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

[Daniel Lezcano]

On 01/10/2016 11:46 PM, Nicolas Pitre wrote:
> On Sun, 10 Jan 2016, Daniel Lezcano wrote:
>
> > 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@xxxxxxxxxx>
> >
> > [ ... ]
> >
> > > > +
> > > > +		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.
>
> You don't need the current test if the interval is already limited
> earlier on.  Predictions that would otherwise trip that test will target
> a time in the past and be discarded.

Yes, but that wake up source should be discarded in the process of the 
selection, so ignored in the loop, otherwise it can end up as the next 
event (which is obviously wrong) and discarded at the end by returning 
KTIME_MAX, instead of giving the opportunity to find another interrupt 
as the next event with a greater value.



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