Date: Tue, 17 Nov 2015 05:49:46 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>, 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>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry
Message-ID: <20151117134946.GT5184@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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 <20151116223210.GA21522@cloud>
 <20151116232640.GM5184@linux.vnet.ibm.com>
 <20151117014103.GA6629@x>
 <20151117025342.GP5184@linux.vnet.ibm.com>
 <564A978A.8060905@linux.intel.com>
 <20151117050403.GQ5184@linux.vnet.ibm.com>
 <20151117102449.GR3816@twins.programming.kicks-ass.net>
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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@infradead.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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