linux-kernel - Re: [PATCH 3/4] sched: introduce synchronized idle injection

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Date:	Wed, 18 Nov 2015 09:36:22 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 3/4] sched: introduce synchronized idle injection


* Jacob Pan <jacob.jun.pan@...ux.intel.com> wrote:

> With increasingly constrained power and thermal budget, it's often necessary to 
> cap power via throttling. Throttling individual CPUs or devices at random times 
> can help power capping but may not be optimal in terms of energy efficiency. 
> Frequency scaling is also limited by certain range before losing energy 
> efficiency.
> 
> In general, the optimal solution in terms of energy efficiency is to align idle 
> periods such that more shared circuits can be power gated to enter lower power 
> states. Combined with energy efficient frequency point, idle injection provides 
> a way to scale power and performance efficiently.
> 
> 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?

I.e. what mechanism will drive the throttling in the typical case?

> Since only CFS class is targeted, other high priority tasks are not affected, 
> such as EDF and RT tasks as well as softirq and interrupts.
> 
> Hotpath in CFS pick_next_task is optimized by Peter Zijlstra, where a new 
> runnable flag is introduced to combine forced idle and nr_running.

> +config CFS_IDLE_INJECT
> +	bool "Synchronized CFS idle injection"
> +	depends on NO_HZ_IDLE && HIGH_RES_TIMERS
> +	default n
> +	help
> +	  This feature let scheduler inject synchronized idle time across all online
> +	  CPUs. Idle injection affects normal tasks only, yeilds to RT and interrupts.
> +	  Effecitvely, CPUs can be duty cycled between running at the most power
> +	  efficient performance state and deep idle states.

So there are 3 typos in this single paragraph alone ...

I also think that naming it 'idle injection' is pretty euphemistic: this is forced 
idling, right? So why not name it CFS_FORCED_IDLE?

What will such throttling do to latencies, as observed by user-space tasks? What's 
the typical expected frequency of the throttling frequency that you are targeting?

Thanks,

	Ingo
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