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Commit b878dc00 authored by Anton Blanchard's avatar Anton Blanchard Committed by Benjamin Herrenschmidt
Browse files

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: default avatarAnton Blanchard <anton@samba.org>
Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
parent dd04c63c

arch/powerpc/kernel/sysfs.c

+8 −9
Original line number Diff line number Diff line
@@ -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)) {

	snooze = simple_strtol(str, NULL, 10);

	for_each_possible_cpu(cpu)

		per_cpu(smt_snooze_delay, cpu) = snooze;

	}



	return 1;

}

arch/powerpc/platforms/pseries/setup.c

+5 −5
Original line number Diff line number Diff line
@@ -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();