rcu: Force nohz_full tick on upon irq enter instead of exit

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@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9fda33864edeb638304576333629e65862f8e0d6..8dc878406c71c5de7773a7c190bc893e9ffa545c 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -651,12 +651,6 @@ 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;
@@ -831,6 +825,11 @@ 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,