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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>
This commit is contained in:
Paul E. McKenney 2020-03-18 17:16:37 -07:00
parent 238dbce39e
commit 9ae58d7bd1
3 changed files with 22 additions and 2 deletions
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3
include/linux/rcupdate_trace.h
View File

@ -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);
}

18
kernel/rcu/Kconfig
View File

@ -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"

3
kernel/rcu/tasks.h
View File

@ -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)