linux-kernel - Re: [RFC][PATCH] sched: Kick bandwidth timer immediately on start up

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Date:	Tue, 23 Feb 2016 15:29:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	John Kacur <jkacur@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH] sched: Kick bandwidth timer immediately on start up

On Tue, Feb 16, 2016 at 06:37:46PM -0500, Steven Rostedt wrote:
> Index: linux-trace.git/kernel/sched/rt.c
> ===================================================================
> --- linux-trace.git.orig/kernel/sched/rt.c	2016-02-16 16:31:12.041035819 -0500
> +++ linux-trace.git/kernel/sched/rt.c	2016-02-16 16:31:19.997905282 -0500
> @@ -58,7 +58,15 @@ static void start_rt_bandwidth(struct rt
>  	raw_spin_lock(&rt_b->rt_runtime_lock);
>  	if (!rt_b->rt_period_active) {
>  		rt_b->rt_period_active = 1;
> -		hrtimer_forward_now(&rt_b->rt_period_timer, rt_b->rt_period);
> +		/*
> +		 * SCHED_DEADLINE updates the bandwidth, as a run away
> +		 * RT task with a DL task could hog a CPU. But DL does
> +		 * not reset the period. If a deadline task was running
> +		 * without an RT task running, it can cause RT tasks to
> +		 * throttle when they start up. Kick the timer right away
> +		 * to update the period.
> +		 */
> +		hrtimer_forward_now(&rt_b->rt_period_timer, ns_to_ktime(0));

That's a bit icky, but yeah, probably the best we can do without making
a giant mess of things..

>  		hrtimer_start_expires(&rt_b->rt_period_timer, HRTIMER_MODE_ABS_PINNED);
>  	}
>  	raw_spin_unlock(&rt_b->rt_runtime_lock);