linux-kernel - Re: [PATCH v5 1/4] sched/fair: add util

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Message-ID: <20180306190241.GH25201@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Mar 2018 20:02:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT

On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote:
> +struct util_est {
> +	unsigned int			enqueued;
> +	unsigned int			ewma;
> +#define UTIL_EST_WEIGHT_SHIFT		2
> +};

> +	ue = READ_ONCE(p->se.avg.util_est);

> +	WRITE_ONCE(p->se.avg.util_est, ue);

That is actually quite dodgy... and relies on the fact that we have the
8 byte case in __write_once_size() and __read_once_size()
unconditionally. It then further relies on the compiler DTRT for 32bit
platforms, which is generating 2 32bit loads/stores.

The advantage is of course that it will use single u64 loads/stores
where available.