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

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Message-ID: <20151110085055.67bdb1fa@icelake>
Date:	Tue, 10 Nov 2015 08:50:55 -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 17:36:46 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.  
> 
> Why? the timer will fire and observe the new value for reprogramming
> the next period. All you need to do is to ensure whole values are
> written/read -- ie. avoid load/store tearing.
Different per CPU timer may intercept parameter changes at slightly
different time, so there is a race condition such that some CPUs may
catch the period change later by one period, which results in a correct
period change but at a different time, i.e. out of sync.

Jacob
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