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 <an...@samba.org>
Signed-off-by: Shreyas B. Prabhu <shre...@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
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