rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic queue selection

This commit introduces a ->percpu_enqueue_shift field to the rcu_tasks
structure, and uses it to shift down the CPU number in order to
select a rcu_tasks_percpu structure.  This field is currently set to a
sufficiently large shift count to always select the CPU-0 instance of
the rcu_tasks_percpu structure, and later commits will adjust this.

Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 30048db7aa49d050c97153ffee23999526f6bcb9..2a5fe3e04b3631f8f3753069e67268811291e416 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -52,6 +52,7 @@ struct rcu_tasks_percpu {
  * @postgp_func: This flavor's post-grace-period function (optional).
  * @call_func: This flavor's call_rcu()-equivalent function.
  * @rtpcpu: This flavor's rcu_tasks_percpu structure.
+ * @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
  * @name: This flavor's textual name.
  * @kname: This flavor's kthread name.
  */
@@ -75,6 +76,7 @@ struct rcu_tasks {
        postgp_func_t postgp_func;
        call_rcu_func_t call_func;
        struct rcu_tasks_percpu __percpu *rtpcpu;
+       int percpu_enqueue_shift;
        char *name;
        char *kname;
 };
@@ -91,6 +93,7 @@ static struct rcu_tasks rt_name =                                                     \
        .call_func = call,                                                              \
        .rtpcpu = &rt_name ## __percpu,                                                 \
        .name = n,                                                                      \
+       .percpu_enqueue_shift = ilog2(CONFIG_NR_CPUS),                                  \
        .kname = #rt_name,                                                              \
 }
 
@@ -169,6 +172,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
        unsigned long flags;
 
        raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
+       rtp->percpu_enqueue_shift = ilog2(nr_cpu_ids);
        for_each_possible_cpu(cpu) {
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 
@@ -195,7 +199,8 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
        rhp->next = NULL;
        rhp->func = func;
        local_irq_save(flags);
-       rtpcp = per_cpu_ptr(rtp->rtpcpu, 0 /* smp_processor_id() */);
+       rtpcp = per_cpu_ptr(rtp->rtpcpu,
+                           smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift));
        raw_spin_lock(&rtpcp->cbs_pcpu_lock);
        if (!rtpcp->cbs_tail) {
                raw_spin_unlock(&rtpcp->cbs_pcpu_lock); // irqs remain disabled.