intel_idle: enable interrupts before C1 on Xeons

Enable local interrupts before requesting C1 on the last two generations
of Intel Xeon platforms: Sky Lake, Cascade Lake, Cooper Lake, Ice Lake.
This decreases average C1 interrupt latency by about 5-10%, as measured
with the 'wult' tool.

The '->enter()' function of the driver enters C-states with local
interrupts disabled by executing the 'monitor' and 'mwait' pair of
instructions. If an interrupt happens, the CPU exits the C-state and
continues executing instructions after 'mwait'. It does not jump to
the interrupt handler, because local interrupts are disabled. The
cpuidle subsystem enables interrupts a bit later, after doing some
housekeeping.

With this patch, we enable local interrupts before requesting C1. In
this case, if the CPU wakes up because of an interrupt, it will jump
to the interrupt handler right away. The cpuidle housekeeping will be
done after the pending interrupt(s) are handled.

Enabling interrupts before entering a C-state has measurable impact
for faster C-states, like C1. Deeper, but slower C-states like C6 do
not really benefit from this sort of change, because their latency is
a lot higher comparing to the delay added by cpuidle housekeeping.

This change was also tested with cyclictest and dbench. In case of Ice
Lake, the average cyclictest latency decreased by 5.1%, and the average
'dbench' throughput increased by about 0.8%. Both tests were run for 4
hours with only C1 enabled (all other idle states, including 'POLL',
were disabled). CPU frequency was pinned to HFM, and uncore frequency
was pinned to the maximum value. The other platforms had similar
single-digit percentage improvements.

It is worth noting that this patch affects 'cpuidle' statistics a tiny
bit.  Before this patch, C1 residency did not include the interrupt
handling time, but with this patch, it will include it. This is similar
to what happens in case of the 'POLL' state, which also runs with
interrupts enabled.
Suggested-by: Len Brown <len.brown@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/idle/intel_idle.c

@@ -88,6 +88,12 @@ static struct cpuidle_state *cpuidle_state_table __initdata;
 
 static unsigned int mwait_substates __initdata;
 
+/*
+ * Enable interrupts before entering the C-state. On some platforms and for
+ * some C-states, this may measurably decrease interrupt latency.
+ */
+#define CPUIDLE_FLAG_IRQ_ENABLE		BIT(14)
+
 /*
  * Enable this state by default even if the ACPI _CST does not list it.
  */
@@ -127,6 +133,9 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
 	unsigned long eax = flg2MWAIT(state->flags);
 	unsigned long ecx = 1; /* break on interrupt flag */
 
+	if (state->flags & CPUIDLE_FLAG_IRQ_ENABLE)
+		local_irq_enable();
+
 	mwait_idle_with_hints(eax, ecx);
 
 	return index;
@@ -698,7 +707,7 @@ static struct cpuidle_state skx_cstates[] __initdata = {
 	{
 		.name = "C1",
 		.desc = "MWAIT 0x00",
-		.flags = MWAIT2flg(0x00),
+		.flags = MWAIT2flg(0x00) | CPUIDLE_FLAG_IRQ_ENABLE,
 		.exit_latency = 2,
 		.target_residency = 2,
 		.enter = &intel_idle,
@@ -727,7 +736,7 @@ static struct cpuidle_state icx_cstates[] __initdata = {
 	{
 		.name = "C1",
 		.desc = "MWAIT 0x00",
-		.flags = MWAIT2flg(0x00),
+		.flags = MWAIT2flg(0x00) | CPUIDLE_FLAG_IRQ_ENABLE,
 		.exit_latency = 1,
 		.target_residency = 1,
 		.enter = &intel_idle,