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From: Arjan van de Ven on 22 May 2010 01:30
On Fri, 21 May 2010 12:43:09 -0700
Andrew Morton <akpm(a)linux-foundation.org> wrote:

> On Wed, 19 May 2010 12:51:50 -0700
> Frank Rowand <frank.rowand(a)am.sony.com> wrote:
>
> > Aplogies if this is a duplicate, my outgoing email seems to have not
> > been working.
> >
> > On 05/09/10 16:04, Arjan van de Ven wrote:
> >
> > > +/*
> > > + * Try detecting repeating patterns by keeping track of the last
> > > 8
> > > + * intervals, and checking if the standard deviation of that set
> > > + * of points is below a threshold. If it is... then use the
> > > + * average of these 8 points as the estimated value.
> > > + */
> > > +static void detect_repeating_patterns(struct menu_device *data)
> > > +{
> > > + int i;
> > > + uint64_t avg = 0;
> > > + uint64_t stddev = 0; /* contains the square of the std
> > > deviation */ +
> > > + /* first calculate average and standard deviation of the
> > > past */
> > > + for (i = 0; i < INTERVALS; i++)
> > > + avg += data->intervals[i];
> > > +
> > > + /* if the avg is beyond the known next tick, it's
> > > worthless */
> > > + if (avg > data->expected_us)
> > > + return;
> > > +
> >
> > Should the following division by INTERVALS be moved up 6 lines to
> > before "if (avg > data->expected_us)"?
>
> Quite possibly.

yeah that's a bug; I'll make a patch to fix it

>
> > > + avg = avg / INTERVALS;
> > > + for (i = 0; i < INTERVALS; i++)
> > > + stddev += (data->intervals[i] - avg) *
> > > + (data->intervals[i] - avg);
> > > +
> > > + stddev = stddev / INTERVALS;
> > > +
> > > + /*
> > > + * now.. if stddev is small.. then assume we have a
> > > + * repeating pattern and predict we keep doing this.
> > > + */
> > > +
> > > + if (avg && stddev < STDDEV_THRESH)
> > > + data->predicted_us = avg;
> > > +}
>
> wakey wakey, Arjan.

sorry, seems this email got lost in 400 "Suspend block API" emails ;(


>
>
> Also, expected_us is 32-bit and predicted_us is 64-bit. Was that
> rational?

predicted holds intermediate results of some calculations.. and those
overflow 32 bit...


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From: Arjan van de Ven on 22 May 2010 12:30
On Fri, 21 May 2010 12:43:09 -0700
Andrew Morton <akpm(a)linux-foundation.org> wrote:
> > Should the following division by INTERVALS be moved up 6 lines to
> > before "if (avg > data->expected_us)"?
>
> Quite possibly.

From 7350a9007eee802eaa9e4c1878dd5a2ea4d6f7f9 Mon Sep 17 00:00:00 2001
From: Arjan van de Ven <arjan(a)linux.intel.com>
Date: Sat, 22 May 2010 09:29:29 -0700
Subject: [PATCH] cpuidle: fix bug in the pattern detector as reported by Frank Rowand

Frank Rowand found a bug in the new pattern detector code where
the average value was looked at before the sum of values got divided
by the number of samples.

Signed-off-by: Arjan van de Ven <arjan(a)linux.intel.com>
CC: akpm(a)linux-foundation.org
CC: frank.rowand(a)am.sony.com
---
drivers/cpuidle/governors/menu.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index f3d4783..52ff8aa 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -205,12 +205,12 @@ static void detect_repeating_patterns(struct menu_device *data)
/* first calculate average and standard deviation of the past */
for (i = 0; i < INTERVALS; i++)
avg += data->intervals[i];
+ avg = avg / INTERVALS;

/* if the avg is beyond the known next tick, it's worthless */
if (avg > data->expected_us)
return;

- avg = avg / INTERVALS;
for (i = 0; i < INTERVALS; i++)
stddev += (data->intervals[i] - avg) *
(data->intervals[i] - avg);
--
1.6.2.5

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