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rcu: Make kernel-mode nohz_full CPUs invoke the RCU core processing 

If a nohz_full CPU is idle or executing in userspace, it makes good sense
to keep it out of RCU core processing.  After all, the RCU grace-period
kthread can see its quiescent states and all of its callbacks are
offloaded, so there is nothing for RCU core processing to do.

However, if a nohz_full CPU is executing in kernel space, the RCU
grace-period kthread cannot do anything for it, so such a CPU must report
its own quiescent states.  This commit therefore makes nohz_full CPUs
skip RCU core processing only if the scheduler-clock interrupt caught
them in idle or in userspace.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2019-09-14 03:39:22 -07:00
parent ed93dfc6bc
commit dd7dafd1ad
1 changed files with 5 additions and 5 deletions
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10
kernel/rcu/tree.c
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@ -497,7 +497,7 @@ module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next
module_param(rcu_kick_kthreads, bool, 0644);
static void force_qs_rnp(int (*f)(struct rcu_data *rdp));
static int rcu_pending(void);
static int rcu_pending(int user);
/*
* Return the number of RCU GPs completed thus far for debug & stats.
@ -2267,7 +2267,7 @@ void rcu_sched_clock_irq(int user)
__this_cpu_write(rcu_data.rcu_urgent_qs, false);
}
rcu_flavor_sched_clock_irq(user);
if (rcu_pending())
if (rcu_pending(user))
invoke_rcu_core();
trace_rcu_utilization(TPS("End scheduler-tick"));
@ -2816,7 +2816,7 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
* CPU-local state are performed first. However, we must check for CPU
* stalls first, else we might not get a chance.
*/
static int rcu_pending(void)
static int rcu_pending(int user)
{
bool gp_in_progress;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
@ -2829,8 +2829,8 @@ static int rcu_pending(void)
if (rcu_nocb_need_deferred_wakeup(rdp))
return 1;
/* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
if (rcu_nohz_full_cpu())
/* Is this a nohz_full CPU in userspace or idle? (Ignore RCU if so.) */
if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
return 0;
/* Is the RCU core waiting for a quiescent state from this CPU? */