MFi386: revision 225936
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.
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svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=225977
@ -1117,7 +1117,7 @@ void
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cpu_halt(void)
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{
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for (;;)
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__asm__ ("hlt");
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halt();
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}
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static int idle_mwait = 1; /* Use MONITOR/MWAIT for short idle. */
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@ -1136,9 +1136,22 @@ cpu_idle_hlt(int busy)
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state = (int *)PCPU_PTR(monitorbuf);
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*state = STATE_SLEEPING;
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/*
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* We must absolutely guarentee that hlt is the next instruction
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* after sti or we introduce a timing window.
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* Since we may be in a critical section from cpu_idle(), if
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* an interrupt fires during that critical section we may have
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* a pending preemption. If the CPU halts, then that thread
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* may not execute until a later interrupt awakens the CPU.
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* To handle this race, check for a runnable thread after
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* disabling interrupts and immediately return if one is
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* found. Also, we must absolutely guarentee that hlt is
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* the next instruction after sti. This ensures that any
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* interrupt that fires after the call to disable_intr() will
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* immediately awaken the CPU from hlt. Finally, please note
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* that on x86 this works fine because of interrupts enabled only
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* after the instruction following sti takes place, while IF is set
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* to 1 immediately, allowing hlt instruction to acknowledge the
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* interrupt.
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*/
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disable_intr();
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if (sched_runnable())
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@ -1164,11 +1177,19 @@ cpu_idle_mwait(int busy)
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state = (int *)PCPU_PTR(monitorbuf);
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*state = STATE_MWAIT;
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if (!sched_runnable()) {
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cpu_monitor(state, 0, 0);
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if (*state == STATE_MWAIT)
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cpu_mwait(0, MWAIT_C1);
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/* See comments in cpu_idle_hlt(). */
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disable_intr();
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if (sched_runnable()) {
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enable_intr();
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*state = STATE_RUNNING;
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return;
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}
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cpu_monitor(state, 0, 0);
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if (*state == STATE_MWAIT)
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__asm __volatile("sti; mwait" : : "a" (MWAIT_C1), "c" (0));
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else
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enable_intr();
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*state = STATE_RUNNING;
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}
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@ -1180,6 +1201,12 @@ cpu_idle_spin(int busy)
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state = (int *)PCPU_PTR(monitorbuf);
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*state = STATE_RUNNING;
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/*
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* The sched_runnable() call is racy but as long as there is
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* a loop missing it one time will have just a little impact if any
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* (and it is much better than missing the check at all).
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*/
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for (i = 0; i < 1000; i++) {
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if (sched_runnable())
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return;
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