linux-kernel - Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle period

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Date:	Thu, 21 Jan 2016 11:03:36 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	tglx@...utronix.de, peterz@...radead.org, rafael@...nel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	vincent.guittot@...aro.org
Subject: Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle
 period


Hi Nico,


On 01/20/2016 06:46 PM, Nicolas Pitre wrote:
> On Wed, 20 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@...aro.org>
>> ---

[ ... ]

>> +struct stats {
>> +	u64           sum;                     /* sum of values */
>> +	u32           values[STATS_NR_VALUES]; /* array of values */
>> +	unsigned char w_ptr;                   /* current window pointer */
>
> Why did you change this from an unsigned int?
>
> This won't provide any memory space saving given that the structure has
> to be padded up to the next 64-bit boundary.

Ok, I will change it back to unsigned int.

[ ... ]

>> +	for (i = 0; i < STATS_NR_VALUES; i++) {
>> +		s64 diff = s->values[i] - mean;
>> +		variance += (u64)diff * diff;
>> +	}
>
> This is completely wrong.  Even more wrong than it used to be.  I must
> have expressed myself badly about this last time.
>
> To avoid any confusion, here's what the code should be:
>
> 	int i;
> 	u64 variance = 0;
>
> 	for (i = 0; i < STATS_NR_VALUES; i++) {
> 		s32 diff = s->values[i] - mean;
> 		variance += (s64)diff * diff;
> 	}
>
> 	[...]

Aah, ok :)

[ ... ]

>> +	if (diff > (1 << 20)) {
>
> You could use the USEC_PER_SEC constant here. It is already widely used
> and would make the code even more obvious.

Indeed.

[ ... ]

>> +		/*
>> +		 * 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.
>> +		 */
>
> This comment is wrong.  It is relevant in sched_irq_timing_handler() but
> not here.  Instead this should be something like:
>
> 		/*
> 		 * This interrupt last triggered more than a second ago.
> 		 * It is definitely not predictable for our purpose anymore.
> 		 */

Ok.

[ ... ]

>> +		interval = w->stats.values[w->stats.w_ptr];
>> +		if ((u64)((interval - mean) * (interval - mean)) > variance)
>
> s/u64/s64/ please.

Noted.

Thanks Nico for the review.


   -- Daniel

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