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Subject[PATCH v3 tip/core/rcu 22/34] rcu-tasks: Provide boot parameter to delay IPIs until late in grace period
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

This commit provides a rcupdate.rcu_task_ipi_delay kernel boot parameter
that specifies how old the RCU tasks trace grace period must be before
the grace-period kthread starts sending IPIs. This delay allows more
tasks to pass through rcu_tasks_qs() quiescent states, thus reducing
(or even eliminating) the number of IPIs that must be sent.

On a short rcutorture test setting this kernel boot parameter to HZ/2
resulted in zero IPIs for all 877 RCU-tasks trace grace periods that
elapsed during that test.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
kernel/rcu/tasks.h | 15 ++++++++++-----
2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index df2baf9..6f3b3be 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4246,6 +4246,13 @@
only normal grace-period primitives. No effect
on CONFIG_TINY_RCU kernels.

+ rcupdate.rcu_task_ipi_delay= [KNL]
+ Set time in jiffies during which RCU tasks will
+ avoid sending IPIs, starting with the beginning
+ of a given grace period. Setting a large
+ number avoids disturbing real-time workloads,
+ but lengthens grace periods.
+
rcupdate.rcu_task_stall_timeout= [KNL]
Set timeout in jiffies for RCU task stall warning
messages. Disable with a value less than or equal
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index fa9c069..a034f48 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -74,6 +74,11 @@ static struct rcu_tasks rt_name = \
/* Track exiting tasks in order to allow them to be waited for. */
DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);

+/* Avoid IPIing CPUs early in the grace period. */
+#define RCU_TASK_IPI_DELAY (HZ / 2)
+static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
+module_param(rcu_task_ipi_delay, int, 0644);
+
/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
@@ -713,6 +718,10 @@ DECLARE_WAIT_QUEUE_HEAD(trc_wait); // List of holdout tasks.
// Record outstanding IPIs to each CPU. No point in sending two...
static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);

+void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
+ "RCU Tasks Trace");
+
/* If we are the last reader, wake up the grace-period kthread. */
void rcu_read_unlock_trace_special(struct task_struct *t)
{
@@ -825,7 +834,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,

// If currently running, send an IPI, either way, add to list.
trc_add_holdout(t, bhp);
- if (task_curr(t)) {
+ if (task_curr(t) && time_after(jiffies, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
// The task is currently running, so try IPIing it.
cpu = task_cpu(t);

@@ -994,10 +1003,6 @@ void exit_tasks_rcu_finish_trace(struct task_struct *t)
rcu_read_unlock_trace_special(t);
}

-void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
-DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
- "RCU Tasks Trace");
-
/**
* call_rcu_tasks_trace() - Queue a callback trace task-based grace period
* @rhp: structure to be used for queueing the RCU updates.
--
2.9.5
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