[PATCH] ia64: cpu_idle performance bug fix

Our performance validation on 2.6.15-rc1 caught a disastrous performance
regression on ia64 with netperf (-98%) and volanomark (-58%) compares to
previous kernel version 2.6.14-git7.  See the following chart (result
group 1 & 2).

  http://kernel-perf.sourceforge.net/results.machine_id=26.html

We have root caused it to commit 64c7c8f88559624abdbe12b5da6502e8879f8d28

This changeset broke the ia64 task resched notification.  In
sched.c:resched_task(), a reschedule IPI is conditioned upon
TIF_POLLING_NRFLAG.  However, the above changeset unconditionally set
the polling thread flag for idle tasks regardless whether pal_halt_light
is in use or not.  As a result, resched IPI is not sent from
resched_task().  And since the default behavior on ia64 is to use
pal_halt_light, we end up delaying the rescheduling task until next
timer tick, and thus cause the performance regression.

This fixes the performance bug.  I'm glad our performance suite is
turning up bad performance bug like this in time.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index e92ea64..4305d2b 100644 (file)
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -202,12 +202,9 @@ default_idle (void)
 {
        local_irq_enable();
        while (!need_resched()) {
-               if (can_do_pal_halt) {
-                       local_irq_disable();
-                       if (!need_resched())
-                               safe_halt();
-                       local_irq_enable();
-               } else
+               if (can_do_pal_halt)
+                       safe_halt();
+               else
                        cpu_relax();
        }
 }
@@ -272,10 +269,14 @@ cpu_idle (void)
 {
        void (*mark_idle)(int) = ia64_mark_idle;
        int cpu = smp_processor_id();
-       set_thread_flag(TIF_POLLING_NRFLAG);
 
        /* endless idle loop with no priority at all */
        while (1) {
+               if (can_do_pal_halt)
+                       clear_thread_flag(TIF_POLLING_NRFLAG);
+               else
+                       set_thread_flag(TIF_POLLING_NRFLAG);
+
                if (!need_resched()) {
                        void (*idle)(void);
 #ifdef CONFIG_SMP