[PATCH v3] cpuidle: Fix last_residency division

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 using a better approximation for division by 1000.

Reported-by: Anton Blanchard <redacted>
Bisected-by: Shilpasri G Bhat [off-list ref]
Suggested-by David Laight [off-list ref]
Signed-off-by: Shreyas B. Prabhu <redacted>
---
Changes in v3
=============
 - Using approximation suggested by David

Changes in v2
=============
 - Fixing it in the cpuidle core code instead of driver code.

 drivers/cpuidle/cpuidle.c | 11 +++--------
 drivers/cpuidle/cpuidle.h | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index a4d0059..e9a7f74 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c

@@ -174,7 +174,6 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 	struct cpuidle_state *target_state = &drv->states[index];
 	bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
 	u64 time_start, time_end;
-	s64 diff;
 
 	/*
 	 * Tell the time framework to switch to a broadcast timer because our

@@ -218,14 +217,10 @@ 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, convert it to
+	 * microsecond based time.
 	 */
-	diff = (time_end - time_start) >> 10;
-	if (diff > INT_MAX)
-		diff = INT_MAX;
-
-	dev->last_residency = (int) diff;
+	dev->last_residency = convert_nsec_to_usec(time_end - time_start);
 
 	if (entered_state >= 0) {
 		/* Update cpuidle counters */

diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
index f87f399..c8ea5ad 100644
--- a/drivers/cpuidle/cpuidle.h
+++ b/drivers/cpuidle/cpuidle.h

@@ -68,4 +68,27 @@ static inline void cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
 }
 #endif
 
+/*
+ * Used for calculating last_residency in usec. Optimized for case
+ * where last_residency in nsecs is < INT_MAX/2 by using faster
+ * approximation. Approximated value has less than 1% error.
+ */
+static inline int convert_nsec_to_usec(u64 nsec)
+{
+	if (likely(nsec < INT_MAX / 2)) {
+		int usec = (int)nsec;
+
+		usec += usec >> 5;
+		usec = usec >> 10;
+		return usec;
+	} else {
+		u64 usec = div_u64(nsec, 1000);
+
+		if (usec > INT_MAX)
+			usec = INT_MAX;
+		return (int)usec;
+	}
+}
+
+
 #endif /* __DRIVER_CPUIDLE_H */

-- 
2.1.4

On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:

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 using a better approximation for division by 1000.

Reported-by: Anton Blanchard <redacted>
Bisected-by: Shilpasri G Bhat [off-list ref]
Suggested-by David Laight [off-list ref]
Signed-off-by: Shreyas B. Prabhu <redacted>

[Cc'ed Nicolas Pitre]

19 more quoted lines

UINT_MAX ?

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 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

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quoted

+/*
+ * Used for calculating last_residency in usec. Optimized for case
+ * where last_residency in nsecs is < INT_MAX/2 by using faster
+ * approximation. Approximated value has less than 1% error.
+ */
+static inline int convert_nsec_to_usec(u64 nsec)
+{
+    if (likely(nsec < INT_MAX / 2)) {

UINT_MAX ?

I don't think I can use UINT_MAX here since usec += usec >> 5 can
overflow. Also using INT_MAX / 2 instead of INT_MAX since potentially
usec += usec >> 5 can be negative and usec >> 10 will retain the sign bit.

quoted

+        int usec = (int)nsec;
+
+        usec += usec >> 5;
+        usec = usec >> 10;
+        return usec;
+    } else {
+        u64 usec = div_u64(nsec, 1000);
+
+        if (usec > INT_MAX)
+            usec = INT_MAX;
+        return (int)usec;
+    }
+}

Thanks,
Shreyas

On Wed, 29 Jun 2016, Daniel Lezcano wrote:

On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:

quoted

diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
index f87f399..c8ea5ad 100644
--- a/drivers/cpuidle/cpuidle.h
+++ b/drivers/cpuidle/cpuidle.h

@@ -68,4 +68,27 @@ static inline void

cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
  }
  #endif

+/*
+ * Used for calculating last_residency in usec. Optimized for case
+ * where last_residency in nsecs is < INT_MAX/2 by using faster
+ * approximation. Approximated value has less than 1% error.
+ */
+static inline int convert_nsec_to_usec(u64 nsec)
+{
+	if (likely(nsec < INT_MAX / 2)) {

UINT_MAX ?

Actually this can be better than that.

quoted

+		int usec = (int)nsec;

First, you'll want an unsigned type. Given the provided argument is u64, 
we can assume there won't be any negative values here.

Then it would be wise to use a type with an explicit width, like U32.

quoted

+		usec += usec >> 5;
+		usec = usec >> 10;
+		return usec;

And now you want to maximize the available range. So not to overflow the 
first addition, we must respect:

	usec + (usec >> 5) <= 0xffffffff 
	usec + usec/32 <= 0xffffffff
	usec <= (0xffffffff * 32) / 33

Therefore:

	nsec <= 0xf83e0f82

This is much better than INT_MAX/2.

quoted

+	} else {
+		u64 usec = div_u64(nsec, 1000);
+
+		if (usec > INT_MAX)
+			usec = INT_MAX;
+		return (int)usec;
+	}
+}

-- 
 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

On Wed, 29 Jun 2016, Nicolas Pitre wrote:

On Wed, 29 Jun 2016, Daniel Lezcano wrote:

quoted

On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:

quoted

diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
index f87f399..c8ea5ad 100644
--- a/drivers/cpuidle/cpuidle.h
+++ b/drivers/cpuidle/cpuidle.h

@@ -68,4 +68,27 @@ static inline void

cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
  }
  #endif

+/*
+ * Used for calculating last_residency in usec. Optimized for case
+ * where last_residency in nsecs is < INT_MAX/2 by using faster
+ * approximation. Approximated value has less than 1% error.
+ */
+static inline int convert_nsec_to_usec(u64 nsec)
+{
+	if (likely(nsec < INT_MAX / 2)) {

UINT_MAX ?

Actually this can be better than that.

quoted

quoted

+		int usec = (int)nsec;

First, you'll want an unsigned type. Given the provided argument is u64, 
we can assume there won't be any negative values here.

Then it would be wise to use a type with an explicit width, like U32.

quoted

quoted

+		usec += usec >> 5;
+		usec = usec >> 10;
+		return usec;

And now you want to maximize the available range. So not to overflow the 
first addition, we must respect:

	usec + (usec >> 5) <= 0xffffffff 

6 more quoted lines

And to be sure, you should use 0xf83e0f82UL to avoid any potential sign 
extension.


Nicolas

On 06/29/2016 08:31 PM, Nicolas Pitre wrote:

On Wed, 29 Jun 2016, Daniel Lezcano wrote:

quoted

On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:

quoted

diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
index f87f399..c8ea5ad 100644
--- a/drivers/cpuidle/cpuidle.h
+++ b/drivers/cpuidle/cpuidle.h

@@ -68,4 +68,27 @@ static inline void

cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
  }
  #endif

+/*
+ * Used for calculating last_residency in usec. Optimized for case
+ * where last_residency in nsecs is < INT_MAX/2 by using faster
+ * approximation. Approximated value has less than 1% error.
+ */
+static inline int convert_nsec_to_usec(u64 nsec)
+{
+	if (likely(nsec < INT_MAX / 2)) {

UINT_MAX ?

Actually this can be better than that.

quoted

quoted

+		int usec = (int)nsec;

First, you'll want an unsigned type. Given the provided argument is u64, 
we can assume there won't be any negative values here.

Then it would be wise to use a type with an explicit width, like U32.

Cool. I wanted to avoid multiple casts. i.e u64 -> u32 -> int. But I
guess there is no real need to avoid it.

Sending v4 with your suggestions.

Thanks,
Shreyas