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nohz_full: Add Kconfig parameter for scalable detection of all-idle state

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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>
[ paulmck: Update help text per Frederic Weisbecker. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2013-06-21 11:08:45 -07:00
parent 8bdf7a252b
commit b44379af1c
1 changed files with 23 additions and 0 deletions
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23
kernel/time/Kconfig
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@ -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" also includes dynticks CPUs as long as they are
running non-idle tasks. Because the underlying adaptive-tick
support cannot distinguish between all CPUs being idle and
all CPUs each running a single task in dynticks 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