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    SubjectRe: [PATCH RFC tip/core/rcu 27/28] rcu: Allow dyntick-idle mode for CPUs with callbacks
    On Wed, Nov 02, 2011 at 09:47:44PM -0700, Josh Triplett wrote:
    > On Wed, Nov 02, 2011 at 01:30:48PM -0700, Paul E. McKenney wrote:
    > > /*
    > > - * Check to see if any future RCU-related work will need to be done
    > > - * by the current CPU, even if none need be done immediately, returning
    > > - * 1 if so. This function is part of the RCU implementation; it is -not-
    > > - * an exported member of the RCU API.
    > > + * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
    > > + * callbacks on this CPU, (2) this CPU has not yet attempted to enter
    > > + * dyntick-idle mode, and (3) this CPU is in the process of attempting to
    > > + * enter dyntick-idle mode. Otherwise, if we have recently tried and failed
    >
    > This sentence doesn't quite work; "if either...and..." should become
    > "either...or" or "if...and".

    Good eyes -- "or" it is!

    > > + * to enter dyntick-idle mode, we refuse to try to enter it. After all,
    > > + * it is better to incur scheduling-clock interrupts than to spin
    > > + * continuously for the same time duration!
    > > + */
    > > +int rcu_needs_cpu(int cpu)
    > > +{
    > > + /* If no callbacks, RCU doesn't need the CPU. */
    > > + if (!rcu_cpu_has_callbacks(cpu))
    > > + return 0;
    > > + /* Otherwise, RCU needs the CPU only if it recently tried and failed. */
    > > + return per_cpu(rcu_dyntick_holdoff, cpu) == jiffies;
    >
    > Sigh, one more use of jiffies. :(

    Your suggested alternative? I need something cheap, doesn't need to
    be accurate to more than a few milliseconds, needs to be synchronized
    across all CPUs.

    Thanx, Paul



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