linux-kernel - Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

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Message-ID: <6163696.37NBKbymtj@kreacher>
Date:   Sun, 10 Nov 2019 18:24:04 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

On Sunday, November 10, 2019 5:48:21 PM CET Rafael J. Wysocki wrote:
> On Fri, Nov 8, 2019 at 6:04 PM Doug Smythies <dsmythies@...us.net> wrote:
> >
> > On 2019.11.08 01:45 Rafael J. Wysocki wrote:
> > > On Fri, Nov 8, 2019 at 9:45 AM Doug Smythies <dsmythies@...us.net> wrote:
> > >
> > ...
> > >> I have been running this v2 today, with both menu and teo
> > >> governors. Acquiring some baseline reference data to compare
> > >> to now. The menu governor response seems different (Supporting
> > >> information/graphs will come later).
> > >
> > > That may be good or bad, depending in what way it is different. :-)
> >
> > My thinking was that the differences should be minimal between
> > the baseline (linux-next as of 2019.11.07) and plus your two patches.
> > Because this was a change of units, but not functionality.
> > Such is the case with the teo governor, but not the menu governor.
> > I have not tried the ladder or haltpoll governors, and don't intend to.
> >
> > Now to attempt to isolate the issue in the code, which might take
> > considerable time.
> 
> Thanks!
> 
> It looks like I have overlooked a unit conversion in menu or done a
> unit conversion twice somewhere.

I have found a bug, which should be addressed by the patch below.

If it still doesn't reduce the discrepancy, we'll need to look further.

---
 drivers/cpuidle/governors/menu.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/menu.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/menu.c
+++ linux-pm/drivers/cpuidle/governors/menu.c
@@ -516,8 +516,8 @@ static void menu_update(struct cpuidle_d
 	new_factor -= new_factor / DECAY;
 
 	if (data->next_timer_ns > 0 && measured_ns < MAX_INTERESTING)
-		new_factor += RESOLUTION * div64_u64(measured_ns,
-						     data->next_timer_ns);
+		new_factor += div64_u64(RESOLUTION * measured_ns,
+					data->next_timer_ns);
 	else
 		/*
 		 * we were idle so long that we count it as a perfect