Commit 9ae58d7b authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 238dbce3

include/linux/rcupdate_trace.h

+2 −1
Original line number Diff line number Diff line
@@ -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);

}

kernel/rcu/Kconfig

+18 −0
Original line number Diff line number Diff line
@@ -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"

kernel/rcu/tasks.h

+2 −1
Original line number Diff line number Diff line
@@ -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)