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Commit 676c3dc2 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Apply Dave Jones's NOCB Kconfig help feedback 



The Kconfig help text for the RCU_NOCB_CPU_NONE, RCU_NOCB_CPU_ZERO,
and RCU_NOCB_CPU_ALL Kconfig options was unclear, so this commit
adds a bit more detail.

Reported-by: default avatarDave Jones <davej@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 4982969d

init/Kconfig

+25 −9
Original line number Diff line number Diff line
@@ -682,9 +682,10 @@ choice 
	prompt "Build-forced no-CBs CPUs"

	default RCU_NOCB_CPU_NONE

	help

	  This option allows no-CBs CPUs to be specified at build time.

	  Additional no-CBs CPUs may be specified by the rcu_nocbs=

	  boot parameter.

	  This option allows no-CBs CPUs (whose RCU callbacks are invoked

	  from kthreads rather than from softirq context) to be specified

	  at build time.  Additional no-CBs CPUs may be specified by

	  the rcu_nocbs= boot parameter.



config RCU_NOCB_CPU_NONE

	bool "No build_forced no-CBs CPUs"
@@ -692,25 +693,40 @@ config RCU_NOCB_CPU_NONE 
	help

	  This option does not force any of the CPUs to be no-CBs CPUs.

	  Only CPUs designated by the rcu_nocbs= boot parameter will be

	  no-CBs CPUs.

	  no-CBs CPUs, whose RCU callbacks will be invoked by per-CPU

	  kthreads whose names begin with "rcuo".  All other CPUs will

	  invoke their own RCU callbacks in softirq context.



	  Select this option if you want to choose no-CBs CPUs at

	  boot time, for example, to allow testing of different no-CBs

	  configurations without having to rebuild the kernel each time.



config RCU_NOCB_CPU_ZERO

	bool "CPU 0 is a build_forced no-CBs CPU"

	depends on RCU_NOCB_CPU && !NO_HZ_FULL

	help

	  This option forces CPU 0 to be a no-CBs CPU.  Additional CPUs

	  may be designated as no-CBs CPUs using the rcu_nocbs= boot

	  parameter will be no-CBs CPUs.

	  This option forces CPU 0 to be a no-CBs CPU, so that its RCU

	  callbacks are invoked by a per-CPU kthread whose name begins

	  with "rcuo".	Additional CPUs may be designated as no-CBs

	  CPUs using the rcu_nocbs= boot parameter will be no-CBs CPUs.

	  All other CPUs will invoke their own RCU callbacks in softirq

	  context.



	  Select this if CPU 0 needs to be a no-CBs CPU for real-time

	  or energy-efficiency reasons.

	  or energy-efficiency reasons, but the real reason it exists

	  is to ensure that randconfig testing covers mixed systems.



config RCU_NOCB_CPU_ALL

	bool "All CPUs are build_forced no-CBs CPUs"

	depends on RCU_NOCB_CPU

	help

	  This option forces all CPUs to be no-CBs CPUs.  The rcu_nocbs=

	  boot parameter will be ignored.

	  boot parameter will be ignored.  All CPUs' RCU callbacks will

	  be executed in the context of per-CPU rcuo kthreads created for

	  this purpose.  Assuming that the kthreads whose names start with

	  "rcuo" are bound to "housekeeping" CPUs, this reduces OS jitter

	  on the remaining CPUs, but might decrease memory locality during

	  RCU-callback invocation, thus potentially degrading throughput.



	  Select this if all CPUs need to be no-CBs CPUs for real-time

	  or energy-efficiency reasons.