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

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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>
This commit is contained in:
Paul E. McKenney 2022-02-02 16:34:40 -08:00
parent 2bcd18e041
commit 00a8b4b54c
1 changed files with 1 additions and 1 deletions
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2
kernel/rcu/tasks.h
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@ -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);