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Subject[PATCH tip/core/rcu 12/12] rcu: Make kernel-mode nohz_full CPUs invoke the RCU core processing
Date
From: "Paul E. McKenney" <paulmck@linux.ibm.com>

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@linux.ibm.com>
---
kernel/rcu/tree.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1b250d4..9ffe503 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -496,7 +496,7 @@ 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.
@@ -2270,7 +2270,7 @@ 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"));
@@ -2819,7 +2819,7 @@ 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);
@@ -2832,8 +2832,8 @@ 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? */
--
2.9.5
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