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, 8 insertions, 9 deletions

diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
index 158fb731e19..c0d8c2006bf 100644
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -35,7 +35,7 @@ static DEFINE_PER_CPU(struct cpu, cpu_devices);
 #ifdef CONFIG_PPC64
 
 /* Time in microseconds we delay before sleeping in the idle loop */
-DEFINE_PER_CPU(unsigned long, smt_snooze_delay) = { 100 };
+DEFINE_PER_CPU(long, smt_snooze_delay) = { 100 };
 
 static ssize_t store_smt_snooze_delay(struct sys_device *dev,
 				      struct sysdev_attribute *attr,
@@ -44,9 +44,9 @@ static ssize_t store_smt_snooze_delay(struct sys_device *dev,
 {
 	struct cpu *cpu = container_of(dev, struct cpu, sysdev);
 	ssize_t ret;
-	unsigned long snooze;
+	long snooze;
 
-	ret = sscanf(buf, "%lu", &snooze);
+	ret = sscanf(buf, "%ld", &snooze);
 	if (ret != 1)
 		return -EINVAL;
 
@@ -61,7 +61,7 @@ static ssize_t show_smt_snooze_delay(struct sys_device *dev,
 {
 	struct cpu *cpu = container_of(dev, struct cpu, sysdev);
 
-	return sprintf(buf, "%lu\n", per_cpu(smt_snooze_delay, cpu->sysdev.id));
+	return sprintf(buf, "%ld\n", per_cpu(smt_snooze_delay, cpu->sysdev.id));
 }
 
 static SYSDEV_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
@@ -70,15 +70,14 @@ static SYSDEV_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
 static int __init setup_smt_snooze_delay(char *str)
 {
 	unsigned int cpu;
-	int snooze;
+	long snooze;
 
 	if (!cpu_has_feature(CPU_FTR_SMT))
 		return 1;
 
-	if (get_option(&str, &snooze)) {
-		for_each_possible_cpu(cpu)
-			per_cpu(smt_snooze_delay, cpu) = snooze;
-	}
+	snooze = simple_strtol(str, NULL, 10);
+	for_each_possible_cpu(cpu)
+		per_cpu(smt_snooze_delay, cpu) = snooze;
 
 	return 1;
 }