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    SubjectRe: [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@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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