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
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