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rcu-tasks: Use order_base_2() instead of ilog2()

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The ilog2() function can be used to generate a shift count, but it will
generate the same count for a power of two as for one greater than a power
of two.  This results in shift counts that are larger than necessary for
systems with a power-of-two number of CPUs because the CPUs are numbered
from zero, so that the maximum CPU number is one less than that power
of two.

This commit therefore substitutes order_base_2(), which appears to have
been designed for exactly this use case.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-02-02 15:42:36 -08:00
parent da123016ca
commit 2bcd18e041
1 changed files with 3 additions and 3 deletions
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6
kernel/rcu/tasks.h
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@ -123,7 +123,7 @@ static struct rcu_tasks rt_name = \
.call_func = call, \
.rtpcpu = &rt_name ## __percpu, \
.name = n, \
.percpu_enqueue_shift = ilog2(CONFIG_NR_CPUS) + 1, \
.percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS), \
.percpu_enqueue_lim = 1, \
.percpu_dequeue_lim = 1, \
.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex), \
@ -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, ilog2(nr_cpu_ids) + 1);
WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
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);
@ -417,7 +417,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
if (rtp->percpu_enqueue_lim > 1) {
WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids) + 1);
WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
smp_store_release(&rtp->percpu_enqueue_lim, 1);
rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);