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 158fb731e199..c0d8c2006bf4 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;
 }