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authorPaul E. McKenney <paulmck@kernel.org>2021-09-27 14:18:51 -0700
committerPaul E. McKenney <paulmck@kernel.org>2021-11-30 17:24:47 -0800
commite2c73a6860bdf54f2c6bf8cddc34ddc91a1343e1 (patch)
treed8b4e1ad9c9a7131bfe6fe50235e4d66465364cc /Documentation/timers/no_hz.rst
parent24eab6e1ff58d33048c36b5452f5d284c3710d02 (diff)
rcu: Remove the RCU_FAST_NO_HZ Kconfig option
All of the uses of CONFIG_RCU_FAST_NO_HZ=y that I have seen involve systems with RCU callbacks offloaded. In this situation, all that this Kconfig option does is slow down idle entry/exit with an additional allways-taken early exit. If this is the only use case, then this Kconfig option nothing but an attractive nuisance that needs to go away. This commit therefore removes the RCU_FAST_NO_HZ Kconfig option. Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'Documentation/timers/no_hz.rst')
-rw-r--r--Documentation/timers/no_hz.rst10
1 files changed, 3 insertions, 7 deletions
diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
index 20ad23a6c618..f8786be15183 100644
--- a/Documentation/timers/no_hz.rst
+++ b/Documentation/timers/no_hz.rst
@@ -184,16 +184,12 @@ There are situations in which idle CPUs cannot be permitted to
enter either dyntick-idle mode or adaptive-tick mode, the most
common being when that CPU has RCU callbacks pending.
-The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
-to enter dyntick-idle mode or adaptive-tick mode anyway. In this case,
-a timer will awaken these CPUs every four jiffies in order to ensure
-that the RCU callbacks are processed in a timely fashion.
-
-Another approach is to offload RCU callback processing to "rcuo" kthreads
+Avoid this by offloading RCU callback processing to "rcuo" kthreads
using the CONFIG_RCU_NOCB_CPU=y Kconfig option. The specific CPUs to
offload may be selected using The "rcu_nocbs=" kernel boot parameter,
which takes a comma-separated list of CPUs and CPU ranges, for example,
-"1,3-5" selects CPUs 1, 3, 4, and 5.
+"1,3-5" selects CPUs 1, 3, 4, and 5. Note that CPUs specified by
+the "nohz_full" kernel boot parameter are also offloaded.
The offloaded CPUs will never queue RCU callbacks, and therefore RCU
never prevents offloaded CPUs from entering either dyntick-idle mode