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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-04-30 14:49:42 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-06-10 13:44:57 -0700 |
| commit | 676c3dc2033aa43f2e605611dec9be55a921dafd (patch) | |
| tree | 2021f6c6d9b79d8af4b0c9e85d4d6726af01fb49 | |
| parent | 4982969d965ec87b1887c86d2e0b3d81065e1d38 (diff) | |
| download | linux-676c3dc2033aa43f2e605611dec9be55a921dafd.tar.gz linux-676c3dc2033aa43f2e605611dec9be55a921dafd.tar.bz2 linux-676c3dc2033aa43f2e605611dec9be55a921dafd.zip | |
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: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
| -rw-r--r-- | init/Kconfig | 34 |
1 files changed, 25 insertions, 9 deletions
diff --git a/init/Kconfig b/init/Kconfig index 8df5116621ec..f54bc60f2afb 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -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. |