linux-kernel - Re: [PATCH 2/4] timer: relax tick stop in idle entry

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Message-ID: <20151117000031.GO5184@linux.vnet.ibm.com>
Date:	Mon, 16 Nov 2015 16:00:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	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>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry

On Mon, Nov 16, 2015 at 03:40:46PM -0800, Jacob Pan wrote:
> On Mon, 16 Nov 2015 15:32:38 -0800
> Arjan van de Ven <arjan@...ux.intel.com> wrote:
> 
> > On 11/16/2015 3:28 PM, Paul E. McKenney wrote:
> > 
> > > Is this mostly an special-purpose embedded thing, or do you expect
> > > distros to be enabling this?  If the former, I suggest
> > > , but if distros are doing this for
> > > general-purpose workloads, I instead suggest CONFIG_RCU_FAST_NO_HZ.
> > 
> > thermal overload happens a lot on small devices, but sadly also in
> > big datacenters where it is not uncommon to underprovision cooling
> > capacity by a bit (it's one of those "99% of the time you only need
> > THIS much, the 1% you need 30% more" and that more is expensive or
> > even impractical)

Then CONFIG_RCU_FAST_NO_HZ it is.

> CONFIG_RCU_FAST_NO_HZ is more inline with idle injection in that both
> are targeted energy efficiency.
> Checking ubuntu and fedora, seems both have CONFIG_RCU_NOCB_CPU_ALL=y.

Ah, that is right -- they both do CONFIG_NO_HZ_FULL=y, which does imply
CONFIG_RCU_NOCB_CPU_ALL=y.  But SUSE does not, last I knew.

But it should be easy to set up Kconfig for this.  Make your new Kconfig
option select CONFIG_RCU_FAST_NO_HZ unless CONFIG_RCU_NOCB_CPU_ALL is
already set.  Alternatively, CONFIG_RCU_FAST_NO_HZ could be set up
something like the following:

	depends on (NO_HZ_COMMON && SMP && RCU_EXPERT) || CONFIG_THERM_THROT
	default CONFIG_THERM_THROT

But the "select RCU_FAST_NO_HZ if !RCU_NOCB_CPU_ALL" is probably cleaner.

Anyway, again, the details can be settled later.

							Thanx, Paul

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