Re: [PATCH] x86: Simplify cpu_idle_wait

[Venki Pallipadi]

On Fri, Feb 08, 2008 at 11:28:48AM +0100, Andi Kleen wrote:
> 
> > -	set_cpus_allowed(current, tmp);
> > +	smp_mb();
> > +	/* kick all the CPUs so that they exit out of pm_idle */
> > +	smp_call_function(do_nothing, NULL, 0, 0);
> 
> I think the last argument (wait) needs to be 1 to make sure it is 
> synchronous (for 32/64) Otherwise the patch looks great.

Yes. Below is the updated patch


Earlier commit 40d6a146629b98d8e322b6f9332b182c7cbff3df
added smp_call_function in cpu_idle_wait() to kick cpus that are in tickless
idle. Looking at cpu_idle_wait code at that time, code seemed to be
over-engineered for a case which is rarely used (while changing idle handler).

Below is a simplified version of cpu_idle_wait, which just makes
a dummy smp_call_function to all cpus, to make them come out of old idle handler
and start using the new idle handler. It eliminates code in the idle loop
to handle cpu_idle_wait.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>

Index: linux-2.6.25-rc/arch/x86/kernel/process_32.c
===================================================================
--- linux-2.6.25-rc.orig/arch/x86/kernel/process_32.c
+++ linux-2.6.25-rc/arch/x86/kernel/process_32.c
@@ -82,7 +82,6 @@ unsigned long thread_saved_pc(struct tas
  */
 void (*pm_idle)(void);
 EXPORT_SYMBOL(pm_idle);
-static DEFINE_PER_CPU(unsigned int, cpu_idle_state);
 
 void disable_hlt(void)
 {
@@ -181,9 +180,6 @@ void cpu_idle(void)
 		while (!need_resched()) {
 			void (*idle)(void);
 
-			if (__get_cpu_var(cpu_idle_state))
-				__get_cpu_var(cpu_idle_state) = 0;
-
 			check_pgt_cache();
 			rmb();
 			idle = pm_idle;
@@ -208,40 +204,19 @@ static void do_nothing(void *unused)
 {
 }
 
+/*
+ * cpu_idle_wait - Used to ensure that all the CPUs discard old value of
+ * pm_idle and update to new pm_idle value. Required while changing pm_idle
+ * handler on SMP systems.
+ *
+ * Caller must have changed pm_idle to the new value before the call. Old
+ * pm_idle value will not be used by any CPU after the return of this function.
+ */
 void cpu_idle_wait(void)
 {
-	unsigned int cpu, this_cpu = get_cpu();
-	cpumask_t map, tmp = current->cpus_allowed;
-
-	set_cpus_allowed(current, cpumask_of_cpu(this_cpu));
-	put_cpu();
-
-	cpus_clear(map);
-	for_each_online_cpu(cpu) {
-		per_cpu(cpu_idle_state, cpu) = 1;
-		cpu_set(cpu, map);
-	}
-
-	__get_cpu_var(cpu_idle_state) = 0;
-
-	wmb();
-	do {
-		ssleep(1);
-		for_each_online_cpu(cpu) {
-			if (cpu_isset(cpu, map) && !per_cpu(cpu_idle_state, cpu))
-				cpu_clear(cpu, map);
-		}
-		cpus_and(map, map, cpu_online_map);
-		/*
-		 * We waited 1 sec, if a CPU still did not call idle
-		 * it may be because it is in idle and not waking up
-		 * because it has nothing to do.
-		 * Give all the remaining CPUS a kick.
-		 */
-		smp_call_function_mask(map, do_nothing, 0, 0);
-	} while (!cpus_empty(map));
-
-	set_cpus_allowed(current, tmp);
+	smp_mb();
+	/* kick all the CPUs so that they exit out of pm_idle */
+	smp_call_function(do_nothing, NULL, 0, 1);
 }
 EXPORT_SYMBOL_GPL(cpu_idle_wait);
 
Index: linux-2.6.25-rc/arch/x86/kernel/process_64.c
===================================================================
--- linux-2.6.25-rc.orig/arch/x86/kernel/process_64.c
+++ linux-2.6.25-rc/arch/x86/kernel/process_64.c
@@ -64,7 +64,6 @@ EXPORT_SYMBOL(boot_option_idle_override)
  */
 void (*pm_idle)(void);
 EXPORT_SYMBOL(pm_idle);
-static DEFINE_PER_CPU(unsigned int, cpu_idle_state);
 
 static ATOMIC_NOTIFIER_HEAD(idle_notifier);
 
@@ -139,41 +138,19 @@ static void do_nothing(void *unused)
 {
 }
 
+/*
+ * cpu_idle_wait - Used to ensure that all the CPUs discard old value of
+ * pm_idle and update to new pm_idle value. Required while changing pm_idle
+ * handler on SMP systems.
+ *
+ * Caller must have changed pm_idle to the new value before the call. Old
+ * pm_idle value will not be used by any CPU after the return of this function.
+ */
 void cpu_idle_wait(void)
 {
-	unsigned int cpu, this_cpu = get_cpu();
-	cpumask_t map, tmp = current->cpus_allowed;
-
-	set_cpus_allowed(current, cpumask_of_cpu(this_cpu));
-	put_cpu();
-
-	cpus_clear(map);
-	for_each_online_cpu(cpu) {
-		per_cpu(cpu_idle_state, cpu) = 1;
-		cpu_set(cpu, map);
-	}
-
-	__get_cpu_var(cpu_idle_state) = 0;
-
-	wmb();
-	do {
-		ssleep(1);
-		for_each_online_cpu(cpu) {
-			if (cpu_isset(cpu, map) &&
-					!per_cpu(cpu_idle_state, cpu))
-				cpu_clear(cpu, map);
-		}
-		cpus_and(map, map, cpu_online_map);
-		/*
-		 * We waited 1 sec, if a CPU still did not call idle
-		 * it may be because it is in idle and not waking up
-		 * because it has nothing to do.
-		 * Give all the remaining CPUS a kick.
-		 */
-		smp_call_function_mask(map, do_nothing, 0, 0);
-	} while (!cpus_empty(map));
-
-	set_cpus_allowed(current, tmp);
+	smp_mb();
+	/* kick all the CPUs so that they exit out of pm_idle */
+	smp_call_function(do_nothing, NULL, 0, 1);
 }
 EXPORT_SYMBOL_GPL(cpu_idle_wait);
 
@@ -215,9 +192,6 @@ void cpu_idle (void)
 		while (!need_resched()) {
 			void (*idle)(void);
 
-			if (__get_cpu_var(cpu_idle_state))
-				__get_cpu_var(cpu_idle_state) = 0;
-
 			tick_nohz_stop_sched_tick();
 
 			rmb();
--
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