linux-kernel - Re: [RFC 4/8] Improve the tracking of active utilisation

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Date:	Fri, 15 Jan 2016 09:32:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luca Abeni <luca.abeni@...tn.it>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [RFC 4/8] Improve the tracking of active utilisation

On Fri, Jan 15, 2016 at 09:10:15AM +0100, Luca Abeni wrote:
> On 01/14/2016 08:47 PM, Peter Zijlstra wrote:
> >On Thu, Jan 14, 2016 at 04:24:49PM +0100, Luca Abeni wrote:
> >
> >>+	if (!hrtimer_is_queued(timer)) {
> >>+		hrtimer_start(timer, act, HRTIMER_MODE_ABS);
> >>+	}
> >>+
> >>+	if (hrtimer_active(timer) == 0) {
> >>+		printk("Problem activating inactive_timer!\n");
> >>+		clear_running_bw(dl_se, dl_rq);
> >>+		if (!dl_task(p)) {
> >>+			__dl_clear_params(p);
> >>+		}
> >>+	} else {
> >>+		get_task_struct(p);
> >
> >Ah, I missed that one. I would suggest putting that right _before_
> >hrtimer_start(), because hrtimer_start() guarantees the callback will
> >run.

> Ok. So, if I understand well, the "if (hrtimer_active(timer) == 0)" check
> is useless (or should be somehow revised)... Right?

Yes, ever since: c6eb3f70d448 ("hrtimer: Get rid of hrtimer softirq")
hrtimer_start() is guaranteed to work and result in a callback, even if
the time is in the past.