[PATCH] sched: resched and cpu_idle rework

Make some changes to the NEED_RESCHED and POLLING_NRFLAG to reduce
confusion, and make their semantics rigid.  Improves efficiency of
resched_task and some cpu_idle routines.

* In resched_task:
- TIF_NEED_RESCHED is only cleared with the task's runqueue lock held,
  and as we hold it during resched_task, then there is no need for an
  atomic test and set there. The only other time this should be set is
  when the task's quantum expires, in the timer interrupt - this is
  protected against because the rq lock is irq-safe.

- If TIF_NEED_RESCHED is set, then we don't need to do anything. It
  won't get unset until the task get's schedule()d off.

- If we are running on the same CPU as the task we resched, then set
  TIF_NEED_RESCHED and no further action is required.

- If we are running on another CPU, and TIF_POLLING_NRFLAG is *not* set
  after TIF_NEED_RESCHED has been set, then we need to send an IPI.

Using these rules, we are able to remove the test and set operation in
resched_task, and make clear the previously vague semantics of
POLLING_NRFLAG.

* In idle routines:
- Enter cpu_idle with preempt disabled. When the need_resched() condition
  becomes true, explicitly call schedule(). This makes things a bit clearer
  (IMO), but haven't updated all architectures yet.

- Many do a test and clear of TIF_NEED_RESCHED for some reason. According
  to the resched_task rules, this isn't needed (and actually breaks the
  assumption that TIF_NEED_RESCHED is only cleared with the runqueue lock
  held). So remove that. Generally one less locked memory op when switching
  to the idle thread.

- Many idle routines clear TIF_POLLING_NRFLAG, and only set it in the inner
  most polling idle loops. The above resched_task semantics allow it to be
  set until before the last time need_resched() is checked before going into
  a halt requiring interrupt wakeup.

  Many idle routines simply never enter such a halt, and so POLLING_NRFLAG
  can be always left set, completely eliminating resched IPIs when rescheduling
  the idle task.

  POLLING_NRFLAG width can be increased, to reduce the chance of resched IPIs.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Con Kolivas <kernel@kolivas.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

20 files changed, 296 insertions, 220 deletions

diff --git a/Documentation/sched-arch.txt b/Documentation/sched-arch.txt
new file mode 100644
index 000000000000..941615a9769b
--- /dev/null
+++ b/Documentation/sched-arch.txt
@@ -0,0 +1,89 @@
+        CPU Scheduler implementation hints for architecture specific code
+
+        Nick Piggin, 2005
+
+Context switch
+==============
+1. Runqueue locking
+By default, the switch_to arch function is called with the runqueue
+locked. This is usually not a problem unless switch_to may need to
+take the runqueue lock. This is usually due to a wake up operation in
+the context switch. See include/asm-ia64/system.h for an example.
+
+To request the scheduler call switch_to with the runqueue unlocked,
+you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
+(typically the one where switch_to is defined).
+
+Unlocked context switches introduce only a very minor performance
+penalty to the core scheduler implementation in the CONFIG_SMP case.
+
+2. Interrupt status
+By default, the switch_to arch function is called with interrupts
+disabled. Interrupts may be enabled over the call if it is likely to
+introduce a significant interrupt latency by adding the line
+`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for
+unlocked context switches. This define also implies
+`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an
+example.
+
+
+CPU idle
+========
+Your cpu_idle routines need to obey the following rules:
+
+1. Preempt should now disabled over idle routines. Should only
+   be enabled to call schedule() then disabled again.
+
+2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
+   be cleared until the running task has called schedule(). Idle
+   threads need only ever query need_resched, and may never set or
+   clear it.
+
+3. When cpu_idle finds (need_resched() == 'true'), it should call
+   schedule(). It should not call schedule() otherwise.
+
+4. The only time interrupts need to be disabled when checking
+   need_resched is if we are about to sleep the processor until
+   the next interrupt (this doesn't provide any protection of
+   need_resched, it prevents losing an interrupt).
+
+        4a. Common problem with this type of sleep appears to be:
+                local_irq_disable();
+                if (!need_resched()) {
+                        local_irq_enable();
+                        *** resched interrupt arrives here ***
+                        __asm__("sleep until next interrupt");
+                }
+
+5. TIF_POLLING_NRFLAG can be set by idle routines that do not
+   need an interrupt to wake them up when need_resched goes high.
+   In other words, they must be periodically polling need_resched,
+   although it may be reasonable to do some background work or enter
+   a low CPU priority.
+
+        5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
+            an interrupt sleep, it needs to be cleared then a memory
+            barrier issued (followed by a test of need_resched with
+            interrupts disabled, as explained in 3).
+
+arch/i386/kernel/process.c has examples of both polling and
+sleeping idle functions.
+
+
+Possible arch/ problems
+=======================
+
+Possible arch problems I found (and either tried to fix or didn't):
+
+h8300 - Is such sleeping racy vs interrupts? (See #4a).
+        The H8/300 manual I found indicates yes, however disabling IRQs
+        over the sleep mean only NMIs can wake it up, so can't fix easily
+        without doing spin waiting.
+
+ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
+
+sh64 - Is sleeping racy vs interrupts? (See #4a)
+
+sparc - IRQs on at this point(?), change local_irq_save to _disable.
+      - TODO: needs secondary CPUs to disable preempt (See #1)
+
diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
index eb20c3afff58..a8682612abc0 100644
--- a/arch/alpha/kernel/process.c
+++ b/arch/alpha/kernel/process.c
@@ -43,21 +43,17 @@
 #include "proto.h"
 #include "pci_impl.h"
 
-void default_idle(void)
-{
-        barrier();
-}
-
 void
 cpu_idle(void)
 {
+        set_thread_flag(TIF_POLLING_NRFLAG);
+
         while (1) {
-                void (*idle)(void) = default_idle;
                 /* FIXME -- EV6 and LCA45 know how to power down
                    the CPU.  */
 
                 while (!need_resched())
-                        idle();
+                        cpu_relax();
                 schedule();
         }
 }
diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 93dd92cc12f8..c0f6a119de3b 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -86,12 +86,16 @@ EXPORT_SYMBOL(pm_power_off);
  */
 void default_idle(void)
 {
-        local_irq_disable();
-        if (!need_resched() && !hlt_counter) {
-                timer_dyn_reprogram();
-                arch_idle();
+        if (hlt_counter)
+                cpu_relax();
+        else {
+                local_irq_disable();
+                if (!need_resched()) {
+                        timer_dyn_reprogram();
+                        arch_idle();
+                }
+                local_irq_enable();
         }
-        local_irq_enable();
 }
 
 /*
diff --git a/arch/i386/kernel/apm.c b/arch/i386/kernel/apm.c
index 86e80c551478..003548b8735f 100644
--- a/arch/i386/kernel/apm.c
+++ b/arch/i386/kernel/apm.c
@@ -769,8 +769,26 @@ static int set_system_power_state(u_short state)
 static int apm_do_idle(void)
 {
         u32     eax;
+        u8      ret = 0;
+        int     idled = 0;
+        int     polling;
+
+        polling = test_thread_flag(TIF_POLLING_NRFLAG);
+        if (polling) {
+                clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
+        }
+        if (!need_resched()) {
+                idled = 1;
+                ret = apm_bios_call_simple(APM_FUNC_IDLE, 0, 0, &eax);
+        }
+        if (polling)
+                set_thread_flag(TIF_POLLING_NRFLAG);
+
+        if (!idled)
+                return 0;
 
-        if (apm_bios_call_simple(APM_FUNC_IDLE, 0, 0, &eax)) {
+        if (ret) {
                 static unsigned long t;
 
                 /* This always fails on some SMP boards running UP kernels.
diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 5296e284ea36..1cb261f225d5 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -99,14 +99,22 @@ EXPORT_SYMBOL(enable_hlt);
  */
 void default_idle(void)
 {
+        local_irq_enable();
+
         if (!hlt_counter && boot_cpu_data.hlt_works_ok) {
-                local_irq_disable();
-                if (!need_resched())
-                        safe_halt();
-                else
-                        local_irq_enable();
+                clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
+                while (!need_resched()) {
+                        local_irq_disable();
+                        if (!need_resched())
+                                safe_halt();
+                        else
+                                local_irq_enable();
+                }
+                set_thread_flag(TIF_POLLING_NRFLAG);
         } else {
-                cpu_relax();
+                while (!need_resched())
+                        cpu_relax();
         }
 }
 #ifdef CONFIG_APM_MODULE
@@ -120,29 +128,14 @@ EXPORT_SYMBOL(default_idle);
  */
 static void poll_idle (void)
 {
-        int oldval;
-
         local_irq_enable();
 
-        /*
-         * Deal with another CPU just having chosen a thread to
-         * run here:
-         */
-        oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-        if (!oldval) {
-                set_thread_flag(TIF_POLLING_NRFLAG);
-                asm volatile(
-                        "2:"
-                        "testl %0, %1;"
-                        "rep; nop;"
-                        "je 2b;"
-                        : : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags));
-
-                clear_thread_flag(TIF_POLLING_NRFLAG);
-        } else {
-                set_need_resched();
-        }
+        asm volatile(
+                "2:"
+                "testl %0, %1;"
+                "rep; nop;"
+                "je 2b;"
+                : : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags));
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -181,6 +174,8 @@ void cpu_idle(void)
 {
         int cpu = smp_processor_id();
 
+        set_thread_flag(TIF_POLLING_NRFLAG);
+
         /* endless idle loop with no priority at all */
         while (1) {
                 while (!need_resched()) {
@@ -246,15 +241,12 @@ static void mwait_idle(void)
 {
         local_irq_enable();
 
-        if (!need_resched()) {
-                set_thread_flag(TIF_POLLING_NRFLAG);
-                do {
-                        __monitor((void *)&current_thread_info()->flags, 0, 0);
-                        if (need_resched())
-                                break;
-                        __mwait(0, 0);
-                } while (!need_resched());
-                clear_thread_flag(TIF_POLLING_NRFLAG);
+        while (!need_resched()) {
+                __monitor((void *)&current_thread_info()->flags, 0, 0);
+                smp_mb();
+                if (need_resched())
+                        break;
+                __mwait(0, 0);
         }
 }
 
diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index 4c621fc3c3b9..640d6908f8ec 100644
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -197,11 +197,15 @@ void
 default_idle (void)
 {
         local_irq_enable();
-        while (!need_resched())
-                if (can_do_pal_halt)
-                        safe_halt();
-                else
+        while (!need_resched()) {
+                if (can_do_pal_halt) {
+                        local_irq_disable();
+                        if (!need_resched())
+                                safe_halt();
+                        local_irq_enable();
+                } else
                         cpu_relax();
+        }
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -263,16 +267,16 @@ void __attribute__((noreturn))
 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 (!need_resched()) {
+                        void (*idle)(void);
 #ifdef CONFIG_SMP
-                if (!need_resched())
                         min_xtp();
 #endif
-                while (!need_resched()) {
-                        void (*idle)(void);
-
                         if (__get_cpu_var(cpu_idle_state))
                                 __get_cpu_var(cpu_idle_state) = 0;
 
@@ -284,19 +288,17 @@ cpu_idle (void)
                         if (!idle)
                                 idle = default_idle;
                         (*idle)();
-                }
-
-                if (mark_idle)
-                        (*mark_idle)(0);
-
+                        if (mark_idle)
+                                (*mark_idle)(0);
 #ifdef CONFIG_SMP
-                normal_xtp();
+                        normal_xtp();
 #endif
+                }
                 preempt_enable_no_resched();
                 schedule();
                 preempt_disable();
                 check_pgt_cache();
-                if (cpu_is_offline(smp_processor_id()))
+                if (cpu_is_offline(cpu))
                         play_dead();
         }
 }
diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
index f482f78de435..fee4f1f09adc 100644
--- a/arch/parisc/kernel/process.c
+++ b/arch/parisc/kernel/process.c
@@ -88,6 +88,8 @@ void default_idle(void)
  */
 void cpu_idle(void)
 {
+        set_thread_flag(TIF_POLLING_NRFLAG);
+
         /* endless idle loop with no priority at all */
         while (1) {
                 while (!need_resched())
diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
index 0130f2619dac..7f8f0cda6a74 100644
--- a/arch/powerpc/platforms/iseries/setup.c
+++ b/arch/powerpc/platforms/iseries/setup.c
@@ -703,13 +703,10 @@ static void iseries_shared_idle(void)
 static void iseries_dedicated_idle(void)
 {
         long oldval;
+        set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
-                oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-                if (!oldval) {
-                        set_thread_flag(TIF_POLLING_NRFLAG);
-
+                if (!need_resched()) {
                         while (!need_resched()) {
                                 ppc64_runlatch_off();
                                 HMT_low();
@@ -722,9 +719,6 @@ static void iseries_dedicated_idle(void)
                         }
 
                         HMT_medium();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
-                } else {
-                        set_need_resched();
                 }
 
                 ppc64_runlatch_on();
diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
index 4854f5eb5c3d..a093a0d4dd69 100644
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -469,6 +469,7 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
                  * more.
                  */
                 clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
 
                 /*
                  * SMT dynamic mode. Cede will result in this thread going
@@ -481,6 +482,7 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
                         cede_processor();
                 else
                         local_irq_enable();
+                set_thread_flag(TIF_POLLING_NRFLAG);
         } else {
                 /*
                  * Give the HV an opportunity at the processor, since we are
@@ -492,11 +494,11 @@ static inline void dedicated_idle_sleep(unsigned int cpu)
 
 static void pseries_dedicated_idle(void)
 { 
-        long oldval;
         struct paca_struct *lpaca = get_paca();
         unsigned int cpu = smp_processor_id();
         unsigned long start_snooze;
         unsigned long *smt_snooze_delay = &__get_cpu_var(smt_snooze_delay);
+        set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
                 /*
@@ -505,10 +507,7 @@ static void pseries_dedicated_idle(void)
                  */
                 lpaca->lppaca.idle = 1;
 
-                oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-                if (!oldval) {
-                        set_thread_flag(TIF_POLLING_NRFLAG);
-
+                if (!need_resched()) {
                         start_snooze = __get_tb() +
                                 *smt_snooze_delay * tb_ticks_per_usec;
 
@@ -531,9 +530,6 @@ static void pseries_dedicated_idle(void)
                         }
 
                         HMT_medium();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
-                } else {
-                        set_need_resched();
                 }
 
                 lpaca->lppaca.idle = 0;
diff --git a/arch/ppc/kernel/idle.c b/arch/ppc/kernel/idle.c
index a6141f05c919..3c4e4cb61074 100644
--- a/arch/ppc/kernel/idle.c
+++ b/arch/ppc/kernel/idle.c
@@ -63,18 +63,18 @@ void cpu_idle(void)
         int cpu = smp_processor_id();
 
         for (;;) {
-                if (ppc_md.idle != NULL)
-                        ppc_md.idle();
-                else
-                        default_idle();
-                if (cpu_is_offline(cpu) && system_state == SYSTEM_RUNNING)
-                        cpu_die();
-                if (need_resched()) {
-                        preempt_enable_no_resched();
-                        schedule();
-                        preempt_disable();
+                while (need_resched()) {
+                        if (ppc_md.idle != NULL)
+                                ppc_md.idle();
+                        else
+                                default_idle();
                 }
 
+                if (cpu_is_offline(cpu) && system_state == SYSTEM_RUNNING)
+                        cpu_die();
+                preempt_enable_no_resched();
+                schedule();
+                preempt_disable();
         }
 }
 
diff --git a/arch/ppc64/kernel/idle.c b/arch/ppc64/kernel/idle.c
index 909ea669af91..715bc0e71e0f 100644
--- a/arch/ppc64/kernel/idle.c
+++ b/arch/ppc64/kernel/idle.c
@@ -34,15 +34,11 @@ extern void power4_idle(void);
 
 void default_idle(void)
 {
-        long oldval;
         unsigned int cpu = smp_processor_id();
+        set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
-                oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-                if (!oldval) {
-                        set_thread_flag(TIF_POLLING_NRFLAG);
-
+                if (!need_resched()) {
                         while (!need_resched() && !cpu_is_offline(cpu)) {
                                 ppc64_runlatch_off();
 
@@ -55,9 +51,6 @@ void default_idle(void)
                         }
 
                         HMT_medium();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
-                } else {
-                        set_need_resched();
                 }
 
                 ppc64_runlatch_on();
diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
index 66ca5757e368..78b64fe5e7c2 100644
--- a/arch/s390/kernel/process.c
+++ b/arch/s390/kernel/process.c
@@ -99,14 +99,15 @@ void default_idle(void)
 {
         int cpu, rc;
 
+        /* CPU is going idle. */
+        cpu = smp_processor_id();
+
         local_irq_disable();
-        if (need_resched()) {
+        if (need_resched()) {
                 local_irq_enable();
-                return;
-        }
+                return;
+        }
 
-        /* CPU is going idle. */
-        cpu = smp_processor_id();
         rc = notifier_call_chain(&idle_chain, CPU_IDLE, (void *)(long) cpu);
         if (rc != NOTIFY_OK && rc != NOTIFY_DONE)
                 BUG();
@@ -119,7 +120,7 @@ void default_idle(void)
         __ctl_set_bit(8, 15);
 
 #ifdef CONFIG_HOTPLUG_CPU
-        if (cpu_is_offline(smp_processor_id()))
+        if (cpu_is_offline(cpu))
                 cpu_die();
 #endif
 
diff --git a/arch/sh/kernel/process.c b/arch/sh/kernel/process.c
index 1cbc26b796ad..fd4f240b833d 100644
--- a/arch/sh/kernel/process.c
+++ b/arch/sh/kernel/process.c
@@ -51,14 +51,13 @@ void enable_hlt(void)
 
 EXPORT_SYMBOL(enable_hlt);
 
-void default_idle(void)
+void cpu_idle(void)
 {
         /* endless idle loop with no priority at all */
         while (1) {
                 if (hlt_counter) {
-                        while (1)
-                                if (need_resched())
-                                        break;
+                        while (!need_resched())
+                                cpu_relax();
                 } else {
                         while (!need_resched())
                                 cpu_sleep();
@@ -70,11 +69,6 @@ void default_idle(void)
         }
 }
 
-void cpu_idle(void)
-{
-        default_idle();
-}
-
 void machine_restart(char * __unused)
 {
         /* SR.BL=1 and invoke address error to let CPU reset (manual reset) */
diff --git a/arch/sh64/kernel/process.c b/arch/sh64/kernel/process.c
index 0c09537449b3..b95d04141855 100644
--- a/arch/sh64/kernel/process.c
+++ b/arch/sh64/kernel/process.c
@@ -307,23 +307,19 @@ __setup("hlt", hlt_setup);
 
 static inline void hlt(void)
 {
-        if (hlt_counter)
-                return;
-
         __asm__ __volatile__ ("sleep" : : : "memory");
 }
 
 /*
  * The idle loop on a uniprocessor SH..
  */
-void default_idle(void)
+void cpu_idle(void)
 {
         /* endless idle loop with no priority at all */
         while (1) {
                 if (hlt_counter) {
-                        while (1)
-                                if (need_resched())
-                                        break;
+                        while (!need_resched())
+                                cpu_relax();
                 } else {
                         local_irq_disable();
                         while (!need_resched()) {
@@ -338,11 +334,7 @@ void default_idle(void)
                 schedule();
                 preempt_disable();
         }
-}
 
-void cpu_idle(void)
-{
-        default_idle();
 }
 
 void machine_restart(char * __unused)
diff --git a/arch/sparc/kernel/process.c b/arch/sparc/kernel/process.c
index c39f4d01096d..ea8647411462 100644
--- a/arch/sparc/kernel/process.c
+++ b/arch/sparc/kernel/process.c
@@ -67,13 +67,6 @@ extern void fpsave(unsigned long *, unsigned long *, void *, unsigned long *);
 struct task_struct *last_task_used_math = NULL;
 struct thread_info *current_set[NR_CPUS];
 
-/*
- * default_idle is new in 2.5. XXX Review, currently stolen from sparc64.
- */
-void default_idle(void)
-{
-}
-
 #ifndef CONFIG_SMP
 
 #define SUN4C_FAULT_HIGH 100
@@ -92,12 +85,11 @@ void cpu_idle(void)
                         static unsigned long fps;
                         unsigned long now;
                         unsigned long faults;
-                        unsigned long flags;
 
                         extern unsigned long sun4c_kernel_faults;
                         extern void sun4c_grow_kernel_ring(void);
 
-                        local_irq_save(flags);
+                        local_irq_disable();
                         now = jiffies;
                         count -= (now - last_jiffies);
                         last_jiffies = now;
@@ -113,13 +105,16 @@ void cpu_idle(void)
                                         sun4c_grow_kernel_ring();
                                 }
                         }
-                        local_irq_restore(flags);
+                        local_irq_enable();
                 }
 
-                while((!need_resched()) && pm_idle) {
-                        (*pm_idle)();
+                if (pm_idle) {
+                        while (!need_resched())
+                                (*pm_idle)();
+                } else {
+                        while (!need_resched())
+                                cpu_relax();
                 }
-
                 preempt_enable_no_resched();
                 schedule();
                 preempt_disable();
@@ -132,15 +127,15 @@ void cpu_idle(void)
 /* This is being executed in task 0 'user space'. */
 void cpu_idle(void)
 {
+        set_thread_flag(TIF_POLLING_NRFLAG);
         /* endless idle loop with no priority at all */
         while(1) {
-                if(need_resched()) {
-                        preempt_enable_no_resched();
-                        schedule();
-                        preempt_disable();
-                        check_pgt_cache();
-                }
-                barrier(); /* or else gcc optimizes... */
+                while (!need_resched())
+                        cpu_relax();
+                preempt_enable_no_resched();
+                schedule();
+                preempt_disable();
+                check_pgt_cache();
         }
 }
 
diff --git a/arch/sparc64/kernel/process.c b/arch/sparc64/kernel/process.c
index 2f89206e008f..02f9dec1d459 100644
--- a/arch/sparc64/kernel/process.c
+++ b/arch/sparc64/kernel/process.c
@@ -85,23 +85,31 @@ void cpu_idle(void)
 
 /*
  * the idle loop on a UltraMultiPenguin...
+ *
+ * TIF_POLLING_NRFLAG is set because we do not sleep the cpu
+ * inside of the idler task, so an interrupt is not needed
+ * to get a clean fast response.
+ *
+ * XXX Reverify this assumption... -DaveM
+ *
+ * Addendum: We do want it to do something for the signal
+ *           delivery case, we detect that by just seeing
+ *           if we are trying to send this to an idler or not.
  */
-#define idle_me_harder()        (cpu_data(smp_processor_id()).idle_volume += 1)
-#define unidle_me()             (cpu_data(smp_processor_id()).idle_volume = 0)
 void cpu_idle(void)
 {
+        cpuinfo_sparc *cpuinfo = &local_cpu_data();
         set_thread_flag(TIF_POLLING_NRFLAG);
+
         while(1) {
                 if (need_resched()) {
-                        unidle_me();
-                        clear_thread_flag(TIF_POLLING_NRFLAG);
+                        cpuinfo->idle_volume = 0;
                         preempt_enable_no_resched();
                         schedule();
                         preempt_disable();
-                        set_thread_flag(TIF_POLLING_NRFLAG);
                         check_pgt_cache();
                 }
-                idle_me_harder();
+                cpuinfo->idle_volume++;
 
                 /* The store ordering is so that IRQ handlers on
                  * other cpus see our increasing idleness for the buddy
diff --git a/arch/sparc64/kernel/smp.c b/arch/sparc64/kernel/smp.c
index 8aca4b1dc04e..797a65493fb8 100644
--- a/arch/sparc64/kernel/smp.c
+++ b/arch/sparc64/kernel/smp.c
@@ -1152,20 +1152,9 @@ void __init smp_cpus_done(unsigned int max_cpus)
                (bogosum/(5000/HZ))%100);
 }
 
-/* This needn't do anything as we do not sleep the cpu
- * inside of the idler task, so an interrupt is not needed
- * to get a clean fast response.
- *
- * XXX Reverify this assumption... -DaveM
- *
- * Addendum: We do want it to do something for the signal
- *           delivery case, we detect that by just seeing
- *           if we are trying to send this to an idler or not.
- */
 void smp_send_reschedule(int cpu)
 {
-        if (cpu_data(cpu).idle_volume == 0)
-                smp_receive_signal(cpu);
+        smp_receive_signal(cpu);
 }
 
 /* This is a nop because we capture all other cpus
diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 571f9fe490ce..59be85d9a4bc 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -86,12 +86,22 @@ EXPORT_SYMBOL(enable_hlt);
  */
 void default_idle(void)
 {
+        local_irq_enable();
+
         if (!atomic_read(&hlt_counter)) {
-                local_irq_disable();
-                if (!need_resched())
-                        safe_halt();
-                else
-                        local_irq_enable();
+                clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
+                while (!need_resched()) {
+                        local_irq_disable();
+                        if (!need_resched())
+                                safe_halt();
+                        else
+                                local_irq_enable();
+                }
+                set_thread_flag(TIF_POLLING_NRFLAG);
+        } else {
+                while (!need_resched())
+                        cpu_relax();
         }
 }
 
@@ -102,30 +112,16 @@ void default_idle(void)
  */
 static void poll_idle (void)
 {
-        int oldval;
-
         local_irq_enable();
 
-        /*
-         * Deal with another CPU just having chosen a thread to
-         * run here:
-         */
-        oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-        if (!oldval) {
-                set_thread_flag(TIF_POLLING_NRFLAG); 
-                asm volatile(
-                        "2:"
-                        "testl %0,%1;"
-                        "rep; nop;"
-                        "je 2b;"
-                        : :
-                        "i" (_TIF_NEED_RESCHED), 
-                        "m" (current_thread_info()->flags));
-                clear_thread_flag(TIF_POLLING_NRFLAG);
-        } else {
-                set_need_resched();
-        }
+        asm volatile(
+                "2:"
+                "testl %0,%1;"
+                "rep; nop;"
+                "je 2b;"
+                : :
+                "i" (_TIF_NEED_RESCHED),
+                "m" (current_thread_info()->flags));
 }
 
 void cpu_idle_wait(void)
@@ -187,6 +183,8 @@ static inline void play_dead(void)
  */
 void cpu_idle (void)
 {
+        set_thread_flag(TIF_POLLING_NRFLAG);
+
         /* endless idle loop with no priority at all */
         while (1) {
                 while (!need_resched()) {
@@ -221,15 +219,12 @@ static void mwait_idle(void)
 {
         local_irq_enable();
 
-        if (!need_resched()) {
-                set_thread_flag(TIF_POLLING_NRFLAG);
-                do {
-                        __monitor((void *)&current_thread_info()->flags, 0, 0);
-                        if (need_resched())
-                                break;
-                        __mwait(0, 0);
-                } while (!need_resched());
-                clear_thread_flag(TIF_POLLING_NRFLAG);
+        while (!need_resched()) {
+                __monitor((void *)&current_thread_info()->flags, 0, 0);
+                smp_mb();
+                if (need_resched())
+                        break;
+                __mwait(0, 0);
         }
 }
 
diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index 161db4acfb91..573b6a97bb1f 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -167,6 +167,19 @@ acpi_processor_power_activate(struct acpi_processor *pr,
         return;
 }
 
+static void acpi_safe_halt(void)
+{
+        int polling = test_thread_flag(TIF_POLLING_NRFLAG);
+        if (polling) {
+                clear_thread_flag(TIF_POLLING_NRFLAG);
+                smp_mb__after_clear_bit();
+        }
+        if (!need_resched())
+                safe_halt();
+        if (polling)
+                set_thread_flag(TIF_POLLING_NRFLAG);
+}
+
 static atomic_t c3_cpu_count;
 
 static void acpi_processor_idle(void)
@@ -177,7 +190,7 @@ static void acpi_processor_idle(void)
         int sleep_ticks = 0;
         u32 t1, t2 = 0;
 
-        pr = processors[raw_smp_processor_id()];
+        pr = processors[smp_processor_id()];
         if (!pr)
                 return;
 
@@ -197,8 +210,13 @@ static void acpi_processor_idle(void)
         }
 
         cx = pr->power.state;
-        if (!cx)
-                goto easy_out;
+        if (!cx) {
+                if (pm_idle_save)
+                        pm_idle_save();
+                else
+                        acpi_safe_halt();
+                return;
+        }
 
         /*
          * Check BM Activity
@@ -278,7 +296,8 @@ static void acpi_processor_idle(void)
                 if (pm_idle_save)
                         pm_idle_save();
                 else
-                        safe_halt();
+                        acpi_safe_halt();
+
                 /*
                  * TBD: Can't get time duration while in C1, as resumes
                  *      go to an ISR rather than here.  Need to instrument
@@ -414,16 +433,6 @@ static void acpi_processor_idle(void)
          */
         if (next_state != pr->power.state)
                 acpi_processor_power_activate(pr, next_state);
-
-        return;
-
-      easy_out:
-        /* do C1 instead of busy loop */
-        if (pm_idle_save)
-                pm_idle_save();
-        else
-                safe_halt();
-        return;
 }
 
 static int acpi_processor_set_power_policy(struct acpi_processor *pr)
diff --git a/kernel/sched.c b/kernel/sched.c
index 0f2def822296..ac3f5cc3bb51 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -864,21 +864,28 @@ static void deactivate_task(struct task_struct *p, runqueue_t *rq)
 #ifdef CONFIG_SMP
 static void resched_task(task_t *p)
 {
-        int need_resched, nrpolling;
+        int cpu;
 
         assert_spin_locked(&task_rq(p)->lock);
 
-        /* minimise the chance of sending an interrupt to poll_idle() */
-        nrpolling = test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
-        need_resched = test_and_set_tsk_thread_flag(p,TIF_NEED_RESCHED);
-        nrpolling |= test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
+        if (unlikely(test_tsk_thread_flag(p, TIF_NEED_RESCHED)))
+                return;
+
+        set_tsk_thread_flag(p, TIF_NEED_RESCHED);
+
+        cpu = task_cpu(p);
+        if (cpu == smp_processor_id())
+                return;
 
-        if (!need_resched && !nrpolling && (task_cpu(p) != smp_processor_id()))
-                smp_send_reschedule(task_cpu(p));
+        /* NEED_RESCHED must be visible before we test POLLING_NRFLAG */
+        smp_mb();
+        if (!test_tsk_thread_flag(p, TIF_POLLING_NRFLAG))
+                smp_send_reschedule(cpu);
 }
 #else
 static inline void resched_task(task_t *p)
 {
+        assert_spin_locked(&task_rq(p)->lock);
         set_tsk_need_resched(p);
 }
 #endif