[Xen-devel] [PATCH v2] cpuidle: Improve perf for certain workloads

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

The existing mechanism of using interrupt frequency as a heuristic does
not work well for certain workloads.  As an example, synchronous dd on a
small block size uses deep C-states because much of the time is spent
doing processing so the interrupt frequency is not too high, but when an
IOP is submitted, the interrupt occurs soon after going idle.  This
causes exit latency to be a significant factor.

To fix this, add a new factor which limits the exit latency to be no
more than 10% of the decaying measured idle time.  This improves
performance for workloads with a medium interrupt frequency but a short
idle duration.

In the workload given previously, throughput improves by 20% with this
patch.

This is not ported from the Linux menu governor since that uses load
average and number of IO wait processes to satisfy latency constraints.
If a process is in IO wait state, it compares the exit latency with the
predicted residency reduced by a factor of 10, which is somewhat similar
to what this patch does.

A side effect of this patch is to correctly limit the maximum idle time
used in the correction factor calculation. Previously data->measured_us
was used, and it was never set.

Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
---
 xen/arch/x86/acpi/cpuidle_menu.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/acpi/cpuidle_menu.c b/xen/arch/x86/acpi/cpuidle_menu.c
index 6952776..4afaa8d 100644
--- a/xen/arch/x86/acpi/cpuidle_menu.c
+++ b/xen/arch/x86/acpi/cpuidle_menu.c
@@ -36,6 +36,7 @@
 #define RESOLUTION 1024
 #define DECAY 4
 #define MAX_INTERESTING 50000
+#define LATENCY_MULTIPLIER 10
 
 /*
  * Concepts and ideas behind the menu governor
@@ -88,6 +89,9 @@
  * the average interrupt interval is, the smaller C state latency should be
  * and thus the less likely a busy CPU will hit such a deep C state.
  *
+ * As an additional rule to reduce the performance impact, menu tries to
+ * limit the exit latency duration to be no more than 10% of the decaying
+ * measured idle time.
  */
 
 struct perf_factor{
@@ -102,6 +106,7 @@ struct menu_device
     int             last_state_idx;
     unsigned int    expected_us;
     u64             predicted_us;
+    u64             latency_factor;
     unsigned int    measured_us;
     unsigned int    exit_us;
     unsigned int    bucket;
@@ -199,6 +204,10 @@ static int menu_select(struct acpi_processor_power *power)
 
     io_interval = avg_intr_interval_us();
 
+    data->latency_factor = DIV_ROUND(
+            data->latency_factor * (DECAY - 1) + data->measured_us,
+            DECAY);
+
     /*
      * if the correction factor is 0 (eg first time init or cpu hotplug
      * etc), we actually want to start out with a unity factor.
@@ -220,6 +229,8 @@ static int menu_select(struct acpi_processor_power *power)
             break;
         if (s->latency * IO_MULTIPLIER > io_interval)
             break;
+        if (s->latency * LATENCY_MULTIPLIER > data->latency_factor)
+            break;
         /* TBD: we need to check the QoS requirment in future */
         data->exit_us = s->latency;
         data->last_state_idx = i;
@@ -231,18 +242,16 @@ static int menu_select(struct acpi_processor_power *power)
 static void menu_reflect(struct acpi_processor_power *power)
 {
     struct menu_device *data = &__get_cpu_var(menu_devices);
-    unsigned int last_idle_us = power->last_residency;
-    unsigned int measured_us;
     u64 new_factor;
 
-    measured_us = last_idle_us;
+    data->measured_us = power->last_residency;
 
     /*
      * We correct for the exit latency; we are assuming here that the
      * exit latency happens after the event that we're interested in.
      */
-    if (measured_us > data->exit_us)
-        measured_us -= data->exit_us;
+    if (data->measured_us > data->exit_us)
+        data->measured_us -= data->exit_us;
 
     /* update our correction ratio */
 
@@ -250,7 +259,7 @@ static void menu_reflect(struct acpi_processor_power *power)
         * (DECAY - 1) / DECAY;
 
     if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
-        new_factor += RESOLUTION * measured_us / data->expected_us;
+        new_factor += RESOLUTION * data->measured_us / data->expected_us;
     else
         /*
          * we were idle so long that we count it as a perfect
-- 
1.9.3


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