Messages in this thread | | | Subject | Re: [RFC PATCH v2 3/8] cpuidle: add a new predict interface | From | "Li, Aubrey" <> | Date | Mon, 16 Oct 2017 16:04:15 +0800 |
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On 2017/10/14 8:45, Rafael J. Wysocki wrote: > On Saturday, September 30, 2017 9:20:29 AM CEST Aubrey Li wrote: >> For the governor has predict functionality, add a new predict >> interface in cpuidle framework to call and use it. > > Care to describe how it is intended to work? > > Also this patch uses data structures introduced in the previous one > (and the previous one didn't use them), so maybe merge the two?
okay, will refine in the next version.
> >> --- >> drivers/cpuidle/cpuidle.c | 34 ++++++++++++++++++++++++++++++++++ >> drivers/cpuidle/governors/menu.c | 7 +++++++ >> include/linux/cpuidle.h | 3 +++ >> kernel/sched/idle.c | 1 + >> 4 files changed, 45 insertions(+) >> >> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c >> index 4066308..ef6f7dd 100644 >> --- a/drivers/cpuidle/cpuidle.c >> +++ b/drivers/cpuidle/cpuidle.c >> @@ -336,6 +336,40 @@ void cpuidle_entry_end(void) >> } >> >> /** >> + * cpuidle_predict - predict whether the coming idle is a fast idle or not >> + */ >> +void cpuidle_predict(void) >> +{ >> + struct cpuidle_device *dev = cpuidle_get_device(); >> + unsigned int overhead_threshold; >> + >> + if (!dev) >> + return; >> + >> + overhead_threshold = dev->idle_stat.overhead; >> + >> + if (cpuidle_curr_governor->predict) { >> + dev->idle_stat.predicted_us = cpuidle_curr_governor->predict(); >> + /* >> + * notify idle governor to avoid reduplicative >> + * prediction computation >> + */ > > This comment is hard to understand. > > What does it mean, really? >
predict() does a lot of computation. If it's called in cpuidle_predict(), we don't want it to be called in menu governor again. So here I use a flag to tell menu governor if predict computation is already done for the coming idle this time.
>> + dev->idle_stat.predicted = true; >> + if (dev->idle_stat.predicted_us < overhead_threshold) { >> + /* >> + * notify tick subsystem to keep ticking >> + * for the coming idle >> + */ >> + dev->idle_stat.fast_idle = true; >> + } else >> + dev->idle_stat.fast_idle = false; > > What about > > dev->idle_stat.fast_idle = dev->idle_stat.predicted_us < overhead_threshold;
Ack
> > Also why don't you use dev->idle_stat.overhead directly here?
Yeah, I should merge a few patches, because dev->idle_stat.overhead is referenced many times, for irq timings, for scheduler idle statistics and for idle governor.
> > And what is the basic idea here? Why do we compare the predicted > idle duration with the entry/exit overhead from the previous cycle > (if I understood this correctly, that is)?
entry/exit overhead is an average here, and the variance should not be big.
> >> + } else { >> + dev->idle_stat.predicted = false; >> + dev->idle_stat.fast_idle = false; >> + } >> +} >> + >> +/** >> * cpuidle_install_idle_handler - installs the cpuidle idle loop handler >> */ >> void cpuidle_install_idle_handler(void) >> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c >> index 6bed197..90b2a10 100644 >> --- a/drivers/cpuidle/governors/menu.c >> +++ b/drivers/cpuidle/governors/menu.c >> @@ -344,6 +344,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev) >> if (unlikely(latency_req == 0)) >> return 0; >> >> + /*don't predict again if idle framework already did it */ >> + if (!dev->idle_stat.predicted) >> + menu_predict(); >> + else >> + dev->idle_stat.predicted = false; > > We provide the callback which is going to be used by the core if present, > so why would the core not use it after all?
There is a case that in the loop
while (!need_resched()) { cpuidle_idle_call() }
The CPU receives a timer interrupt and wake up and find nothing to be scheduled and back to call menu then mwait to sleep again.
Under this case, cpuidle_predict() is not reached but the idle data for the prediction needs to be updated.
Thanks, -Aubrey
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