Re: [Patch]cpuidle: Save current cpu as local variable instead of calling smp_processor_id() in loop

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 18, 2016 at 01:47:21PM +0200, Peter Zijlstra wrote:
> 
> Another optimization you could look at is removing that rmb(); I don't
> actually think its needed, but you'd need to find why it was added and
> then check it was in fact needed back then, and then check if it is in
> fact still needed now.
> 
> Its a bit of a trek through git history, but lfence / dsb ld are
> expensive instructions.
> 

OK; so I was curious ... :-)

the rmb() was introduced by the below commit (found in tglx/history.git).

which at the time ordered the cpu_idle_state vs pm_idle loads.

---
commit f2f1b44c75c478f541d87234c98606d7686c4209
Author: Zwane Mwaikambo <zwane@arm.linux.org.uk>
Date:   Tue Jan 4 05:35:06 2005 -0800

    [PATCH] Remove RCU abuse in cpu_idle()
    
    Introduce cpu_idle_wait() on architectures requiring modification of
    pm_idle from modules, this will ensure that all processors have updated
    their cached values of pm_idle upon exit.  This patch is to address the bug
    report at http://bugme.osdl.org/show_bug.cgi?id=1716 and replaces the
    current code fix which is in violation of normal RCU usage as pointed out
    by Stephen, Dipankar and Paul.
    
    Signed-off-by: Zwane Mwaikambo <zwane@arm.linux.org.uk>
    Signed-off-by: Andrew Morton <akpm@osdl.org>
    Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/i386/kernel/apm.c b/arch/i386/kernel/apm.c
index 904217f5361e..47712b5d5814 100644
--- a/arch/i386/kernel/apm.c
+++ b/arch/i386/kernel/apm.c
@@ -2369,7 +2369,7 @@ static void __exit apm_exit(void)
 		 * (pm_idle), Wait for all processors to update cached/local
 		 * copies of pm_idle before proceeding.
 		 */
-		synchronize_kernel();
+		cpu_idle_wait();
 	}
 	if (((apm_info.bios.flags & APM_BIOS_DISENGAGED) == 0)
 	    && (apm_info.connection_version > 0x0100)) {
diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 7efce8e4c376..7b79a9b6a04f 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -72,6 +72,7 @@ unsigned long thread_saved_pc(struct task_struct *tsk)
  * Powermanagement idle function, if any..
  */
 void (*pm_idle)(void);
+static cpumask_t cpu_idle_map;
 
 void disable_hlt(void)
 {
@@ -93,7 +94,7 @@ EXPORT_SYMBOL(enable_hlt);
  */
 void default_idle(void)
 {
-	if (!hlt_counter && current_cpu_data.hlt_works_ok) {
+	if (!hlt_counter && boot_cpu_data.hlt_works_ok) {
 		local_irq_disable();
 		if (!need_resched())
 			safe_halt();
@@ -144,29 +145,44 @@ static void poll_idle (void)
  */
 void cpu_idle (void)
 {
+	int cpu = smp_processor_id();
+
 	/* endless idle loop with no priority at all */
 	while (1) {
 		while (!need_resched()) {
 			void (*idle)(void);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
+
+			if (cpu_isset(cpu, cpu_idle_map))
+				cpu_clear(cpu, cpu_idle_map);
+			rmb();
 			idle = pm_idle;
 
 			if (!idle)
 				idle = default_idle;
 
-			irq_stat[smp_processor_id()].idle_timestamp = jiffies;
+			irq_stat[cpu].idle_timestamp = jiffies;
 			idle();
-			rcu_read_unlock();
 		}
 		schedule();
 	}
 }
 
+void cpu_idle_wait(void)
+{
+	int cpu;
+	cpumask_t map;
+
+	for_each_online_cpu(cpu)
+		cpu_set(cpu, cpu_idle_map);
+
+	wmb();
+	do {
+		ssleep(1);
+		cpus_and(map, cpu_idle_map, cpu_online_map);
+	} while (!cpus_empty(map));
+}
+EXPORT_SYMBOL_GPL(cpu_idle_wait);
+
 /*
  * This uses new MONITOR/MWAIT instructions on P4 processors with PNI,
  * which can obviate IPI to trigger checking of need_resched.
diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index 078075f7be3f..f96bfb005268 100644
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -25,6 +25,7 @@
 #include <linux/unistd.h>
 #include <linux/efi.h>
 #include <linux/interrupt.h>
+#include <linux/delay.h>
 
 #include <asm/cpu.h>
 #include <asm/delay.h>
@@ -46,6 +47,7 @@
 #include "sigframe.h"
 
 void (*ia64_mark_idle)(int);
+static cpumask_t cpu_idle_map;
 
 unsigned long boot_option_idle_override = 0;
 EXPORT_SYMBOL(boot_option_idle_override);
@@ -225,10 +227,28 @@ static inline void play_dead(void)
 }
 #endif /* CONFIG_HOTPLUG_CPU */
 
+
+void cpu_idle_wait(void)
+{
+        int cpu;
+        cpumask_t map;
+
+        for_each_online_cpu(cpu)
+                cpu_set(cpu, cpu_idle_map);
+
+        wmb();
+        do {
+                ssleep(1);
+                cpus_and(map, cpu_idle_map, cpu_online_map);
+        } while (!cpus_empty(map));
+}
+EXPORT_SYMBOL_GPL(cpu_idle_wait);
+
 void __attribute__((noreturn))
 cpu_idle (void *unused)
 {
 	void (*mark_idle)(int) = ia64_mark_idle;
+	int cpu = smp_processor_id();
 
 	/* endless idle loop with no priority at all */
 	while (1) {
@@ -241,17 +261,14 @@ cpu_idle (void *unused)
 
 			if (mark_idle)
 				(*mark_idle)(1);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
+
+			if (cpu_isset(cpu, cpu_idle_map))
+				cpu_clear(cpu, cpu_idle_map);
+			rmb();
 			idle = pm_idle;
 			if (!idle)
 				idle = default_idle;
 			(*idle)();
-			rcu_read_unlock();
 		}
 
 		if (mark_idle)
diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 1ce2e1f9a316..129a20d3888c 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -61,6 +61,7 @@ EXPORT_SYMBOL(boot_option_idle_override);
  * Powermanagement idle function, if any..
  */
 void (*pm_idle)(void);
+static cpumask_t cpu_idle_map;
 
 void disable_hlt(void)
 {
@@ -123,6 +124,23 @@ static void poll_idle (void)
 	}
 }
 
+
+void cpu_idle_wait(void)
+{
+        int cpu;
+        cpumask_t map;
+
+        for_each_online_cpu(cpu)
+                cpu_set(cpu, cpu_idle_map);
+
+        wmb();
+        do {
+                ssleep(1);
+                cpus_and(map, cpu_idle_map, cpu_online_map);
+        } while (!cpus_empty(map));
+}
+EXPORT_SYMBOL_GPL(cpu_idle_wait);
+
 /*
  * The idle thread. There's no useful work to be
  * done, so just try to conserve power and have a
@@ -131,21 +149,20 @@ static void poll_idle (void)
  */
 void cpu_idle (void)
 {
+	int cpu = smp_processor_id();
+
 	/* endless idle loop with no priority at all */
 	while (1) {
 		while (!need_resched()) {
 			void (*idle)(void);
-			/*
-			 * Mark this as an RCU critical section so that
-			 * synchronize_kernel() in the unload path waits
-			 * for our completion.
-			 */
-			rcu_read_lock();
+
+			if (cpu_isset(cpu, cpu_idle_map))
+				cpu_clear(cpu, cpu_idle_map);
+			rmb();
 			idle = pm_idle;
 			if (!idle)
 				idle = default_idle;
 			idle();
-			rcu_read_unlock();
 		}
 		schedule();
 	}
diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index d9e4433177d3..47eb7463c009 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -988,7 +988,7 @@ int acpi_processor_power_exit(struct acpi_processor *pr, struct acpi_device *dev
 		 * (pm_idle), Wait for all processors to update cached/local
 		 * copies of pm_idle before proceeding.
 		 */
-		synchronize_kernel();
+		cpu_idle_wait();
 	}
 
 	return_VALUE(0);
diff --git a/include/asm-i386/system.h b/include/asm-i386/system.h
index 9bb0f63ff64e..c705fa77b138 100644
--- a/include/asm-i386/system.h
+++ b/include/asm-i386/system.h
@@ -466,5 +466,6 @@ void disable_hlt(void);
 void enable_hlt(void);
 
 extern int es7000_plat;
+void cpu_idle_wait(void);
 
 #endif
diff --git a/include/asm-ia64/system.h b/include/asm-ia64/system.h
index fdefa16fabc4..f28b920e9089 100644
--- a/include/asm-ia64/system.h
+++ b/include/asm-ia64/system.h
@@ -284,6 +284,7 @@ do {						\
 
 #define ia64_platform_is(x) (strcmp(x, platform_name) == 0)
 
+void cpu_idle_wait(void);
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */
diff --git a/include/asm-x86_64/system.h b/include/asm-x86_64/system.h
index 55ac71fd4a60..05acc62c2b61 100644
--- a/include/asm-x86_64/system.h
+++ b/include/asm-x86_64/system.h
@@ -326,6 +326,8 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 /* For spinlocks etc */
 #define local_irq_save(x) 	do { warn_if_not_ulong(x); __asm__ __volatile__("# local_irq_save \n\t pushfq ; popq %0 ; cli":"=g" (x): /* no input */ :"memory"); } while (0)
 
+void cpu_idle_wait(void);
+
 /*
  * disable hlt during certain critical i/o operations
  */