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    SubjectRe: [patch 00/20] timer: Refactor the timer wheel
    On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote:
    > The current timer wheel has some drawbacks:
    >
    > 1) Cascading
    >
    > Cascading can be an unbound operation and is completely pointless in most
    > cases because the vast majority of the timer wheel timers are canceled or
    > rearmed before expiration.
    >
    > 2) No fast lookup of the next expiring timer
    >
    > In NOHZ scenarios the first timer soft interrupt after a long NOHZ period
    > must fast forward the base time to current jiffies. As we have no way to
    > find the next expiring timer fast, the code loops and increments the base
    > time by one and checks for expired timers in each step. I've observed loops
    > lasting 1 ms!
    >
    > There are some other issues caused by the above, but they are minor compare to
    > those.

    For SMP configurations, this passes light rcutorture testing. For UP
    builds, it complains about undefined symbols. Builds succeed with
    the following kneejerk patch. Am retesting rcutorture.

    Thanx, Paul

    ------------------------------------------------------------------------

    commit 87dbd35cf4034f2b664b6e9d60decd2bdbfc416f
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Date: Mon Jun 13 09:11:41 2016 -0700

    Fix undefined get_nohz_timer_target() in non-SMP builds

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 6e42ada26345..322e52415a29 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -348,6 +348,10 @@ extern int get_nohz_timer_target(void);
    #else
    static inline void nohz_balance_enter_idle(int cpu) { }
    static inline void set_cpu_sd_state_idle(void) { }
    +static inline int get_nohz_timer_target(void)
    +{
    + return raw_smp_processor_id();
    +}
    #endif

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