Commit b0afa0f0 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu-tasks: Provide boot parameter to delay IPIs until late in grace period 



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: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 88092d0c

Documentation/admin-guide/kernel-parameters.txt

+7 −0
Original line number Diff line number Diff line
@@ -4286,6 +4286,13 @@ 
			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

kernel/rcu/tasks.h

+9 −4
Original line number Diff line number Diff line
@@ -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)

{
@@ -998,10 +1007,6 @@ 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.