[PATCH tip/core/rcu 09/12] rcu: Force nohz_full tick on upon irq enter instead of exit

paulmck
2019-10-03T01:39:13Z

From: "Paul E. McKenney" <paulmck@linux.ibm.com>

There is interrupt-exit code that forces on the tick for nohz_full CPUs
failing to respond to the current grace period in a timely fashion.
However, this code must compare ->dynticks_nmi_nesting to the value 2
in the interrupt-exit fastpath. This commit therefore moves this code
to the interrupt-entry fastpath, where a lighter-weight comparison to
zero may be used.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
[ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
kernel/rcu/tree.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 621cc06..1601fa6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -650,12 +650,6 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
*/
if (rdp->dynticks_nmi_nesting != 1) {
trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
- if (tick_nohz_full_cpu(rdp->cpu) &&
- rdp->dynticks_nmi_nesting == 2 &&
- rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
- rdp->rcu_forced_tick = true;
- tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
- }
WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
rdp->dynticks_nmi_nesting - 2);
return;
@@ -830,6 +824,11 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
rcu_cleanup_after_idle();

incby = 1;
+ } else if (tick_nohz_full_cpu(rdp->cpu) &&
+ rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
+ rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+ rdp->rcu_forced_tick = true;
+ tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
}
trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
rdp->dynticks_nmi_nesting,
--
2.9.5