Busy idle loop when profiling

- the Intel architecture cycle counter (performance counter) does not
  count when the CPU is idle therefore we use busy loop instead of
  halting the cpu when there is nothing to schedule

- the downside is that handling interrupts may be accounted as idle
  time if a sample is taken before we get out of the nested trap and
  pick a new process

kernel/arch/i386/arch_clock.c

@@ -11,6 +11,7 @@
 #include "kernel/interrupt.h"
 #include <minix/u64.h>
 #include "glo.h"
+#include "profile.h"
 
 
 #ifdef CONFIG_APIC
@@ -272,6 +273,9 @@ PUBLIC void context_stop_idle(void)
 
 	if (is_idle)
 		restart_local_timer();
+
+	if (sprofiling)
+		get_cpulocal_var(idle_interrupted) = 1;
 }
 
 PUBLIC u64_t ms_2_cpu_time(unsigned ms)

kernel/arch/i386/klib.S

@@ -1030,6 +1030,14 @@ ENTRY(read_ebp)
 	mov	%ebp, %eax
 	ret
 
+ENTRY(interrupts_enable)
+	sti
+	ret
+
+ENTRY(interrupts_disable)
+	cli
+	ret
+
 
 /*
  * void switch_k_stack(void * esp, void (* continuation)(void));

kernel/cpulocals.h

@@ -76,6 +76,9 @@ DECLARE_CPULOCAL(struct proc *, run_q_head[NR_SCHED_QUEUES]); /* ptrs to ready l
 DECLARE_CPULOCAL(struct proc *, run_q_tail[NR_SCHED_QUEUES]); /* ptrs to ready list tails */
 DECLARE_CPULOCAL(int, cpu_is_idle); /* let the others know that you are idle */
 
+DECLARE_CPULOCAL(volatile int, idle_interrupted); /* to interrupt busy-idle
+						     while profiling */
+
 DECLARE_CPULOCAL(u64_t ,tsc_ctr_switch); /* when did we switched time accounting */
 
 /* last values read from cpu when sending ooq msg to scheduler */

kernel/proc.c

@@ -43,6 +43,7 @@
 #include "vm.h"
 #include "clock.h"
 #include "spinlock.h"
+#include "profile.h"
 
 #include "arch_proto.h"
 
@@ -214,7 +215,18 @@ PRIVATE void idle(void)
 
 	/* start accounting for the idle time */
 	context_stop(proc_addr(KERNEL));
-	halt_cpu();
+	if (!sprofiling)
+		halt_cpu();
+	else {
+		volatile int * v;
+
+		v = get_cpulocal_var_ptr(idle_interrupted);
+		interrupts_enable();
+		while (!*v)
+			arch_pause();
+		interrupts_disable();
+		*v = 0;
+	}
 	/*
 	 * end of accounting for the idle task does not happen here, the kernel
 	 * is handling stuff for quite a while before it gets back here!

kernel/proto.h

@@ -103,6 +103,9 @@ _PROTOTYPE( void rm_irq_handler, (const irq_hook_t *hook)		);
 _PROTOTYPE( void enable_irq, (const irq_hook_t *hook)			);
 _PROTOTYPE( int disable_irq, (const irq_hook_t *hook)			);
 
+_PROTOTYPE(void interrupts_enable, (void));
+_PROTOTYPE(void interrupts_disable, (void));
+
 /* debug.c */
 _PROTOTYPE( int runqueues_ok, (void) );
 #ifndef CONFIG_SMP