On 18/05/2015 14:29, Andriy Gapon wrote:
> On 09/05/2015 15:28, Konstantin Belousov wrote:
>> +void
>> +acpi_cpu_idle_mwait(uint32_t mwait_hint)
>> +{
>> + int *state;
>> +
>> + state = (int *)PCPU_PTR(monitorbuf);
>> + /*
>> + * XXXKIB. Software coordination mode should be supported,
>> + * but all Intel CPUs provide hardware coordination.
>> + */
>> + cpu_monitor(state, 0, 0);
>> + cpu_mwait(MWAIT_INTRBREAK, mwait_hint);
>> +}
>> +
>
> Kostik,
>
> it's been a while since I studied this code, so please pardon me if I am
> asking
> something obvious or silly.
>
> I wonder why this function does not set 'state' before monitor + mwait.
> As far as I can see, all other idling functions do that. And
> cpu_idle_wakeup()
> compares the state to STATE_MWAIT before changing it.
>
> So, I am concerned that if the state happens to be anything other than
> STATE_MWAIT when acpi_cpu_idle_mwait() is called, then cpu_idle_wakeup() won't
> wake up the idled CPU. It seems that if the state is not STATE_SLEEPING then
> an
> IPI won't be sent either. Actually, that leaves STATE_RUNNING is the only
> problematic case, but that's probably the state that the CPU would have before
> idling.
>
After having written the above I realized what I overlooked:
acpi_cpu_idle_mwait() is called from the ACPI idle method, so the state must
already be set to STATE_SLEEPING.
So, looks like the wake-up would always be done by an IPI...
Just in case, here's what I had in my old local code:
void
acpi_cpu_mwait_cx(u_int hints)
{
int *state;
state = (int *)PCPU_PTR(monitorbuf);
KASSERT(*state == STATE_SLEEPING,
("cpu_mwait_cx: wrong monitorbuf state"));
*state = STATE_MWAIT;
cpu_monitor(state, 0, 0);
if (*state == STATE_MWAIT)
cpu_mwait(MWAIT_INTR_BRK, hints);
/*
* We should exit on any event that interrupts mwait,
* because that event might be a wanted interrupt.
*/
*state = STATE_RUNNING;
}
This code also accounted for a time window between the CPU wanting to go idle
and it calling mwait. During that window other CPU could want to wake up the
first CPU.
--
Andriy Gapon
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