| Message ID | 1467183971-12327-1-git-send-email-shreyas@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | Changes Requested, archived |
| Headers | show |
>> >> +/* >> + * 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 -- To unsubscribe from this list: send the line "unsubscribe linux-pm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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) <= 0xffffffff usec + usec/32 <= 0xffffffff usec <= (0xffffffff * 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; > > + } > > +} > > > > -- > <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: > > > 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) <= 0xffffffff > usec + usec/32 <= 0xffffffff > usec <= (0xffffffff * 32) / 33 > > Therefore: > > nsec <= 0xf83e0f82 And to be sure, you should use 0xf83e0f82UL to avoid any potential sign extension. Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-pm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 -- To unsubscribe from this list: send the line "unsubscribe linux-pm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
============= - 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 */
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 <anton@samba.org> Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com> Suggested-by David Laight <david.laight@aculab.com> Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com> --- Changes in v3