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Date: Tue, 10 Nov 2015 08:50:55 -0800
From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>, Andi Kleen <andi.kleen@intel.com>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Eduardo Valentin <edubezval@gmail.com>,
        Punit Agrawal <punit.agrawal@arm.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        jacob.jun.pan@linux.intel.com
Subject: Re: [RFC PATCH v2 3/3] sched: introduce synchronized idle injection
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On Tue, 10 Nov 2015 17:36:46 +0100
Peter Zijlstra <peterz@infradead.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