powerpc: Use smt_snooze_delay=-1 to always busy loop

Right now if we want to busy loop and not give up any time to the hypervisor
we put a very large value into smt_snooze_delay. This is sometimes useful
when running a single partition and you want to avoid any latencies due
to the hypervisor or CPU power state transitions. While this works, it's a bit
ugly - how big a number is enough now we have NO_HZ and can be idle for a very
long time.

The patch below makes smt_snooze_delay signed, and a negative value means loop
forever:

echo -1 > /sys/devices/system/cpu/cpu0/smt_snooze_delay

This change shouldn't affect the existing userspace tools (eg ppc64_cpu), but
I'm cc-ing Nathan just to be sure.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
index 6710761bf60f..a6d19e3a505e 100644
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -496,13 +496,14 @@ static int __init pSeries_probe(void)
 }
 
 
-DECLARE_PER_CPU(unsigned long, smt_snooze_delay);
+DECLARE_PER_CPU(long, smt_snooze_delay);
 
 static void pseries_dedicated_idle_sleep(void)
 { 
 	unsigned int cpu = smp_processor_id();
 	unsigned long start_snooze;
 	unsigned long in_purr, out_purr;
+	long snooze = __get_cpu_var(smt_snooze_delay);
 
 	/*
 	 * Indicate to the HV that we are idle. Now would be
@@ -517,13 +518,12 @@ static void pseries_dedicated_idle_sleep(void)
 	 * has been checked recently.  If we should poll for a little
 	 * while, do so.
 	 */
-	if (__get_cpu_var(smt_snooze_delay)) {
-		start_snooze = get_tb() +
-			__get_cpu_var(smt_snooze_delay) * tb_ticks_per_usec;
+	if (snooze) {
+		start_snooze = get_tb() + snooze * tb_ticks_per_usec;
 		local_irq_enable();
 		set_thread_flag(TIF_POLLING_NRFLAG);
 
-		while (get_tb() < start_snooze) {
+		while ((snooze < 0) || (get_tb() < start_snooze)) {
 			if (need_resched() || cpu_is_offline(cpu))
 				goto out;
 			ppc64_runlatch_off();