[PATCH v3 2/2] x86/mwait-idle: squash stats update when not actually entering C-state

[Jan Beulich <jbeulich@xxxxxxxx>]

While we don't want to skip calling update_idle_stats(), arrange for it
to not increment the overall time spent in the state we didn't really
enter.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: If we wanted to also move the tracing, then I think the part ahead
     of the if() also would need moving. At that point we could as well
     move update_last_cx_stat(), too, which afaict would allow skipping
     update_idle_stats() on the "else" path (which therefore would go
     away). Yet then, with the setting of power->safe_state moved up a
     little (which imo it should have been anyway) the two
     cpu_is_haltable() invocations would only have the lapic_timer_off()
     invocation left in between. This would then seem to call for simply
     ditching the 2nd one - acpi-idle also doesn't have a 2nd instance.
TBD: For the tracing I wonder if that really needs to come ahead of the
     local_irq_enable(). Maybe trace_exit_reason() needs to, but quite
     certainly TRACE_6D() doesn't.
---
v3: Also move cstate_restore_tsc() invocation and split ones to
    update_idle_stats().
v2: New.

--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -854,17 +854,23 @@ static void mwait_idle(void)
                mwait_idle_with_hints(cx->address, MWAIT_ECX_INTERRUPT_BREAK);
 
                local_irq_disable();
-       }
 
-       after = alternative_call(cpuidle_get_tick);
+               after = alternative_call(cpuidle_get_tick);
+
+               cstate_restore_tsc();
+
+               /* Now back in C0. */
+               update_idle_stats(power, cx, before, after);
+       } else {
+               /* Never left C0. */
+               after = alternative_call(cpuidle_get_tick);
+               update_idle_stats(power, cx, after, after);
+       }
 
-       cstate_restore_tsc();
        trace_exit_reason(irq_traced);
        TRACE_6D(TRC_PM_IDLE_EXIT, cx->type, after,
                irq_traced[0], irq_traced[1], irq_traced[2], irq_traced[3]);
 
-       /* Now back in C0. */
-       update_idle_stats(power, cx, before, after);
        local_irq_enable();
 
        if (!(lapic_timer_reliable_states & (1 << cx->type)))