[RFC][PATCH 3/3] cpuidle: count double the exit latency

[Rik van Riel <riel@redhat.com>]

In some workloads, for example web servers talking to database servers,
a task on one CPU will wake up a task on another CPU, wait for that
other CPU to handle the job, and get the result back.  Each of those
wakeups is likely to incur the c-state exit latency.

By only counting the exit latency once, even though the round trip
involves two c-state exits, we can end up going into too deep a
c-state for the workload.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 drivers/cpuidle/governors/menu.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 47b1150..ccdb348 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -378,10 +378,13 @@ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 	/*
 	 * We correct for the exit latency; we are assuming here that the
 	 * exit latency happens after the event that we're interested in.
+	 * Pessimistically assume that we got woken up by another CPU,
+	 * so we count its exit latency, too.
 	 */
-	if (measured_us > data->exit_us)
-		measured_us -= data->exit_us;
-
+	if (measured_us > 2 * data->exit_us)
+		measured_us -= 2 * data->exit_us;
+	else
+		measured_us = 0;
 
 	/* update our correction ratio */