Date: Fri, 27 Nov 2015 10:17:02 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Len Brown <len.brown@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Eduardo Valentin <edubezval@gmail.com>, Paul Turner <pjt@google.com>
Subject: Re: [PATCH 3/4] sched: introduce synchronized idle injection
Message-ID: <20151127091702.GA27790@gmail.com>
References: <1447444387-23525-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1447444387-23525-4-git-send-email-jacob.jun.pan@linux.intel.com>
 <20151118083622.GA25217@gmail.com>
 <20151118061026.03a28616@yairi>
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* Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:

> On Wed, 18 Nov 2015 09:36:22 +0100
> Ingo Molnar <mingo@kernel.org> wrote:
> 
> > > 
> > > This patch introduces a scheduler based idle injection method, it
> > > works by blocking CFS runqueue synchronously and periodically. The
> > > actions on all online CPUs are orchestrated by per CPU hrtimers.
> > > 
> > > Two sysctl knobs are given to the userspace for selecting the
> > > percentage of idle time as well as the forced idle duration for each
> > > idle period injected.  
> > 
> > What's the purpose of these knobs? Just testing, or will some
> > user-space daemon set them dynamically?
> > 
> yes, it is to be used by userspace daemon such as thermal daemon.
> Though there are interests from in kernel thermal governor but that is
> another story.

Yeah, so let me make this very clear: for a kernel scheduling feature to be 
self-sufficient is not 'another story', but a must-have aspect for this feature
to become upstream acceptable.

We don't add scheduler features that rely on pushing 'policy' to user-space. 
That's poor design with many disadvantages. This feature should offer a reasonable 
and automatic in-kernel default behavior with numbers that prove that it works.

Keeping an essential part of the feature in user-space earns a NAK from me.

Thanks,

	Ingo
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