linux-kernel - Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

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Message-ID: <20190507133528.ia4p3medvtg4z5az@queper01-lin>
Date:   Tue, 7 May 2019 14:35:28 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Luca Abeni <luca.abeni@...tannapisa.it>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

Hi Luca,

On Monday 06 May 2019 at 06:48:32 (+0200), Luca Abeni wrote:
> +static inline int dl_task_fit(const struct sched_dl_entity *dl_se,
> +			      int cpu, u64 *c)
> +{
> +	u64 cap = (arch_scale_cpu_capacity(NULL, cpu) * arch_scale_freq_capacity(cpu)) >> SCHED_CAPACITY_SHIFT;

I'm a little bit confused by this use of arch_scale_freq_capacity()
here. IIUC this means you would say a big DL task doesn't fit on a big
CPU just because it happens to be running at a low frequency when this
function is called. Is this what we want ?

If the frequency is low, we can (probably) raise it to accommodate this
DL task so perhaps we should say it fits ?

> +	s64 rel_deadline = dl_se->dl_deadline;
> +	u64 rem_runtime  = dl_se->dl_runtime;
> +
> +	if (c)
> +		*c = cap;
> +
> +	if ((rel_deadline * cap) >> SCHED_CAPACITY_SHIFT < rem_runtime)
> +		return 0;
> +
> +	return 1;
> +}

Thanks,
Quentin