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Date: Mon, 9 Apr 2018 11:40:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: linux-kernel@vger.kernel.org, Quentin Perret <quentin.perret@arm.com>,
	Thara Gopinath <thara.gopinath@linaro.org>,
	linux-pm@vger.kernel.org,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Chris Redpath <chris.redpath@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC PATCH 3/6] sched: Add over-utilization/tipping point
 indicator
Message-ID: <20180409094001.GZ4043@hirez.programming.kicks-ass.net>
References: <20180320094312.24081-1-dietmar.eggemann@arm.com>
 <20180320094312.24081-4-dietmar.eggemann@arm.com>
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(I know there is a new version out; but I was reading through this to
catch up with the discussion)

On Tue, Mar 20, 2018 at 09:43:09AM +0000, Dietmar Eggemann wrote:
> +static inline int sd_overutilized(struct sched_domain *sd)
> +{
> +	return READ_ONCE(sd->shared->overutilized);
> +}
> +
> +static inline void update_overutilized_status(struct rq *rq)
> +{
> +	struct sched_domain *sd;
> +
> +	rcu_read_lock();
> +	sd = rcu_dereference(rq->sd);
> +	if (sd && !sd_overutilized(sd) && cpu_overutilized(rq->cpu))
> +		WRITE_ONCE(sd->shared->overutilized, 1);
> +	rcu_read_unlock();
> +}
> +#else

I think you ought to go have a look at the end of
kernel/sched/topology.c:sd_init(), where it says:

	/*
	 * For all levels sharing cache; connect a sched_domain_shared
	 * instance.
	 */
	if (sd->flags & SD_SHARE_PKG_RESOURCES) {
		sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
		atomic_inc(&sd->shared->ref);
		atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
	}

Because if I read all this correctly, your code assumes sd->shared
exists unconditionally, while the quoted bit only ensures it does so <=
LLC.