linux-kernel - Re: [PATCH 04/23] sched,acpi_pad: Convert to sched_set

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Date:   Thu, 23 Apr 2020 10:46:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        rostedt@...dmis.org, qais.yousef@....com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, bsegall@...gle.com, mgorman@...e.de,
        rafael.j.wysocki@...el.com
Subject: Re: [PATCH 04/23] sched,acpi_pad: Convert to sched_set_fifo*()

On Wed, Apr 22, 2020 at 06:45:36PM +0200, Dietmar Eggemann wrote:
> On 22/04/2020 13:27, Peter Zijlstra wrote:
> > Because SCHED_FIFO is a broken scheduler model (see previous patches)
> > take away the priority field, the kernel can't possibly make an
> > informed decision.
> > 
> > In this case, use fifo_low, because it only cares about being above
> > SCHED_NORMAL. Effectively no change in behaviour.
> > 
> > XXX: this driver is still complete crap; why isn't it using proper
> > idle injection or at the very least play_idle() ?
> > 
> > Cc: rafael.j.wysocki@...el.com
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Reviewed-by: Ingo Molnar <mingo@...nel.org>
> > ---
> >  drivers/acpi/acpi_pad.c |    3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > --- a/drivers/acpi/acpi_pad.c
> > +++ b/drivers/acpi/acpi_pad.c
> > @@ -136,12 +136,11 @@ static unsigned int idle_pct = 5; /* per
> >  static unsigned int round_robin_time = 1; /* second */
> >  static int power_saving_thread(void *data)
> >  {
> > -	struct sched_param param = {.sched_priority = 1};
> >  	int do_sleep;
> >  	unsigned int tsk_index = (unsigned long)data;
> >  	u64 last_jiffies = 0;
> >  
> > -	sched_setscheduler(current, SCHED_RR, &param);
> 
> I was wondering what happened to the SCHED_RR cases but as I can see now
> they are handled here and in the next patch.

Oh right; I completely forgot to mention that in the Changelog didn't I
:-(

In this case, this driver is a broken piece of crap and doing fake idle
with RR is just plain idiotic. Also note the WARNs in play_idle().

Also, rjw, what was the point of renmaing play_idle() to
play_idle_precise() if there is only one anyway? The changelog talks
about adding play_idle_precise() but the patch (rightfully) doesn't add
another version but replaces the existing one. But why change the name?!