linux-kernel - Re: [PATCH] sched: write better comments for weight calculations

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Message-ID: <20170315144317.GF18557@e110439-lin>
Date:   Wed, 15 Mar 2017 14:43:17 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Juri Lelli <Juri.Lelli@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched: write better comments for weight calculations

On 15-Mar 05:35, Joel Fernandes wrote:
> On Wed, Mar 15, 2017 at 5:04 AM, Patrick Bellasi
> <patrick.bellasi@....com> wrote:
> > Few comments inline, otherwise LGTM.
> 
> Ok, I'll take that as an Acked-by with the following comment addressed
> if that's Ok with you.

Well, I cannot really ACK anything... you should defenitively ask
someone else in CC for such a tag ;-)

FWIW, if you like, you can add instead a Reviewed-by tag.

Cheers Patrick

> >
> > On 10-Mar 12:47, Joel Fernandes wrote:
> >> This patch rewrites comments related task priorities and CPU usage
> >> along with an example to show how it works.
> >>
> >> Cc: Juri Lelli <Juri.Lelli@....com>
> >> Cc: Patrick Bellasi <patrick.bellasi@....com>
> >> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
> >> ---
> >>  kernel/sched/core.c | 27 +++++++++++++++++++--------
> >>  1 file changed, 19 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >> index c56fb57f2991..2175bf663f3d 100644
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -8823,16 +8823,27 @@ void dump_cpu_task(int cpu)
> >>  }
> >>
> >>  /*
> >> - * Nice levels are multiplicative, with a gentle 10% change for every
> >> - * nice level changed. I.e. when a CPU-bound task goes from nice 0 to
> >> - * nice 1, it will get ~10% less CPU time than another CPU-bound task
> >> - * that remained on nice 0.
> >> + * Nice levels are multiplicative, with a gentle 10% relative change
> >> + * for every nice level changed. I.e. if there were 2 CPU-bound tasks
> >> + * of equal nice value and one of them goes from a nice level of 0 to 1
> >> + * then the task at nice level 1 will get ~5% less CPU time than before
> >> + * the change and the task that remained at nice level 0 will get ~5%
> >> + * more CPU time.
> >>   *
> >>   * The "10% effect" is relative and cumulative: from _any_ nice level,
> >> - * if you go up 1 level, it's -10% CPU usage, if you go down 1 level
> >> - * it's +10% CPU usage. (to achieve that we use a multiplier of 1.25.
> >> - * If a task goes up by ~10% and another task goes down by ~10% then
> >> - * the relative distance between them is ~25%.)
> >> + * if you go up 1 level, it's -10% relative CPU usage, if you go down
> >> + * by 1 level it's +10% CPU usage.
> >                           ^
> >                        relative
> >> + * To achieve that, we use a multiplier of 1.25.
> >
> >
> > The following sentence:
> >
> >> + * If a task goes up by ~5% and another task goes down by ~5%
> >> + * then the relative distance between their weights is ~25% as shown
> >> + * in the following example:
> >
> > is still confusing to me, mainly because we are mixing the "shares
> > percentage" with the CPU usage percentage.
> >
> > What about this:
> >
> >       If two tasks have a 25% relative distance between their weights
> >       then they will get a 10% difference in CPU usage as shown in the
> >       following example.
> 
> I agree your statement is clearer and I will use it in the repost.
> 
> J.

-- 
#include <best/regards.h>

Patrick Bellasi