Re: [ck] [PATCH] dynamic-tick patch modified for SMP

Srivatsa Vaddagiri
2005-08-13T11:38:34Z

On Sat, Aug 13, 2005 at 04:51:07PM +1000, Con Kolivas wrote:
> I'm sorry to say this doesn't appear to skip any ticks on my single P4 with
> SMP/SMT enabled.

Con,
I had enabled skipping ticks only in default_idle routine. So if
you have a different idle route (which I suspect is the case), it will not
skip ticks (since dyn_tick_reprogram_timer will not be called).
Can you try this patch?

---

linux-2.6.13-rc6-root/arch/i386/kernel/process.c | 7 ++++---
linux-2.6.13-rc6-root/kernel/dyn-tick.c | 8 ++++++--
2 files changed, 10 insertions(+), 5 deletions(-)

diff -puN kernel/dyn-tick.c~dynamic-tick-smp-fix kernel/dyn-tick.c
--- linux-2.6.13-rc6/kernel/dyn-tick.c~dynamic-tick-smp-fix 2005-08-13 15:53:56.000000000 +0530
+++ linux-2.6.13-rc6-root/kernel/dyn-tick.c 2005-08-13 15:56:12.000000000 +0530
@@ -37,16 +37,18 @@ spinlock_t dyn_tick_lock;

/*
* Arch independent code needed to reprogram next timer interrupt.
- * Gets called with IRQs disabled from cpu_idle() before entering idle loop.
+ * Gets called from cpu_idle() before entering idle loop.
*/
unsigned long dyn_tick_reprogram_timer(void)
{
int cpu = smp_processor_id();
- unsigned long delta;
+ unsigned long delta, flags;

if (!DYN_TICK_IS_SET(DYN_TICK_ENABLED))
return 0;

+ local_irq_save(flags);
+
if (rcu_pending(cpu) || local_softirq_pending())
return 0;

@@ -76,6 +78,8 @@ unsigned long dyn_tick_reprogram_timer(v

write_sequnlock(&xtime_lock);

+ local_irq_restore(flags);
+
return delta;
}

diff -puN arch/i386/kernel/process.c~dynamic-tick-smp-fix arch/i386/kernel/process.c
--- linux-2.6.13-rc6/arch/i386/kernel/process.c~dynamic-tick-smp-fix 2005-08-13 15:53:56.000000000 +0530
+++ linux-2.6.13-rc6-root/arch/i386/kernel/process.c 2005-08-13 15:55:20.000000000 +0530
@@ -104,10 +104,9 @@ void default_idle(void)
{
if (!hlt_counter && boot_cpu_data.hlt_works_ok) {
local_irq_disable();
- if (!need_resched()) {
- dyn_tick_reprogram_timer();
+ if (!need_resched())
safe_halt();
- } else
+ else
local_irq_enable();
} else {
cpu_relax();
@@ -202,6 +201,8 @@ void cpu_idle(void)
if (cpu_is_offline(cpu))
play_dead();

+ dyn_tick_reprogram_timer();
+
__get_cpu_var(irq_stat).idle_timestamp = jiffies;
idle();
}
_

I have tested this patch on my Laptop (P4) that HZ goes down to ~25 with
dyn-ticks enabled (but Power consumption goes _up_ as Ted had noted earlier
- I need to try some of the ACPI patches that were pointed out in the thread).

--

Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017