: Remove on_null_domain() and redundant
 checks
Message-ID: <20240404072745.GA35684@noisy.programming.kicks-ass.net>
References: <20240403150543.2793354-1-pierre.gondois@arm.com>
 <20240403150543.2793354-7-pierre.gondois@arm.com>
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In-Reply-To: <20240403150543.2793354-7-pierre.gondois@arm.com>

On Wed, Apr 03, 2024 at 05:05:38PM +0200, Pierre Gondois wrote:
> CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
> isolation mask. The two following checks are equialent:
> - !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
> - on_null_domain(rq)
> 
> Remove on_null_domain() and the redundant checks.
> 
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> ---
>  kernel/sched/fair.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3e0f2a0f153f..9657c8f2176b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>  
>  }
>  
> -static inline int on_null_domain(struct rq *rq)
> -{
> -	return unlikely(!rcu_dereference_sched(rq->sd));
> -}
> -
>  #ifdef CONFIG_NO_HZ_COMMON
>  /*
>   * NOHZ idle load balancing (ILB) details:
> @@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
>  	SCHED_WARN_ON(rq != this_rq());
>  
>  	/* If we're a completely isolated CPU, we don't play: */
> -	if (on_null_domain(rq))
> +	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
>  		return;
>  
>  	if (likely(!rq->nohz_tick_stopped))

This seems broken, the whole null domain can happen with cpusets, but
this housekeeping nonsense is predicated on CPU_ISOLATION and none of
that is mandatory for CPUSETS.