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Subject[PATCH rcu 13/18] rcu-tasks: Add rcupdate.rcu_task_enqueue_lim to set initial queueing
Date
This commit adds a rcupdate.rcu_task_enqueue_lim module parameter that
sets the initial number of callback queues to use for the RCU Tasks
family of RCU implementations. This paramter allows testing of various
fanout values.

Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
.../admin-guide/kernel-parameters.txt | 7 ++++++
kernel/rcu/tasks.h | 24 ++++++++++++++-----
2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9725c546a0d46..9b09fc5dfe665 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4805,6 +4805,13 @@
period to instead use normal non-expedited
grace-period processing.

+ rcupdate.rcu_task_enqueue_lim= [KNL]
+ Set the number of callback queues to use for the
+ RCU Tasks family of RCU flavors. The default
+ of -1 allows this to be automatically (and
+ dynamically) adjusted. This parameter is intended
+ for use in testing.
+
rcupdate.rcu_task_ipi_delay= [KNL]
Set time in jiffies during which RCU tasks will
avoid sending IPIs, starting with the beginning
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 61a606569868b..2b148f6743150 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -130,6 +130,9 @@ module_param(rcu_task_ipi_delay, int, 0644);
static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
module_param(rcu_task_stall_timeout, int, 0644);

+static int rcu_task_enqueue_lim __read_mostly = -1;
+module_param(rcu_task_enqueue_lim, int, 0444);
+
/* RCU tasks grace-period state for debugging. */
#define RTGS_INIT 0
#define RTGS_WAIT_WAIT_CBS 1
@@ -192,10 +195,19 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
{
int cpu;
unsigned long flags;
+ int lim;

raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
- rtp->percpu_enqueue_shift = ilog2(nr_cpu_ids);
- rtp->percpu_enqueue_lim = 1;
+ if (rcu_task_enqueue_lim < 0)
+ rcu_task_enqueue_lim = nr_cpu_ids;
+ else if (rcu_task_enqueue_lim == 0)
+ rcu_task_enqueue_lim = 1;
+ lim = rcu_task_enqueue_lim;
+
+ if (lim > nr_cpu_ids)
+ lim = nr_cpu_ids;
+ WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids / lim));
+ smp_store_release(&rtp->percpu_enqueue_lim, lim);
for_each_possible_cpu(cpu) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

@@ -211,7 +223,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled.
}
raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
-
+ pr_info("%s: Setting shift to %d and lim to %d.\n", __func__, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim));
}

// Enqueue a callback for the specified flavor of Tasks RCU.
@@ -307,7 +319,7 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
unsigned long flags;
int needgpcb = 0;

- for (cpu = 0; cpu < rtp->percpu_enqueue_lim; cpu++) {
+ for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_enqueue_lim); cpu++) {
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

/* Advance and accelerate any new callbacks. */
@@ -339,11 +351,11 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu

cpu = rtpcp->cpu;
cpunext = cpu * 2 + 1;
- if (cpunext < rtp->percpu_enqueue_lim) {
+ if (cpunext < smp_load_acquire(&rtp->percpu_enqueue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
cpunext++;
- if (cpunext < rtp->percpu_enqueue_lim) {
+ if (cpunext < smp_load_acquire(&rtp->percpu_enqueue_lim)) {
rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
queue_work_on(cpunext, system_wq, &rtpcp_next->rtp_work);
}
--
2.31.1.189.g2e36527f23
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