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Subject[PATCH v3 tip/core/rcu 25/34] rcu-tasks: Add Kconfig option to mediate smp_mb() vs. IPI
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

This commit provides a new TASKS_TRACE_RCU_READ_MB Kconfig option that
enables use of read-side memory barriers by both rcu_read_lock_trace()
and rcu_read_unlock_trace() when the are executed with the
current->trc_reader_special.b.need_mb flag set. This flag is currently
never set. Doing that is the subject of a later commit.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
include/linux/rcupdate_trace.h | 3 ++-
kernel/rcu/Kconfig | 18 ++++++++++++++++++
kernel/rcu/tasks.h | 3 ++-
3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index c42b365c..4c25a41 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -50,7 +50,8 @@ static inline void rcu_read_lock_trace(void)
struct task_struct *t = current;

WRITE_ONCE(t->trc_reader_nesting, READ_ONCE(t->trc_reader_nesting) + 1);
- if (t->trc_reader_special.b.need_mb)
+ if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) &&
+ t->trc_reader_special.b.need_mb)
smp_mb(); // Pairs with update-side barriers
rcu_lock_acquire(&rcu_trace_lock_map);
}
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index cb1d18e..0ebe15a 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -234,4 +234,22 @@ config RCU_NOCB_CPU
Say Y here if you want to help to debug reduced OS jitter.
Say N here if you are unsure.

+config TASKS_TRACE_RCU_READ_MB
+ bool "Tasks Trace RCU readers use memory barriers in user and idle"
+ depends on RCU_EXPERT
+ default PREEMPT_RT || NR_CPUS < 8
+ help
+ Use this option to further reduce the number of IPIs sent
+ to CPUs executing in userspace or idle during tasks trace
+ RCU grace periods. Given that a reasonable setting of
+ the rcupdate.rcu_task_ipi_delay kernel boot parameter
+ eliminates such IPIs for many workloads, proper setting
+ of this Kconfig option is important mostly for aggressive
+ real-time installations and for battery-powered devices,
+ hence the default chosen above.
+
+ Say Y here if you hate IPIs.
+ Say N here if you hate read-side memory barriers.
+ Take the default if you are unsure.
+
endmenu # "RCU Subsystem"
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index f9a828c..b18298b 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -734,7 +734,8 @@ void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
{
int nq = t->trc_reader_special.b.need_qs;

- if (t->trc_reader_special.b.need_mb)
+ if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) &&
+ t->trc_reader_special.b.need_mb)
smp_mb(); // Pairs with update-side barriers.
// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
if (nq)
--
2.9.5
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