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MFi386: revision 225936

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Add some improvements in the idle table callbacks:
  - Replace instances of manual assembly instruction "hlt" call
    with halt() function calling.
  - In cpu_idle_mwait() avoid races in check to sched_runnable() using
    the same pattern used in cpu_idle_hlt() with the 'hlt' instruction.
  - Add comments explaining the logic behind the pattern used in
    cpu_idle_hlt() and other idle callbacks.
This commit is contained in:
Yoshihiro Takahashi 2011-10-04 13:24:22 +00:00
parent 5166b1fbe8
commit 09713cafdf
1 changed files with 34 additions and 7 deletions
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41
sys/pc98/pc98/machdep.c
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@ -1117,7 +1117,7 @@ void
cpu_halt(void)
{
for (;;)
__asm__ ("hlt");
halt();
}
static int idle_mwait = 1; /* Use MONITOR/MWAIT for short idle. */
@ -1136,9 +1136,22 @@ cpu_idle_hlt(int busy)
state = (int *)PCPU_PTR(monitorbuf);
*state = STATE_SLEEPING;
/*
* We must absolutely guarentee that hlt is the next instruction
* after sti or we introduce a timing window.
* Since we may be in a critical section from cpu_idle(), if
* an interrupt fires during that critical section we may have
* a pending preemption. If the CPU halts, then that thread
* may not execute until a later interrupt awakens the CPU.
* To handle this race, check for a runnable thread after
* disabling interrupts and immediately return if one is
* found. Also, we must absolutely guarentee that hlt is
* the next instruction after sti. This ensures that any
* interrupt that fires after the call to disable_intr() will
* immediately awaken the CPU from hlt. Finally, please note
* that on x86 this works fine because of interrupts enabled only
* after the instruction following sti takes place, while IF is set
* to 1 immediately, allowing hlt instruction to acknowledge the
* interrupt.
*/
disable_intr();
if (sched_runnable())
@ -1164,11 +1177,19 @@ cpu_idle_mwait(int busy)
state = (int *)PCPU_PTR(monitorbuf);
*state = STATE_MWAIT;
if (!sched_runnable()) {
cpu_monitor(state, 0, 0);
if (*state == STATE_MWAIT)
cpu_mwait(0, MWAIT_C1);
/* See comments in cpu_idle_hlt(). */
disable_intr();
if (sched_runnable()) {
enable_intr();
*state = STATE_RUNNING;
return;
}
cpu_monitor(state, 0, 0);
if (*state == STATE_MWAIT)
__asm __volatile("sti; mwait" : : "a" (MWAIT_C1), "c" (0));
else
enable_intr();
*state = STATE_RUNNING;
}
@ -1180,6 +1201,12 @@ cpu_idle_spin(int busy)
state = (int *)PCPU_PTR(monitorbuf);
*state = STATE_RUNNING;
/*
* The sched_runnable() call is racy but as long as there is
* a loop missing it one time will have just a little impact if any
* (and it is much better than missing the check at all).
*/
for (i = 0; i < 1000; i++) {
if (sched_runnable())
return;