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 <an...@samba.org>
Bisected-by: Shilpasri G Bhat <shilpa.b...@linux.vnet.ibm.com>
Suggested-by David Laight <david.lai...@aculab.com>
Signed-off-by: Shreyas B. Prabhu <shre...@linux.vnet.ibm.com>
[Cc'ed Nicolas Pitre]
---
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)) {
UINT_MAX ?
+ 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;
+ }
+}
--
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