linux-kernel - Re: [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets

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Message-ID: <20190121101508.GH27931@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Jan 2019 11:15:08 +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 03/16] sched/core: uclamp: Map TASK's clamp values
 into CPU's clamp buckets

On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:
> +/*
> + * Number of utilization clamp buckets.
> + *
> + * The first clamp bucket (bucket_id=0) is used to track non clamped tasks, i.e.
> + * util_{min,max} (0,SCHED_CAPACITY_SCALE). Thus we allocate one more bucket in
> + * addition to the compile time configured number.
> + */
> +#define UCLAMP_BUCKETS (CONFIG_UCLAMP_BUCKETS_COUNT + 1)
> +
> +/*
> + * Utilization clamp bucket
> + * @value:		clamp value tracked by a clamp bucket
> + * @bucket_id:		the bucket index used by the fast-path
> + * @mapped:		the bucket index is valid
> + *
> + * A utilization clamp bucket maps a:
> + *   clamp value (value), i.e.
> + *   util_{min,max} value requested from userspace
> + * to a:
> + *   clamp bucket index (bucket_id), i.e.
> + *   index of the per-cpu RUNNABLE tasks refcounting array
> + *
> + * The mapped bit is set whenever a task has been mapped on a clamp bucket for
> + * the first time. When this bit is set, any:
> + *   uclamp_bucket_inc() - for a new clamp value
> + * is matched by a:
> + *   uclamp_bucket_dec() - for the old clamp value
> + */
> +struct uclamp_se {
> +	unsigned int value		: bits_per(SCHED_CAPACITY_SCALE);
> +	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
> +	unsigned int mapped		: 1;
> +};

Do we want something like:

	BUILD_BUG_ON(sizeof(struct uclamp_se) == sizeof(unsigned int));

And/or put a limit on CONFIG_UCLAMP_BUCKETS_COUNT that guarantees that ?