softirq: A single rcu_bh_qs() call per softirq set is enough

Calling rcu_bh_qs() after every softirq action is not really needed.
What RCU needs is at least one rcu_bh_qs() per softirq round to note a
quiescent state was passed for rcu_bh.

Note for Paul and myself : this could be inlined as a single instruction
and avoid smp_processor_id()
(sone this_cpu_write(rcu_bh_data.passed_quiesce, 1))

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/softirq.c

@@ -232,7 +232,6 @@ asmlinkage void __do_softirq(void)
 	bool in_hardirq;
 	__u32 pending;
 	int softirq_bit;
-	int cpu;
 
 	/*
 	 * Mask out PF_MEMALLOC s current task context is borrowed for the
@@ -247,7 +246,6 @@ asmlinkage void __do_softirq(void)
 	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 	in_hardirq = lockdep_softirq_start();
 
-	cpu = smp_processor_id();
 restart:
 	/* Reset the pending bitmask before enabling irqs */
 	set_softirq_pending(0);
@@ -276,11 +274,11 @@ asmlinkage void __do_softirq(void)
 			       prev_count, preempt_count());
 			preempt_count_set(prev_count);
 		}
-		rcu_bh_qs(cpu);
 		h++;
 		pending >>= softirq_bit;
 	}
 
+	rcu_bh_qs(smp_processor_id());
 	local_irq_disable();
 
 	pending = local_softirq_pending();