On Mon, May 18, 2015 at 02:40:56PM +0300, Andriy Gapon wrote:
> 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.
Right. And there is very similar, but having some small important
difference, function cpu_idle_mwait(). My plan was to give the commit
some time for wider testing, then axe cpu_idle_mwait and corresponding
machdep.idle selector, with the move of the fast wakeup code under
acpi_cpu_idle_mwait(). Additional reason to wait was the use of
cpu_idle_mwait() as the 'fast' idle method for busy CPU, whatever this
means.
I think I would commit the patch below shortly, with 'Submitted by: avg'.
diff --git a/sys/x86/x86/cpu_machdep.c b/sys/x86/x86/cpu_machdep.c
index 7925713..f07b97e 100644
--- a/sys/x86/x86/cpu_machdep.c
+++ b/sys/x86/x86/cpu_machdep.c
@@ -101,6 +101,10 @@ __FBSDID("$FreeBSD$");
#include <vm/vm_pager.h>
#include <vm/vm_param.h>
+#define STATE_RUNNING 0x0
+#define STATE_MWAIT 0x1
+#define STATE_SLEEPING 0x2
+
/*
* Machine dependent boot() routine
*
@@ -134,13 +138,24 @@ 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.
*/
+
+ state = (int *)PCPU_PTR(monitorbuf);
+ KASSERT(*state == STATE_SLEEPING,
+ ("cpu_mwait_cx: wrong monitorbuf state"));
+ *state = STATE_MWAIT;
cpu_monitor(state, 0, 0);
- cpu_mwait(MWAIT_INTRBREAK, mwait_hint);
+ if (*state == STATE_MWAIT)
+ cpu_mwait(MWAIT_INTRBREAK, mwait_hint);
+
+ /*
+ * We should exit on any event that interrupts mwait, because
+ * that event might be a wanted interrupt.
+ */
+ *state = STATE_RUNNING;
}
/* Get current clock frequency for the given cpu id. */
@@ -231,10 +246,6 @@ static int idle_mwait = 1; /* Use MONITOR/MWAIT
for short idle. */
SYSCTL_INT(_machdep, OID_AUTO, idle_mwait, CTLFLAG_RWTUN, &idle_mwait,
0, "Use MONITOR/MWAIT for short idle");
-#define STATE_RUNNING 0x0
-#define STATE_MWAIT 0x1
-#define STATE_SLEEPING 0x2
-
#ifndef PC98
static void
cpu_idle_acpi(sbintime_t sbt)
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