While sending interrupts to a cpu to repeatedly wake a thread, on occasion that thread will take a full timer tick cycle (4002 usec in my case) to wakeup.
The problem concerns a race condition in the code around the safe_halt() call in the default_idle() routine. Setting 'nohalt' on the kernel command line causes the long wakeups to disappear.
void
default_idle (void)
{
local_irq_enable();
while (!need_resched()) {
--> if (can_do_pal_halt)
--> safe_halt();
else
A timer tick could arrive between the check for !need_resched and the actual call to safe_halt() (which does a pal call to PAL_HALT_LIGHT). By the time the timer tick completes, a thread that might now need to run could get held up for as long as a timer tick waiting for the halted cpu.
I'm proposing that we disable irq's and check need_resched again before calling safe_halt(). Does anyone see any problem with this approach?
Signed-off-by: Dimitri Sivanich <sivanich@xxxxxxx>
Index: linux/arch/ia64/kernel/process.c
===================================================================
--- linux.orig/arch/ia64/kernel/process.c 2007-08-02 15:05:56.427236082 -0500
+++ linux/arch/ia64/kernel/process.c 2007-08-06 19:42:20.147944967 -0500
@@ -198,9 +198,13 @@ default_idle (void)
{
local_irq_enable();
while (!need_resched()) {
- if (can_do_pal_halt)
- safe_halt();
- else
+ if (can_do_pal_halt) {
+ local_irq_disable();
+ if (!need_resched()) {
+ safe_halt();
+ }
+ local_irq_enable();
+ } else
cpu_relax();
}
}
-
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