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 precise division via ktime_us_delta.
Reported-by: Anton Blanchard <an...@samba.org>
Bisected-by: Shilpasri G Bhat <shilpa.b...@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shre...@linux.vnet.ibm.com>
---
Changes in v5
=============
- Replacing approximation division with ktime_us_delta.
Changes in v4
=============
- Increasing the threshold upto which approximation can be used.
- Removed explicit cast. Instead added a comment saying why cast
is safe.
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 | 12 ++++--------
1 file changed, 4 insertions(+), 8 deletions(-)
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index a4d0059..c73207a 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -173,7 +173,7 @@ 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;
+ ktime_t time_start, time_end;
s64 diff;
/*
@@ -195,13 +195,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev,
struct cpuidle_driver *drv,
sched_idle_set_state(target_state);
trace_cpu_idle_rcuidle(index, dev->cpu);
- time_start = local_clock();
+ time_start = ns_to_ktime(local_clock());
stop_critical_timings();
entered_state = target_state->enter(dev, drv, index);
start_critical_timings();
- time_end = local_clock();
+ time_end = ns_to_ktime(local_clock());
trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
/* The cpu is no longer idle or about to enter idle. */
@@ -217,11 +217,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct
cpuidle_driver *drv,
if (!cpuidle_state_is_coupled(drv, index))
local_irq_enable();
- /*
- * local_clock() returns the time in nanosecond, let's shift
- * by 10 (divide by 1024) to have microsecond based time.
- */
- diff = (time_end - time_start) >> 10;
+ diff = ktime_us_delta(time_end, time_start);
if (diff > INT_MAX)
diff = INT_MAX;
--
2.1.4
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