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    Subject[PATCH RFC nohz_full v2 1/7] nohz_full: Add Kconfig parameter for scalable detection of all-idle state
    Date
    From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

    At least one CPU must keep the scheduling-clock tick running for
    timekeeping purposes whenever there is a non-idle CPU. However, with
    the new nohz_full adaptive-idle machinery, it is difficult to distinguish
    between all CPUs really being idle as opposed to all non-idle CPUs being
    in adaptive-ticks mode. This commit therefore adds a Kconfig parameter
    as a first step towards enabling a scalable detection of full-system
    idle state.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    ---
    kernel/time/Kconfig | 23 +++++++++++++++++++++++
    1 file changed, 23 insertions(+)

    diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
    index 70f27e8..a613c2a 100644
    --- a/kernel/time/Kconfig
    +++ b/kernel/time/Kconfig
    @@ -134,6 +134,29 @@ config NO_HZ_FULL_ALL
    Note the boot CPU will still be kept outside the range to
    handle the timekeeping duty.

    +config NO_HZ_FULL_SYSIDLE
    + bool "Detect full-system idle state for full dynticks system"
    + depends on NO_HZ_FULL
    + default n
    + help
    + At least one CPU must keep the scheduling-clock tick running
    + for timekeeping purposes whenever there is a non-idle CPU,
    + where "non-idle" includes CPUs with a single runnable task
    + in adaptive-idle mode. Because the underlying adaptive-tick
    + support cannot distinguish between all CPUs being idle and
    + all CPUs each running a single task in adaptive-idle mode,
    + the underlying support simply ensures that there is always
    + a CPU handling the scheduling-clock tick, whether or not all
    + CPUs are idle. This Kconfig option enables scalable detection
    + of the all-CPUs-idle state, thus allowing the scheduling-clock
    + tick to be disabled when all CPUs are idle. Note that scalable
    + detection of the all-CPUs-idle state means that larger systems
    + will be slower to declare the all-CPUs-idle state.
    +
    + Say Y if you would like to help debug all-CPUs-idle detection.
    +
    + Say N if you are unsure.
    +
    config NO_HZ
    bool "Old Idle dynticks config"
    depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
    --
    1.8.1.5


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