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Commit eb75767b authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU 



Because RCU's quiescent-state-forcing mechanism is used to drive the
full-system-idle state machine, and because this mechanism is executed
by RCU's grace-period kthreads, this commit forces these kthreads to
run on the timekeeping CPU (tick_do_timer_cpu).  To do otherwise would
mean that the RCU grace-period kthreads would force the system into
non-idle state every time they drove the state machine, which would
be just a bit on the futile side.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent 0edd1b17

kernel/rcutree.c

+1 −0
Original line number Diff line number Diff line
@@ -1303,6 +1303,7 @@ static int rcu_gp_init(struct rcu_state *rsp) 
	struct rcu_data *rdp;

	struct rcu_node *rnp = rcu_get_root(rsp);



	rcu_bind_gp_kthread();

	raw_spin_lock_irq(&rnp->lock);

	rsp->gp_flags = 0; /* Clear all flags: New grace period. */

kernel/rcutree.h

+1 −0
Original line number Diff line number Diff line
@@ -560,6 +560,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, 
static bool is_sysidle_rcu_state(struct rcu_state *rsp);

static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,

				  unsigned long maxj);

static void rcu_bind_gp_kthread(void);

static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);



#endif /* #ifndef RCU_TREE_NONCORE */

kernel/rcutree_plugin.h

+20 −1
Original line number Diff line number Diff line
@@ -2531,7 +2531,8 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, 
	if (!*isidle || rdp->rsp != rcu_sysidle_state ||

	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)

		return;

	/* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */

	if (rcu_gp_in_progress(rdp->rsp))

		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);



	/* Pick up current idle and NMI-nesting counter and check. */

	cur = atomic_read(&rdtp->dynticks_idle);
@@ -2556,6 +2557,20 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) 
	return rsp == rcu_sysidle_state;

}



/*

 * Bind the grace-period kthread for the sysidle flavor of RCU to the

 * timekeeping CPU.

 */

static void rcu_bind_gp_kthread(void)

{

	int cpu = ACCESS_ONCE(tick_do_timer_cpu);



	if (cpu < 0 || cpu >= nr_cpu_ids)

		return;

	if (raw_smp_processor_id() != cpu)

		set_cpus_allowed_ptr(current, cpumask_of(cpu));

}



/*

 * Return a delay in jiffies based on the number of CPUs, rcu_node

 * leaf fanout, and jiffies tick rate.  The idea is to allow larger
@@ -2766,6 +2781,10 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) 
	return false;

}



static void rcu_bind_gp_kthread(void)

{

}



static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,

				  unsigned long maxj)

{