linux-kernel - Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting

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Message-ID: <20190121145929.GI27931@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Jan 2019 15:59:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets
 refcounting

On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:
> @@ -835,6 +954,28 @@ static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
>  	} while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata,
>  					  &uc_map_old.data, uc_map_new.data));
>  
> +	/*
> +	 * Ensure each CPU tracks the correct value for this clamp bucket.
> +	 * This initialization of per-CPU variables is required only when a
> +	 * clamp value is requested for the first time from a slow-path.
> +	 */

I'm confused; why is this needed?

> +	if (unlikely(!uc_map_old.se_count)) {
> +		for_each_possible_cpu(cpu) {
> +			struct uclamp_cpu *uc_cpu =
> +				&cpu_rq(cpu)->uclamp[clamp_id];
> +
> +			/* CPU's tasks count must be 0 for free buckets */
> +			SCHED_WARN_ON(uc_cpu->bucket[bucket_id].tasks);
> +			if (unlikely(uc_cpu->bucket[bucket_id].tasks))
> +				uc_cpu->bucket[bucket_id].tasks = 0;
> +
> +			/* Minimize cache lines invalidation */
> +			if (uc_cpu->bucket[bucket_id].value == bucket_value)
> +				continue;
> +			uc_cpu->bucket[bucket_id].value = bucket_value;
> +		}
> +	}
> +
>  	uc_se->value = clamp_value;
>  	uc_se->bucket_id = bucket_id;
>