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

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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>
This commit is contained in:
Paul E. McKenney 2020-03-17 11:39:26 -07:00
parent 88092d0c99
commit b0afa0f056
2 changed files with 16 additions and 4 deletions
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7
Documentation/admin-guide/kernel-parameters.txt
View file

@ -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

13
kernel/rcu/tasks.h
View file

@ -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.