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

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Message-ID: <20151117134946.GT5184@linux.vnet.ibm.com>
Date:	Tue, 17 Nov 2015 05:49:46 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry

On Tue, Nov 17, 2015 at 04:57:21AM -0800, Jacob Pan wrote:
> On Tue, 17 Nov 2015 11:24:49 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, Nov 16, 2015 at 09:04:03PM -0800, Paul E. McKenney wrote:
> > > On Mon, Nov 16, 2015 at 06:57:14PM -0800, Arjan van de Ven wrote:
> > > > On 11/16/2015 6:53 PM, Paul E. McKenney wrote:
> > > > >Fair point.  When in the five-jiffy throttling state, what can
> > > > >wake up a CPU?  In an earlier version of this proposal, the
> > > > >answer was "nothing", but maybe that has changed.
> > > > 
> > > > device interrupts are likely to wake the cpus.
> > > 
> > > OK, that I cannot help you with.  But presumably if the interrupt
> > > handler does a wakeup (or similar), that is deferred to the end of
> > > the throttling interval?  Timers are also deferred, including
> > > hrtimers?
> > 
> > This throttling thing only throttles 'normal' tasks, real-time tasks
> > will still run.

Heh!  Then RCU will be delayed or not based on the priority of the
grace-period kthreads and softirqd.  ;-)

In addition, this does sound like an excellent test for priority-inversion
situations that might otherwise go unnoticed on overprovisioned systems.
That said, I would expect many types of real-time systems to configure
voltage, frequency, and cooling so as to avoid thermal throttling.

> As an optimization or option, it might be useful to further defer the
> next timer interrupt if it falls within the idle injection period. But
> I guess we don't know if that timer belongs to a normal task or rt.
> Also we there could be more than one 'next' timer interrupts fall into
> that injection idle period.

Understood.  This brings me back to my recommendation that throttling
select RCU_FAST_NO_HZ unless RCU_NOCB_CPU_ALL is already set.

							Thanx, Paul

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