On Tue, Apr 30, 2013 at 12:25:41PM -0700, Paul E. McKenney wrote: > On Tue, Apr 30, 2013 at 02:46:12PM -0400, Dave Jones wrote: > > Additionally, nowhere in any of this text does it say what a "no-CB CPU" is, > > or why I would care, or even what the downsides are for each option. > > In the absence of any Kconfig change, would the following be more helpful? A little. You've now documented the mechanism behind each choice, but there's still no real explanation why I would pick one over the other. The average reader of these texts isn't going to know whether running something from a kthread is a better/worse idea than running from softirq context. Who doesn't like saving energy ? Why would I leave it at the NONE default ? Why is it even an option ? I'm assuming there's a reason we don't pick (one of the) energy efficient options by default (performance?) who knows, there's no explanation. Why would I want to treat CPU0 differently ? What user-visible downsides are there ? Who knows.. > +choice > + prompt "Build-forced no-CBs CPUs" > + default RCU_NOCB_CPU_NONE > + help > + 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" > + depends on RCU_NOCB_CPU > + 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, whose RCU callbacks will be invoked by per-CPU > + rcuo kthreads. All other CPUs will invoke their own RCU > + callbacks in softirq context. > + > +config RCU_NOCB_CPU_ZERO > + bool "CPU 0 is a build_forced no-CBs CPU" > + depends on RCU_NOCB_CPU > + help > + This option forces CPU 0 to be a no-CBs CPU, so that its > + RCU callbacks are invoked by a per-CPU rcuo kthread. > + 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. > + > +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. All CPUs' RCU callbacks will > + be executed in the context of per-CPU rcuo kthreads created > + for this purpose. > + > + Select this if all CPUs need to be no-CBs CPUs for real-time > + or energy-efficiency reasons. I know how much IBMers love their acronyms. I thought you'd invented some new rcu variant for a moment. Perhaps "kthreads named 'rcuo'" would be clearer ? Dave -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
