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    SubjectRe: [PATCH 12/32] nohz: Try not to give the timekeeping duty to a cpuset nohz cpu
    On Tue, Aug 30, 2011 at 05:37:28PM +0200, Peter Zijlstra wrote:
    > On Tue, 2011-08-30 at 17:17 +0200, Frederic Weisbecker wrote:
    > > On Mon, Aug 29, 2011 at 04:55:45PM +0200, Peter Zijlstra wrote:
    > > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
    > > > > Try to give the timekeeing duty to a CPU that doesn't belong
    > > > > to any nohz cpuset when possible, so that we increase the chance
    > > > > for these nohz cpusets to run their CPUs out of periodic tick
    > > > > mode.
    > > >
    > > > You and Dmitiri might want to get together:
    > > >
    > > > lkml.kernel.org/r/20110823195628.GB4533@sgi.com
    > >
    > > Right!
    > >
    > > There is another missing piece in my patchset. If every non adaptive-nohz
    > > CPUs are sleeping, then none is handling the do_timer duty and adaptive nohz
    > > CPUs run with a stale jiffies and walltime.
    >
    > Doesn't nohz already deal with the case of all cpus being idle? In that
    > case the cpu that wakes up first gets to play catch up on irq_enter() or
    > so.

    Sure and that works for the nohz idle case. But that's not enough anymore
    in the case of adaptive nohz CPUs. They can run for a while without the tick
    and if nobody else maintains a tick either then jiffies and walltime are
    not maintained anymore.


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