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    Subject[PATCH 2/2] nohz: Move full nohz kick to its own IPI
    Date
    Now that we have smp_queue_function_single() which can be used to
    safely queue IPIs when interrupts are disabled and without worrying
    about concurrent callers, lets use it for the full dynticks kick to
    notify a CPU that it's exiting single task mode.

    This unbloats a bit the scheduler IPI that the nohz code was abusing
    for its cool "callable anywhere/anytime" properties.

    Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Jens Axboe <axboe@fb.com>
    Cc: Kevin Hilman <khilman@linaro.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    ---
    include/linux/tick.h | 2 ++
    kernel/sched/core.c | 5 +----
    kernel/sched/sched.h | 2 +-
    kernel/time/tick-sched.c | 21 +++++++++++++++++++++
    4 files changed, 25 insertions(+), 5 deletions(-)

    diff --git a/include/linux/tick.h b/include/linux/tick.h
    index b84773c..9d3fcc2 100644
    --- a/include/linux/tick.h
    +++ b/include/linux/tick.h
    @@ -182,6 +182,7 @@ static inline bool tick_nohz_full_cpu(int cpu)
    extern void tick_nohz_init(void);
    extern void __tick_nohz_full_check(void);
    extern void tick_nohz_full_kick(void);
    +extern void tick_nohz_full_kick_cpu(int cpu);
    extern void tick_nohz_full_kick_all(void);
    extern void __tick_nohz_task_switch(struct task_struct *tsk);
    #else
    @@ -190,6 +191,7 @@ static inline bool tick_nohz_full_enabled(void) { return false; }
    static inline bool tick_nohz_full_cpu(int cpu) { return false; }
    static inline void __tick_nohz_full_check(void) { }
    static inline void tick_nohz_full_kick(void) { }
    +static inline void tick_nohz_full_kick_cpu(int cpu) { }
    static inline void tick_nohz_full_kick_all(void) { }
    static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
    #endif
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 9cae286..e4b344e 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -1499,9 +1499,7 @@ void scheduler_ipi(void)
    */
    preempt_fold_need_resched();

    - if (llist_empty(&this_rq()->wake_list)
    - && !tick_nohz_full_cpu(smp_processor_id())
    - && !got_nohz_idle_kick())
    + if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick())
    return;

    /*
    @@ -1518,7 +1516,6 @@ void scheduler_ipi(void)
    * somewhat pessimize the simple resched case.
    */
    irq_enter();
    - tick_nohz_full_check();
    sched_ttwu_pending();

    /*
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index c9007f2..4771063 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1225,7 +1225,7 @@ static inline void inc_nr_running(struct rq *rq)
    if (tick_nohz_full_cpu(rq->cpu)) {
    /* Order rq->nr_running write against the IPI */
    smp_wmb();
    - smp_send_reschedule(rq->cpu);
    + tick_nohz_full_kick_cpu(rq->cpu);
    }
    }
    #endif
    diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    index 9f8af69..582d3f6 100644
    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -230,6 +230,27 @@ void tick_nohz_full_kick(void)
    irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
    }

    +static void nohz_full_kick_queue(struct queue_single_data *qsd)
    +{
    + __tick_nohz_full_check();
    +}
    +
    +static DEFINE_PER_CPU(struct queue_single_data, nohz_full_kick_qsd) = {
    + .func = nohz_full_kick_queue,
    +};
    +
    +void tick_nohz_full_kick_cpu(int cpu)
    +{
    + if (!tick_nohz_full_cpu(cpu))
    + return;
    +
    + if (cpu == smp_processor_id()) {
    + irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
    + } else {
    + smp_queue_function_single(cpu, &per_cpu(nohz_full_kick_qsd, cpu));
    + }
    +}
    +
    static void nohz_full_kick_ipi(void *info)
    {
    __tick_nohz_full_check();
    --
    1.8.3.1


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