doc: Remove obsolete kernel-per-CPU-kthreads RCU_FAST_NO_HZ advice

This document advises building with both CONFIG_NO_HZ=y and
CONFIG_RCU_FAST_NO_HZ=y.  However, CONFIG_NO_HZ=y offloads callbacks from
all nohz_full CPUs, and CPUs with offloaded callbacks do not benefit from
CONFIG_RCU_FAST_NO_HZ=y.  Quite the opposite: CONFIG_RCU_FAST_NO_HZ=y
simply adds a bit of idle entry/exit overhead.

This commit therefore changes that advice to only CONFIG_NO_HZ=y.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/admin-guide/kernel-per-CPU-kthreads.rst b/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
index 5e51ee5b03589fc9b538daaeb2289fd85f5ce609..e4a5fc26f1a9dc78c565b2eeac3a65ce54a5253b 100644 (file)
--- a/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
+++ b/Documentation/admin-guide/kernel-per-CPU-kthreads.rst
@@ -208,7 +208,7 @@ Do at least one of the following:
 2.     Enable RCU to do its processing remotely via dyntick-idle by
        doing all of the following:
 
-       a.      Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
+       a.      Build with CONFIG_NO_HZ=y.
        b.      Ensure that the CPU goes idle frequently, allowing other
                CPUs to detect that it has passed through an RCU quiescent
                state.  If the kernel is built with CONFIG_NO_HZ_FULL=y,