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authorPaul E. McKenney <paulmck@kernel.org>2020-03-18 17:16:37 -0700
committerPaul E. McKenney <paulmck@kernel.org>2020-04-27 11:03:52 -0700
commit9ae58d7bd11f1fc4c96389df11751f8593d8bd33 (patch)
tree2839dd92f0e5f91bedc25cd399b2a0ca619f48a1 /kernel/rcu
parent238dbce39ea467577ce7e41ee3e98748c436ed0f (diff)
rcu-tasks: Add Kconfig option to mediate smp_mb() vs. IPI
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>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/Kconfig18
-rw-r--r--kernel/rcu/tasks.h3
2 files changed, 20 insertions, 1 deletions
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index cb1d18ef343c..0ebe15a84985 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 4857450c0430..4147857007d7 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)