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    Subject[PATCH RFC nohz_full 0/8] Provide infrastructure for full-system idle
    Whenever there is at least one non-idle CPU, it is necessary to
    periodically update timekeeping information. Before NO_HZ_FULL, this
    updating was carried out by the scheduling-clock tick, which ran on
    every non-idle CPU. With the advent of NO_HZ_FULL, it is possible
    to have non-idle CPUs that are not receiving scheduling-clock ticks.
    This possibility is handled by assigning a timekeeping CPU that continues
    taking scheduling-clock ticks.

    Unfortunately, timekeeping CPU continues taking scheduling-clock
    interrupts even when all other CPUs are completely idle, which is
    not so good for energy efficiency and battery lifetime. Clearly, it
    would be good to turn off the timekeeping CPU's scheduling-clock tick
    when all CPUs are completely idle. This is conceptually simple, but
    we also need good performance and scalability on large systems, which
    rules out implementations based on frequently updated global counts of
    non-idle CPUs as well as implementations that frequently scan all CPUs.
    Nevertheless, we need a single global indicator in order to keep the
    overhead of checking acceptably low.

    The chosen approach is to enforce hysteresis on the non-idle to
    full-system-idle transition, with the amount of hysteresis increasing
    linearly with the number of CPUs, thus keeping contention acceptably low.
    This approach piggybacks on RCU's existing force-quiescent-state scanning
    of idle CPUs, which has the advantage of avoiding the scan entirely on
    busy systems that have high levels of multiprogramming. This scan
    take per-CPU idleness information and feeds it into a state machine
    that applies the level of hysteresis required to arrive at a single
    full-system-idle indicator.

    Note that this version pays attention to CPUs that have taken an NMI
    from idle. It is not clear to me that NMI handlers can safely access
    the time on a system that is long-term idle. Unless someone tells me
    that it is somehow safe to access time from an NMI from idle, I will
    remove NMI support in the next version.

    Thanx, Paul

    ------------------------------------------------------------------------

    b/include/linux/rcupdate.h | 18 +
    b/kernel/rcutree.c | 56 ++++-
    b/kernel/rcutree.h | 20 ++
    b/kernel/rcutree_plugin.h | 427 ++++++++++++++++++++++++++++++++++++++++++++-
    b/kernel/time/Kconfig | 23 ++
    5 files changed, 527 insertions(+), 17 deletions(-)



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