Date: Mon, 16 Nov 2015 16:00:31 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        John Stultz <john.stultz@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        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>,
        Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry
Message-ID: <20151117000031.GO5184@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <1447444387-23525-3-git-send-email-jacob.jun.pan@linux.intel.com>
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 <20151116135126.5a50e45d@icelake>
 <20151116223117.GK5184@linux.vnet.ibm.com>
 <20151116151503.47cbd4cf@icelake>
 <20151116232837.GN5184@linux.vnet.ibm.com>
 <564A6796.9090409@linux.intel.com>
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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@linux.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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