From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-x229.google.com (mail-wm0-x229.google.com [IPv6:2a00:1450:400c:c09::229]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3rgN1w4Px0zDr30 for ; Fri, 1 Jul 2016 00:57:40 +1000 (AEST) Received: by mail-wm0-x229.google.com with SMTP id f126so225479626wma.1 for ; Thu, 30 Jun 2016 07:57:39 -0700 (PDT) Subject: Re: [PATCH v4] cpuidle: Fix last_residency division To: "Shreyas B. Prabhu" , rjw@rjwysocki.net References: <1467297253-2171-1-git-send-email-shreyas@linux.vnet.ibm.com> Cc: linux-pm@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, anton@samba.org, mpe@ellerman.id.au, bsingharora@gmail.com, David.Laight@ACULAB.COM, arnd@arndb.de, nicolas.pitre@linaro.org From: Daniel Lezcano Message-ID: <5775335E.2040003@linaro.org> Date: Thu, 30 Jun 2016 16:57:34 +0200 MIME-Version: 1.0 In-Reply-To: <1467297253-2171-1-git-send-email-shreyas@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 06/30/2016 04:34 PM, 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 > --- > 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 | 11 +++-------- > drivers/cpuidle/cpuidle.h | 38 ++++++++++++++++++++++++++++++++++++++ > 2 files changed, 41 insertions(+), 8 deletions(-) > > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c > index a4d0059..f55ad01 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 nanoseconds, 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..a027b35 100644 > --- a/drivers/cpuidle/cpuidle.h > +++ b/drivers/cpuidle/cpuidle.h > @@ -68,4 +68,42 @@ static inline void cpuidle_coupled_unregister_device(struct cpuidle_device *dev) > } > #endif > > +/* > + * To ensure that there is no overflow while approximation > + * for dividing val by 1000, we must respect - > + * val + (val >> 5) <= 0xFFFFFFFF > + * val + val/32 <= 0xFFFFFFFF > + * val <= (0xFFFFFFFF * 32) / 33 > + * val <= 0xF83E0F82 > + * Hence the threshold for val below which we can use the > + * approximation is 0xF83E0F82 > + */ > +#define DIV_APPROXIMATION_THRESHOLD 0xF83E0F82UL > + > +/* > + * Used for calculating last_residency in usec. Optimized for case > + * where last_residency in nsecs is < DIV_APPROXIMATION_THRESHOLD > + * Approximated value has less than 1% error. > + */ > +static inline int convert_nsec_to_usec(u64 nsec) > +{ > + if (likely(nsec < DIV_APPROXIMATION_THRESHOLD)) { > + u32 usec = nsec; > + > + usec += usec >> 5; > + usec = usec >> 10; > + > + /* Can safely cast to int since usec is < INT_MAX */ > + return usec; > + } else { > + u64 usec = div_u64(nsec, 1000); > + > + if (usec > INT_MAX) > + usec = INT_MAX; > + > + /* Can safely cast to int since usec is < INT_MAX */ > + return usec; > + } > +} What bothers me with this division is the benefit of adding an extra ultra optimized division by 1000 in cpuidle.h while we have already ktime_divns which is optimized in ktime.h. Why not: ts = ns_to_ktime(local_clock()); ... te = ns_to_ktime(local_clock()); diff = ktime_us_delta(te, ts); -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog