linux-kernel - Re: [RFC PATCH v2 3/3] sched: introduce synchronized idle injection

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Message-ID: <20151110082859.1493f106@icelake>
Date:	Tue, 10 Nov 2015 08:28:59 -0800
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Len Brown <len.brown@...el.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Eduardo Valentin <edubezval@...il.com>,
	Punit Agrawal <punit.agrawal@....com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	jacob.jun.pan@...ux.intel.com
Subject: Re: [RFC PATCH v2 3/3] sched: introduce synchronized idle injection

On Tue, 10 Nov 2015 15:58:23 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Nov 10, 2015 at 06:01:16AM -0800, Jacob Pan wrote:
> > On Tue, 10 Nov 2015 14:23:24 +0100
> > Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > It looks like what you want is:
> > > 
> > > 	hrtimer_forward(hrt, period);
> > > 
> > > unconditionally.
> 
> > In the ideal world yes. But my thinking was that timers may not be
> > so accurate to deliver interrupts, over the time the timeout error
> > may accumulate so that eventually timers will be out of sync.
> 
> Timers have a global time base. Even if individual deliveries have an
> error, there is no accumulated error.
> 
great! I can get rid of the ktime_roundup(). It seems to work with
	now = hrtimer_cb_get_time(hrt);
	if (status)
		hrtimer_forward(hrt, now, ms_to_ktime(inject_interval));
	else
		hrtimer_forward(hrt, now, ms_to_ktime(duration));

The downside is that we need to restart the timers every time if
user were to change injection parameters, i.e. duration and percent.
Or do locking which might be too expensive. In the previous approach, it
will naturally catch up the parameter change.
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