Re: [PATCH v3] cpuidle: Fix last_residency division
On 06/29/2016 08:31 PM, Nicolas Pitre wrote:
> On Wed, 29 Jun 2016, Daniel Lezcano wrote:
>
>> On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:
>>> 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.
>
>>> + 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
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Re: [PATCH v3] cpuidle: Fix last_residency division
On Wed, 29 Jun 2016, Nicolas Pitre wrote:
> On Wed, 29 Jun 2016, Daniel Lezcano wrote:
>
> > On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:
> > > 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.
>
> > > + 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.
>
> > > + 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) <= 0x
> usec + usec/32 <= 0x
> usec <= (0x * 32) / 33
>
> Therefore:
>
> nsec <= 0xf83e0f82
And to be sure, you should use 0xf83e0f82UL to avoid any potential sign
extension.
Nicolas
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Re: [PATCH v3] cpuidle: Fix last_residency division
On Wed, 29 Jun 2016, Daniel Lezcano wrote:
> On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote:
> > 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.
> > + 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.
> > + 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) <= 0x
usec + usec/32 <= 0x
usec <= (0x * 32) / 33
Therefore:
nsec <= 0xf83e0f82
This is much better than INT_MAX/2.
> > + } else {
> > + u64 usec = div_u64(nsec, 1000);
> > +
> > + if (usec > INT_MAX)
> > + usec = INT_MAX;
> > + return (int)usec;
> > + }
> > +}
>
>
>
> --
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Re: [PATCH v3] cpuidle: Fix last_residency division
>>
>> +/*
>> + * 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.
>
>> +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
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Re: [PATCH v3] cpuidle: Fix last_residency division
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
Bisected-by: Shilpasri G Bhat
Suggested-by David Laight
Signed-off-by: Shreyas B. Prabhu
[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 = >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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