rcu-tasks: Set ->percpu_enqueue_shift to zero upon contention

Currently, call_rcu_tasks_generic() sets ->percpu_enqueue_shift to
order_base_2(nr_cpu_ids) upon encountering sufficient contention.
This does not shift to use of non-CPU-0 callback queues as intended, but
rather continues using only CPU 0's queue.  Although this does provide
some decrease in contention due to spreading work over multiple locks,
it is not the dramatic decrease that was intended.

This commit therefore makes call_rcu_tasks_generic() set
->percpu_enqueue_shift to 0.

Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 670c75cbcb98dab80bfa358d2667048f46b74eb6..ac17348187e486226da495920f710d2b70f10680 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -302,7 +302,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
        if (unlikely(needadjust)) {
                raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
                if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
-                       WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
+                       WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
                        WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
                        smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
                        pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);