[PATCH] improve precision of idle time detection.

Increase the precision of the idle time calculation that is exported
to user space via /sys/devices/system/cpu/cpu<x>/idle_time_us

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 45 insertions, 22 deletions

diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
index 04f8c67a610..1e06436f07c 100644
--- a/arch/s390/kernel/process.c
+++ b/arch/s390/kernel/process.c
@@ -38,6 +38,7 @@
 #include <linux/utsname.h>
 #include <linux/tick.h>
 #include <linux/elfcore.h>
+#include <linux/kernel_stat.h>
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
 #include <asm/system.h>
@@ -79,30 +80,19 @@ DEFINE_PER_CPU(struct s390_idle_data, s390_idle) = {
         .lock = __SPIN_LOCK_UNLOCKED(s390_idle.lock)
 };
 
-static int s390_idle_enter(void)
+void s390_idle_leave(void)
 {
         struct s390_idle_data *idle;
+        unsigned long long idle_time;
 
         idle = &__get_cpu_var(s390_idle);
+        idle_time = S390_lowcore.int_clock - idle->idle_enter;
         spin_lock(&idle->lock);
+        idle->idle_time += idle_time;
+        idle->idle_enter = 0ULL;
         idle->idle_count++;
-        idle->in_idle = 1;
-        idle->idle_enter = get_clock();
         spin_unlock(&idle->lock);
-        vtime_stop_cpu_timer();
-        return NOTIFY_OK;
-}
-
-void s390_idle_leave(void)
-{
-        struct s390_idle_data *idle;
-
         vtime_start_cpu_timer();
-        idle = &__get_cpu_var(s390_idle);
-        spin_lock(&idle->lock);
-        idle->idle_time += get_clock() - idle->idle_enter;
-        idle->in_idle = 0;
-        spin_unlock(&idle->lock);
 }
 
 extern void s390_handle_mcck(void);
@@ -111,16 +101,16 @@ extern void s390_handle_mcck(void);
  */
 static void default_idle(void)
 {
+        struct s390_idle_data *idle = &__get_cpu_var(s390_idle);
+        unsigned long addr;
+        psw_t psw;
+
         /* CPU is going idle. */
         local_irq_disable();
         if (need_resched()) {
                 local_irq_enable();
                 return;
         }
-        if (s390_idle_enter() == NOTIFY_BAD) {
-                local_irq_enable();
-                return;
-        }
 #ifdef CONFIG_HOTPLUG_CPU
         if (cpu_is_offline(smp_processor_id())) {
                 preempt_enable_no_resched();
@@ -138,9 +128,42 @@ static void default_idle(void)
         trace_hardirqs_on();
         /* Don't trace preempt off for idle. */
         stop_critical_timings();
+        vtime_stop_cpu_timer();
+
+        /*
+         * The inline assembly is equivalent to
+         *      idle->idle_enter = get_clock();
+         *      __load_psw_mask(psw_kernel_bits | PSW_MASK_WAIT |
+         *                         PSW_MASK_IO | PSW_MASK_EXT);
+         * The difference is that the inline assembly makes sure that
+         * the stck instruction is right before the lpsw instruction.
+         * This is done to increase the precision.
+         */
+
         /* Wait for external, I/O or machine check interrupt. */
-        __load_psw_mask(psw_kernel_bits | PSW_MASK_WAIT |
-                        PSW_MASK_IO | PSW_MASK_EXT);
+        psw.mask = psw_kernel_bits|PSW_MASK_WAIT|PSW_MASK_IO|PSW_MASK_EXT;
+#ifndef __s390x__
+        asm volatile(
+                "       basr    %0,0\n"
+                "0:     ahi     %0,1f-0b\n"
+                "       st      %0,4(%2)\n"
+                "       stck    0(%3)\n"
+                "       lpsw    0(%2)\n"
+                "1:"
+                : "=&d" (addr), "=m" (idle->idle_enter)
+                : "a" (&psw), "a" (&idle->idle_enter), "m" (psw)
+                : "memory", "cc");
+#else /* __s390x__ */
+        asm volatile(
+                "       larl    %0,1f\n"
+                "       stg     %0,8(%2)\n"
+                "       stck    0(%3)\n"
+                "       lpswe   0(%2)\n"
+                "1:"
+                : "=&d" (addr), "=m" (idle->idle_enter)
+                : "a" (&psw), "a" (&idle->idle_enter), "m" (psw)
+                : "memory", "cc");
+#endif /* __s390x__ */
         start_critical_timings();
 }