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    Subject[PATCH tip/core/rcu 13/26] tick/nohz: Narrow down noise while setting current task's tick dependency
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    Setting a tick dependency on any task, including the case where a task
    sets that dependency on itself, triggers an IPI to all CPUs. That is
    of course suboptimal but it had previously not been an issue because it
    was only used by POSIX CPU timers on nohz_full, which apparently never
    occurs in latency-sensitive workloads in production. (Or users of such
    systems are suffering in silence on the one hand or venting their ire
    on the wrong people on the other.)

    But RCU now sets a task tick dependency on the current task in order
    to fix stall issues that can occur during RCU callback processing.
    Thus, RCU callback processing triggers frequent system-wide IPIs from
    nohz_full CPUs. This is quite counter-productive, after all, avoiding
    IPIs is what nohz_full is supposed to be all about.

    This commit therefore optimizes tasks' self-setting of a task tick
    dependency by using tick_nohz_full_kick() to avoid the system-wide IPI.
    Instead, only the execution of the one task is disturbed, which is
    acceptable given that this disturbance is well down into the noise
    compared to the degree to which the RCU callback processing itself
    disturbs execution.

    Fixes: 6a949b7af82d (rcu: Force on tick when invoking lots of callbacks)
    Reported-by: Matt Fleming <matt@codeblueprint.co.uk>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Cc: stable@kernel.org
    Cc: Paul E. McKenney <paulmck@kernel.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    ---
    kernel/time/tick-sched.c | 22 +++++++++++++++-------
    1 file changed, 15 insertions(+), 7 deletions(-)

    diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    index 3e2dc9b..f0199a4 100644
    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -351,16 +351,24 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
    EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);

    /*
    - * Set a per-task tick dependency. Posix CPU timers need this in order to elapse
    - * per task timers.
    + * Set a per-task tick dependency. RCU need this. Also posix CPU timers
    + * in order to elapse per task timers.
    */
    void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
    {
    - /*
    - * We could optimize this with just kicking the target running the task
    - * if that noise matters for nohz full users.
    - */
    - tick_nohz_dep_set_all(&tsk->tick_dep_mask, bit);
    + if (!atomic_fetch_or(BIT(bit), &tsk->tick_dep_mask)) {
    + if (tsk == current) {
    + preempt_disable();
    + tick_nohz_full_kick();
    + preempt_enable();
    + } else {
    + /*
    + * Some future tick_nohz_full_kick_task()
    + * should optimize this.
    + */
    + tick_nohz_full_kick_all();
    + }
    + }
    }
    EXPORT_SYMBOL_GPL(tick_nohz_dep_set_task);

    --
    2.9.5
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