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Subject: Re: [RFC][PATCH 02/11] sched: SCHED_DEADLINE policy implementation.
From: Peter Zijlstra <peterz@infradead.org>
To: Raistlin <raistlin@linux.it>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
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Date: Tue, 13 Apr 2010 20:22:17 +0200
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On Sun, 2010-02-28 at 20:17 +0100, Raistlin wrote:
> +/*
> + * Here we check if --at time t-- a task (which is probably being
> + * [re]activated or, in general, enqueued) can use its remaining runtime
> + * and its current deadline _without_ exceeding the bandwidth it is
> + * assigned (function returns true if it can).
> + *
> + * For this to hold, we must check if:
> + *   runtime / (deadline - t) < dl_runtime / dl_deadline .
> + */
> +static bool dl_check_bandwidth(struct sched_dl_entity *dl_se, u64 t)
> +{
> +       u64 left, right;
> +
> +       /*
> +        * left and right are the two sides of the equation above,
> +        * after a bit of shuffling to use multiplications instead
> +        * of divisions.
> +        */
> +       left = dl_se->dl_deadline * dl_se->runtime;
> +       right = (dl_se->deadline - t) * dl_se->dl_runtime;
> +
> +       return dl_time_before(left, right);
> +} 

So what happens when we overflow u64?