Re: [RFC PATCH 2/2] sched: idle: IRQ based next prediction for idle period

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 01/06/2016 06:40 PM, Nicolas Pitre wrote:
> On Wed, 6 Jan 2016, Daniel Lezcano wrote:
>
>> Many IRQs are quiet most of the time, or they tend to come in bursts of
>> fairly equal time intervals within each burst. It is therefore possible
>> to detect those IRQs with stable intervals and guestimate when the next
>> IRQ event is most likely to happen.
>>
>> Examples of such IRQs may include audio related IRQs where the FIFO size
>> and/or DMA descriptor size with the sample rate create stable intervals,
>> block devices during large data transfers, etc.  Even network streaming
>> of multimedia content creates patterns of periodic network interface IRQs
>> in some cases.
>>
>> This patch adds code to track the mean interval and variance for each IRQ
>> over a window of time intervals between IRQ events. Those statistics can
>> be used to assist cpuidle in selecting the most appropriate sleep state
>> by predicting the most likely time for the next interrupt.
>>
>> Because the stats are gathered in interrupt context, the core computation
>> is as light as possible.
>>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

[ ... ]

>> +
>> +		diff = ktime_sub(now, w->timestamp);
>> +
>> +		/*
>> +		 * There is no point attempting predictions on interrupts more
>> +		 * than 1 second apart. This has no benefit for sleep state
>> +		 * selection and increases the risk of overflowing our variance
>> +		 * computation. Reset all stats in that case.
>> +		 */
>> +		if (unlikely(ktime_after(diff, ktime_set(1, 0)))) {
>> +			stats_reset(&w->stats);
>> +			continue;
>> +		}
>
> The above is wrong. It is not computing the interval between successive
> interruts but rather the interval between the last interrupt occurrence
> and the present time (i.e. when we're about to go idle).  This won't
> prevent interrupt intervals greater than one second from being summed
> and potentially overflowing the variance if this code is executed less
> than a second after one such IRQ interval.  This test should rather be
> performed in sched_idle_irq().

Hi Nico,

I have been through here again and think we should duplicate the test 
because there are two cases:

1. We did not go idle and the interval measured in sched_idle_irq is 
more than one second, then the stats are reset. I suggest to use an 
approximation of one second: (diff < (1 << 20)) as we are in the fast
path.

2. We are going idle and the latest interrupt happened one second apart 
from now. So we keep the current test.

   -- Daniel



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