Re: [PATCH] sparc: Use generic idle loop

Sam Ravnborg
2013-04-01T09:06:27Z

Hi Srivatsa,

thanks for the feedback!

@davem - I need you to look at this again. I am not sure about the changes
I do in sparc64_yield().

> > +/* the idle loop on a Sparc... ;) */
> > +void arch_cpu_idle(void)
> > {
> > + if (sparc_idle)
> > + (*sparc_idle)();
> > + else
> > + cpu_relax();
> > }
>
> You need to enable interrupts when coming out in the 'else' case,
> because we enter with interrupts disabled and expect to come out
> of arch_cpu_idle() with interrupts enabled. (Otherwise you'll hit
> that WARN_ON() in the generic code). Thomas has handled a similar
> case in the mips patch.

Something like this should do it then:

/* Idle loop support */
void arch_cpu_idle(void)
{
if (sparc_idle)
(*sparc_idle)();
else
local_irq_enable();
}

I dropped cpu_relax() as this is a simple barrier() which is
part of the generic code (using rmb()).

> > -static void sparc64_yield(int cpu)
> > +/* The idle loop on sparc64. */
> > +void arch_cpu_idle_enter()
> > {
> > + int cpu;
> > +
> > + cpu = smp_processor_id();
> > +
> > if (tlb_type != hypervisor) {
> > touch_nmi_watchdog();
> > return;
> > @@ -88,31 +93,10 @@ static void sparc64_yield(int cpu)
> > set_thread_flag(TIF_POLLING_NRFLAG);
> > }
> >
>
> Hmm, this looks weird. The contents of this function should have
> been inside arch_cpu_idle() (not arch_cpu_idle_enter).
Yep - I see now.

> And it is
> unnecessary to set/clear POLLING flags here, since they are handled
> in the generic code.
OK

> (Same is the case with the need_resched and
> the cpu_is_offline check).
There are some comments about race-conditions in the commit where they were added.
I think the out-most pair can be dropped as this is done in generic code but
that the inner ones needs to stay.

I simplified arch_cpu_idle() - former sparc64_yield() based on your comments.
- dropped TIF_POLLING_NRFLAG handling
- dropped smp_mb__after_clear_bit() - this is done in generic code using rmb()
- dropped the out-most while (!need_resched() && !cpu_is_offline(cpu))
- always enable irq on exit (but I am not sure if this is correct)

But I need davem to have a look at this - as I am fooling around in
code that I do not know much about..
So this is *NOT* to be applied.

Sam

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 3d361f2..ee5eacc 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -37,6 +37,7 @@ config SPARC
select GENERIC_SMP_IDLE_THREAD
select GENERIC_CMOS_UPDATE
select GENERIC_CLOCKEVENTS
+ select GENERIC_IDLE_LOOP
select GENERIC_STRNCPY_FROM_USER
select GENERIC_STRNLEN_USER
select MODULES_USE_ELF_RELA
diff --git a/arch/sparc/kernel/hvtramp.S b/arch/sparc/kernel/hvtramp.S
index 9365432..605c960 100644
--- a/arch/sparc/kernel/hvtramp.S
+++ b/arch/sparc/kernel/hvtramp.S
@@ -128,8 +128,7 @@ hv_cpu_startup:

call smp_callin
nop
- call cpu_idle
- mov 0, %o0
+
call cpu_panic
nop

diff --git a/arch/sparc/kernel/process_32.c b/arch/sparc/kernel/process_32.c
index 62eede1..890d7c4 100644
--- a/arch/sparc/kernel/process_32.c
+++ b/arch/sparc/kernel/process_32.c
@@ -64,23 +64,13 @@ 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];

-/*
- * the idle loop on a Sparc... ;)
- */
-void cpu_idle(void)
+/* Idle loop support. */
+void arch_cpu_idle(void)
{
- set_thread_flag(TIF_POLLING_NRFLAG);
-
- /* endless idle loop with no priority at all */
- for (;;) {
- while (!need_resched()) {
- if (sparc_idle)
- (*sparc_idle)();
- else
- cpu_relax();
- }
- schedule_preempt_disabled();
- }
+ if (sparc_idle)
+ (*sparc_idle)();
+ else
+ local_irq_enable();
}

/* XXX cli/sti -> local_irq_xxx here, check this works once SMP is fixed. */
diff --git a/arch/sparc/kernel/process_64.c b/arch/sparc/kernel/process_64.c
index cdb80b2..cecb0d6 100644
--- a/arch/sparc/kernel/process_64.c
+++ b/arch/sparc/kernel/process_64.c
@@ -52,17 +52,12 @@

#include "kstack.h"

-static void sparc64_yield(int cpu)
+/* Idle loop support on sparc64. */
+void arch_cpu_idle(void)
{
if (tlb_type != hypervisor) {
touch_nmi_watchdog();
- return;
- }
-
- clear_thread_flag(TIF_POLLING_NRFLAG);
- smp_mb__after_clear_bit();
-
- while (!need_resched() && !cpu_is_offline(cpu)) {
+ } else {
unsigned long pstate;

/* Disable interrupts. */
@@ -73,7 +68,7 @@ static void sparc64_yield(int cpu)
: "=&r" (pstate)
: "i" (PSTATE_IE));

- if (!need_resched() && !cpu_is_offline(cpu))
+ if (!need_resched() && !cpu_is_offline(smp_processor_id()))
sun4v_cpu_yield();

/* Re-enable interrupts. */
@@ -84,36 +79,16 @@ static void sparc64_yield(int cpu)
: "=&r" (pstate)
: "i" (PSTATE_IE));
}
-
- set_thread_flag(TIF_POLLING_NRFLAG);
+ local_irq_enable();
}

-/* The idle loop on sparc64. */
-void cpu_idle(void)
-{
- int cpu = smp_processor_id();
-
- set_thread_flag(TIF_POLLING_NRFLAG);
-
- while(1) {
- tick_nohz_idle_enter();
- rcu_idle_enter();
-
- while (!need_resched() && !cpu_is_offline(cpu))
- sparc64_yield(cpu);
-
- rcu_idle_exit();
- tick_nohz_idle_exit();
-
#ifdef CONFIG_HOTPLUG_CPU
- if (cpu_is_offline(cpu)) {
- sched_preempt_enable_no_resched();
- cpu_play_dead();
- }
-#endif
- schedule_preempt_disabled();
- }
+void arch_cpu_idle_dead()
+{
+ sched_preempt_enable_no_resched();
+ cpu_play_dead();
}
+#endif

#ifdef CONFIG_COMPAT
static void show_regwindow32(struct pt_regs *regs)
diff --git a/arch/sparc/kernel/smp_32.c b/arch/sparc/kernel/smp_32.c
index 9e7e6d7..e3f2b81 100644
--- a/arch/sparc/kernel/smp_32.c
+++ b/arch/sparc/kernel/smp_32.c
@@ -369,7 +369,7 @@ void __cpuinit sparc_start_secondary(void *arg)
local_irq_enable();

wmb();
- cpu_idle();
+ cpu_startup_entry(CPUHP_ONLINE);

/* We should never reach here! */
BUG();
diff --git a/arch/sparc/kernel/smp_64.c b/arch/sparc/kernel/smp_64.c
index 537eb66..c025ffc 100644
--- a/arch/sparc/kernel/smp_64.c
+++ b/arch/sparc/kernel/smp_64.c
@@ -127,6 +127,8 @@ void __cpuinit smp_callin(void)

/* idle thread is expected to have preempt disabled */
preempt_disable();
+
+ cpu_startup_entry(CPUHP_ONLINE);
}

void cpu_panic(void)
diff --git a/arch/sparc/kernel/trampoline_64.S b/arch/sparc/kernel/trampoline_64.S
index da1b781..2e973a2 100644
--- a/arch/sparc/kernel/trampoline_64.S
+++ b/arch/sparc/kernel/trampoline_64.S
@@ -407,8 +407,7 @@ after_lock_tlb:

call smp_callin
nop
- call cpu_idle
- mov 0, %o0
+
call cpu_panic
nop
1: b,a,pt %xcc, 1b