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> --- 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 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev