powerpc: Tell RCU about idle after hcall tracing

The PowerPC pSeries platform (CONFIG_PPC_PSERIES=y) enables
hypervisor-call tracing for CONFIG_TRACEPOINTS=y kernels.  One of the
hypervisor calls that is traced is the H_CEDE call in the idle loop
that tells the hypervisor that this OS instance no longer needs the
current CPU.  However, tracing uses RCU, so this combination of kernel
configuration variables needs to avoid telling RCU about the current CPU's
idleness until after the H_CEDE-entry tracing completes on the one hand,
and must tell RCU that the the current CPU is no longer idle before the
H_CEDE-exit tracing starts.

In all other cases, it suffices to inform RCU of CPU idleness upon
idle-loop entry and exit.

This commit makes the required adjustments.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

arch/powerpc/kernel/idle.c

@@ -46,6 +46,12 @@ static int __init powersave_off(char *arg)
 }
 __setup("powersave=off", powersave_off);
 
+#if defined(CONFIG_PPC_PSERIES) && defined(CONFIG_TRACEPOINTS)
+static const bool idle_uses_rcu = 1;
+#else
+static const bool idle_uses_rcu;
+#endif
+
 /*
  * The body of the idle task.
  */
@@ -56,6 +62,9 @@ void cpu_idle(void)
 
 	set_thread_flag(TIF_POLLING_NRFLAG);
 	while (1) {
+		if (idle_uses_rcu)
+			tick_nohz_idle_enter();
+		else
 			tick_nohz_idle_enter_norcu();
 		while (!need_resched() && !cpu_should_die()) {
 			ppc64_runlatch_off();
@@ -93,6 +102,9 @@ void cpu_idle(void)
 
 		HMT_medium();
 		ppc64_runlatch_on();
+		if (idle_uses_rcu)
+			tick_nohz_idle_exit();
+		else
 			tick_nohz_idle_exit_norcu();
 		preempt_enable_no_resched();
 		if (cpu_should_die())

arch/powerpc/platforms/pseries/lpar.c

@@ -555,6 +555,8 @@ void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
 
 	(*depth)++;
 	trace_hcall_entry(opcode, args);
+	if (opcode == H_CEDE)
+		rcu_idle_enter();
 	(*depth)--;
 
 out:
@@ -575,6 +577,8 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
 		goto out;
 
 	(*depth)++;
+	if (opcode == H_CEDE)
+		rcu_idle_exit();
 	trace_hcall_exit(opcode, retval, retbuf);
 	(*depth)--;