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

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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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Re: [PATCH 2/4] timer: relax tick stop in idle entry Thomas Gleixner <tglx@linutronix.de> - 2015-11-16 16:10 +0100
  Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-16 23:00 +0100
    Re: [PATCH 2/4] timer: relax tick stop in idle entry Thomas Gleixner <tglx@linutronix.de> - 2015-11-16 23:10 +0100
      Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-17 01:20 +0100
        Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-19 18:50 +0100
        Re: [PATCH 2/4] timer: relax tick stop in idle entry Peter Zijlstra <peterz@infradead.org> - 2015-11-19 20:10 +0100
          Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-19 20:30 +0100
            Re: [PATCH 2/4] timer: relax tick stop in idle entry Peter Zijlstra <peterz@infradead.org> - 2015-11-19 21:00 +0100
              Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-20 00:50 +0100
    Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-16 23:40 +0100
      Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-17 00:10 +0100
      Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-17 00:20 +0100
        Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 00:30 +0100
          Re: [PATCH 2/4] timer: relax tick stop in idle entry Arjan van de Ven <arjan@linux.intel.com> - 2015-11-17 00:40 +0100
            Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-17 00:50 +0100
              Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 01:10 +0100
    Re: [PATCH 2/4] timer: relax tick stop in idle entry Josh Triplett <josh@joshtriplett.org> - 2015-11-16 23:40 +0100
      Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 00:30 +0100
        Re: [PATCH 2/4] timer: relax tick stop in idle entry Josh Triplett <josh@joshtriplett.org> - 2015-11-17 02:50 +0100
          Re: [PATCH 2/4] timer: relax tick stop in idle entry Arjan van de Ven <arjan@linux.intel.com> - 2015-11-17 04:00 +0100
            Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 06:10 +0100
              Re: [PATCH 2/4] timer: relax tick stop in idle entry Peter Zijlstra <peterz@infradead.org> - 2015-11-17 11:30 +0100
                Re: [PATCH 2/4] timer: relax tick stop in idle entry Jacob Pan <jacob.jun.pan@linux.intel.com> - 2015-11-17 14:00 +0100
                Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 14:50 +0100
          Re: [PATCH 2/4] timer: relax tick stop in idle entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 04:00 +0100

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