linux-kernel - Re: [RFC v5 5/9] sched/deadline: do not reclaim the whole CPU bandwidth

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Date:   Fri, 24 Mar 2017 15:00:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [RFC v5 5/9] sched/deadline: do not reclaim the whole CPU
 bandwidth

On Fri, Mar 24, 2017 at 04:52:58AM +0100, luca abeni wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 20c62e7..efa88eb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6716,6 +6716,12 @@ static void sched_dl_do_global(void)
>  		raw_spin_unlock_irqrestore(&dl_b->lock, flags);
>  
>  		rcu_read_unlock_sched();
> +		if (dl_b->bw == -1)
> +			cpu_rq(cpu)->dl.deadline_bw_inv = 1 << 8;
> +		else
> +			cpu_rq(cpu)->dl.deadline_bw_inv =
> +				to_ratio(global_rt_runtime(),
> +					 global_rt_period()) >> 12;

Coding style requires braces here (on both legs of the condition)..

Also, I find deadline_bw_inv an awkward name; would something like
bw_ratio or so be more accurate?

> +	if (global_rt_runtime() == RUNTIME_INF)
> +		dl_rq->deadline_bw_inv = 1 << 8;
> +	else
> +		dl_rq->deadline_bw_inv =
> +			to_ratio(global_rt_runtime(), global_rt_period()) >> 12;

That's almost the same code; do we want a helper function?

>  
>  u64 grub_reclaim(u64 delta, struct rq *rq)
>  {
> +	return (delta * rq->dl.running_bw * rq->dl.deadline_bw_inv) >> 20 >> 8;
>  }

At which point we might want a note about how this doesn't overflow I
suppose.

Also:

	delta *= rq->dl.running_bw;
	delta *= rq->dl.bw_ratio;
	delta >>= 20 + 8;

	return delta;

Might be more readable ?

Alternatively:

	delta = (delta * rq->dl.running_bw) >> 8;
	delta = (delta * rq->dl.bw_ratio) >> 20;

	return delta;

But I doubt we care about those extra 8 bit of space; delta should not
be over 36 bits (~64 seconds) anyway I suppose.