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powerpc: Use smt_snooze_delay=-1 to always busy loop

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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>
This commit is contained in:
Anton Blanchard
2010-05-16 20:02:39 +00:00
committed by Benjamin Herrenschmidt
parent dd04c63c96
commit b878dc0059
2 changed files with 13 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
arch/powerpc/platforms/pseries/setup.c
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@@ -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();