From: "Shreyas B. Prabhu" <shreyas@linux.vnet.ibm.com>
To: rjw@rjwysocki.net
Cc: daniel.lezcano@linaro.org, linux-pm@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org, anton@samba.org,
mpe@ellerman.id.au, bsingharora@gmail.com,
"Shreyas B. Prabhu" <shreyas@linux.vnet.ibm.com>
Subject: [PATCH v2] cpuidle: Fix last_residency division
Date: Fri, 24 Jun 2016 03:23:58 -0500 [thread overview]
Message-ID: <1466756638-2362-1-git-send-email-shreyas@linux.vnet.ibm.com> (raw)
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.
Fix this by replacing right shift by 10 with /1000 while calculating
last_residency.
Reported-by: Anton Blanchard <anton@samba.org>
Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
---
Changes in v2
=============
- Fixing it in the cpuidle core code instead of driver code.
drivers/cpuidle/cpuidle.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index a4d0059..30d67a8 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -218,10 +218,10 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
local_irq_enable();
/*
- * local_clock() returns the time in nanosecond, let's shift
- * by 10 (divide by 1024) to have microsecond based time.
+ * local_clock() returns the time in nanosecond, let's
+ * divide by 1000 to have microsecond based time.
*/
- diff = (time_end - time_start) >> 10;
+ diff = (time_end - time_start) / 1000;
if (diff > INT_MAX)
diff = INT_MAX;
--
2.1.4
next reply other threads:[~2016-06-24 8:24 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-24 8:23 Shreyas B. Prabhu [this message]
2016-06-24 9:00 ` [PATCH v2] cpuidle: Fix last_residency division David Laight
2016-06-24 10:05 ` Daniel Lezcano
2016-06-24 10:11 ` Arnd Bergmann
2016-06-24 16:01 ` Shreyas B Prabhu
2016-06-24 19:43 ` Arnd Bergmann
2016-06-27 8:59 ` David Laight
2016-06-29 7:00 ` Shreyas B Prabhu
2016-06-24 9:30 ` kbuild test robot
2016-06-24 10:27 ` kbuild test robot
2016-06-24 12:10 ` kbuild test robot
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