[Xen-devel] [PATCH v2 2/5] x86/cpuidle: really use C1 for "urgent" CPUs

[Jan Beulich <JBeulich@xxxxxxxx>]

For one on recent AMD CPUs entering C1 (if available at all) requires
use of MWAIT, while HLT (i.e. default_idle()) would put the processor
into as deep as CC6. And then even on other vendors' CPUs we should
avoid entering default_idle() when the intended state can be reached
by using the active idle driver's facilities.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -582,12 +582,15 @@ static void acpi_processor_idle(void)
      u32 exp = 0, pred = 0;
      u32 irq_traced[4] = { 0 };
  
-    if ( max_cstate > 0 && power && !sched_has_urgent_vcpu() &&
+    if ( max_cstate > 0 && power &&
           (next_state = cpuidle_current_governor->select(power)) > 0 )
      {
+        unsigned int max_state = sched_has_urgent_vcpu() ? ACPI_STATE_C1
+                                                         : max_cstate;
+
          do {
              cx = &power->states[next_state];
-        } while ( cx->type > max_cstate && --next_state );
+        } while ( cx->type > max_state && --next_state );
          if ( next_state )
          {
              if ( cx->type == ACPI_STATE_C3 && power->flags.bm_check &&
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -724,11 +724,14 @@ static void mwait_idle(void)
        u64 before, after;
        u32 exp = 0, pred = 0, irq_traced[4] = { 0 };
  
-       if (max_cstate > 0 && power && !sched_has_urgent_vcpu() &&
+       if (max_cstate > 0 && power &&
            (next_state = cpuidle_current_governor->select(power)) > 0) {
+               unsigned int max_state = sched_has_urgent_vcpu() ? ACPI_STATE_C1
+                                                                : max_cstate;
+
                do {
                        cx = &power->states[next_state];
-               } while (cx->type > max_cstate && --next_state);
+               } while (cx->type > max_state && --next_state);
                if (!next_state)
                        cx = NULL;
                else if (tb_init_done)

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