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rcu: Make RCU_FAST_NO_HZ take advantage of numbered callbacks

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Because RCU callbacks are now associated with the number of the grace
period that they must wait for, CPUs can now take advance callbacks
corresponding to grace periods that ended while a given CPU was in
dyntick-idle mode.  This eliminates the need to try forcing the RCU
state machine while entering idle, thus reducing the CPU intensiveness
of RCU_FAST_NO_HZ, which should increase its energy efficiency.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
2012-12-28 11:30:36 -08:00
committed by Paul E. McKenney
parent b11cc5760a
commit c0f4dfd4f9
7 changed files with 157 additions and 321 deletions
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15
init/Kconfig
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@@ -582,13 +582,16 @@ config RCU_FAST_NO_HZ
depends on NO_HZ && SMP
default n
help
This option causes RCU to attempt to accelerate grace periods in
order to allow CPUs to enter dynticks-idle state more quickly.
On the other hand, this option increases the overhead of the
dynticks-idle checking, thus degrading scheduling latency.
This option permits CPUs to enter dynticks-idle state even if
they have RCU callbacks queued, and prevents RCU from waking
these CPUs up more than roughly once every four jiffies (by
default, you can adjust this using the rcutree.rcu_idle_gp_delay
parameter), thus improving energy efficiency. On the other
hand, this option increases the duration of RCU grace periods,
for example, slowing down synchronize_rcu().
Say Y if energy efficiency is critically important, and you don't
care about real-time response.
Say Y if energy efficiency is critically important, and you
don't care about increased grace-period durations.
Say N if you are unsure.