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    SubjectRe: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU
    On Wed, Jan 27, 2010 at 10:43:36AM +0100, Andi Kleen wrote:
    > On Tue, Jan 26, 2010 at 09:20:50PM -0800, Paul E. McKenney wrote:
    > > On Tue, Jan 26, 2010 at 10:30:57PM +0100, Andi Kleen wrote:
    > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
    > > >
    > > > Kind of offtopic to the original patch, but I couldn't
    > > > resist...
    > > >
    > > > > +config RCU_FAST_NO_HZ
    > > > > + bool "Accelerate last non-dyntick-idle CPU's grace periods"
    > > > > + depends on TREE_RCU && NO_HZ && SMP
    > > >
    > > > Having such a thing as a config option doesn't really make
    > > > any sense to me. Who would want to recompile their kernel
    > > > to enable/disable this? If anything it should be runtime, or better
    > > > just unconditionally on.
    > >
    > > It adds significant overhead on entry to dyntick-idle mode for systems
    > > with large numbers of CPUs. :-(
    >
    > Can't you simply check that at runtime then?
    >
    > if (num_possible_cpus() > 20)
    > ...
    >
    > BTW the new small is large. This years high end desktop PC will come with
    > upto 12 CPU threads. It would likely be challenging to find a good
    > number for 20 that holds up with the future.

    And this was another line of reasoning that lead me to the extra kernel
    config parameter.

    > Or better perhaps have some threshold that you don't do it
    > that often, or only do it when you expect to be idle for a long
    > enough time that the CPU can enter deeper idle states
    >
    > (I higher idle states some more wakeups typically don't matter
    > that much)
    >
    > The cpufreq/cstate governour have a reasonable good idea
    > now how "idle" the system is and will be. Maybe you can reuse
    > that information somehow.

    My first thought was to find an existing "I am a small device running on
    battery power" or "low power consumption is critical to me" config
    parameter. I didn't find anything that looked like that. If there was
    one, I would make RCU_FAST_NO_HZ depend on it.

    Or did I miss some kernel parameter or API?

    Thanx, Paul


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