[PATCH] cpuidle/powernv: Fix snooze timeout

[Shreyas B. Prabhu]

Snooze is a poll idle state in powernv and pseries platforms. Snooze
has a timeout so that if a cpu stays in snooze for more than target
residency of the next available idle state, then it would exit thereby
giving chance to the cpuidle governor to re-evaluate and
promote the cpu to a deeper idle state. Therefore whenever snooze exits
due to this timeout, its last_residency will be target_residency of next
deeper state.

commit e93e59ce5b85 ("cpuidle: Replace ktime_get() with local_clock()")
changed the math around last_residency calculation. Specifically, while
converting last_residency value from nanoseconds to microseconds it does
right shift by 10. Due to this, in snooze timeout exit scenarios
last_residency calculated is roughly 2.3% less than target_residency of
next available state. This pattern is picked up get_typical_interval()
in the menu governor and therefore expected_interval in menu_select() is
frequently less than the target_residency of any state but snooze.

Due to this we are entering snooze at a higher rate, thereby affecting
the single thread performance.
Since the math around last_residency is not meant to be precise, fix this
issue setting snooze timeout to 105% of target_residency of next
available idle state.

This also adds comment around why snooze timeout is necessary.

Reported-by: Anton Blanchard <anton at samba.org>
Signed-off-by: Shreyas B. Prabhu <shreyas at linux.vnet.ibm.com>
---
 drivers/cpuidle/cpuidle-powernv.c | 14 ++++++++++++++
 drivers/cpuidle/cpuidle-pseries.c | 13 +++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c
index e12dc30..5835491 100644
--- a/drivers/cpuidle/cpuidle-powernv.c
+++ b/drivers/cpuidle/cpuidle-powernv.c
@@ -268,10 +268,24 @@ static int powernv_idle_probe(void)
 		cpuidle_state_table = powernv_states;
 		/* Device tree can indicate more idle states */
 		max_idle_state = powernv_add_idle_states();
+
+		/*
+		 * Staying in snooze for a long period can degrade the
+		 * perfomance of the sibling cpus. Set timeout for snooze such
+		 * that if the cpu stays in snooze longer than target residency
+		 * of the next available idle state then exit from snooze. This
+		 * gives a chance to the cpuidle governor to re-evaluate and
+		 * promote it to deeper idle states.
+		 */
 		if (max_idle_state > 1) {
 			snooze_timeout_en = true;
 			snooze_timeout = powernv_states[1].target_residency *
 					 tb_ticks_per_usec;
+			/*
+			 * Give a 5% margin since target residency related math
+			 * is not precise in cpuidle core.
+			 */
+			snooze_timeout += snooze_timeout / 20;
 		}
  	} else
  		return -ENODEV;
diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index 07135e0..22de841 100644
--- a/drivers/cpuidle/cpuidle-pseries.c
+++ b/drivers/cpuidle/cpuidle-pseries.c
@@ -250,10 +250,23 @@ static int pseries_idle_probe(void)
 	} else
 		return -ENODEV;
 
+	/*
+	 * Staying in snooze for a long period can degrade the
+	 * perfomance of the sibling cpus. Set timeout for snooze such
+	 * that if the cpu stays in snooze longer than target residency
+	 * of the next available idle state then exit from snooze. This
+	 * gives a chance to the cpuidle governor to re-evaluate and
+	 * promote it to deeper idle states.
+	 */
 	if (max_idle_state > 1) {
 		snooze_timeout_en = true;
 		snooze_timeout = cpuidle_state_table[1].target_residency *
 				 tb_ticks_per_usec;
+		/*
+		 * Give a 5% margin since target residency related math
+		 * is not precise in cpuidle core.
+		 */
+		snooze_timeout += snooze_timeout / 20;
 	}
 	return 0;
 }
-- 
2.1.4