linux-kernel - Re: [PATCH 11/23] sched,spi: Convert to sched_set

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Date:   Wed, 22 Apr 2020 08:47:52 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>, qais.yousef@....com,
        juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Benjamin Segall <bsegall@...gle.com>,
        mgorman@...e.de, Guenter Roeck <groeck@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>
Subject: Re: [PATCH 11/23] sched,spi: Convert to sched_set_fifo*()

On Wed, Apr 22, 2020 at 7:35 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Wed, Apr 22, 2020 at 6:56 AM Mark Brown <broonie@...nel.org> wrote:
> >
> > On Wed, Apr 22, 2020 at 01:27:30PM +0200, 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.
> > >
> > > No effective change.
> >
> > Copying Doug who did this change and Guenter who reviewed it.  This
> > looks fine to me but I've no particular involvement with the code or
> > platforms that are affected here.
>
> Thanks!  Probably the maintainers of cros_ec_spi.c (Benson and Enric)
> should be aware of it, too.  CCing them.
>
> From my point of view, my response is pretty much identical to the one
> I wrote when the priority was reduced from "MAX_RT_PRIO - 1" to
> "MAX_RT_PRIO / 2" [1].  Basically, any priority that keeps us from
> being preempted by tasks that are only high priority for performance
> reasons (like dm crypt and loopback did when I last analyzed) is fine.
> Our priority needs to be high not for performance reasons but for
> correctness reasons (the other side will drop our data if we don't
> respond in time).
>
The crypto engine ends up running at the same priority level, so I am
a bit concerned that this patch series will re-introduce the problem
that Doug's initial patch tried to solve. Though I do notice that it
already _is_ running at the same priority, so maybe the problem has
already been re-introduced with the commit that set the priority to
MAX_RT_PRIO / 2, and we just haven't noticed yet. So I guess this
patch indeed doesn't make a difference.

Reviewed-by: Guenter Roeck <groeck@...omium.org>

> Thus:
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
>
>
> [1] https://lore.kernel.org/r/CAD=FV=UsYF1R6+XRfFFFm6PfmkTsEOfxxgCw2JxCnpyu1kGVLQ@mail.gmail.com
>
>
> > > Cc: broonie@...nel.org
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > Reviewed-by: Ingo Molnar <mingo@...nel.org>
> > > ---
> > >  drivers/platform/chrome/cros_ec_spi.c |    6 +-----
> > >  1 file changed, 1 insertion(+), 5 deletions(-)
> > >
> > > --- a/drivers/platform/chrome/cros_ec_spi.c
> > > +++ b/drivers/platform/chrome/cros_ec_spi.c
> > > @@ -709,9 +709,6 @@ static void cros_ec_spi_high_pri_release
> > >  static int cros_ec_spi_devm_high_pri_alloc(struct device *dev,
> > >                                          struct cros_ec_spi *ec_spi)
> > >  {
> > > -     struct sched_param sched_priority = {
> > > -             .sched_priority = MAX_RT_PRIO / 2,
> > > -     };
> > >       int err;
> > >
> > >       ec_spi->high_pri_worker =
> > > @@ -728,8 +725,7 @@ static int cros_ec_spi_devm_high_pri_all
> > >       if (err)
> > >               return err;
> > >
> > > -     err = sched_setscheduler_nocheck(ec_spi->high_pri_worker->task,
> > > -                                      SCHED_FIFO, &sched_priority);
> > > +     err = sched_set_fifo(ec_spi->high_pri_worker->task);
> > >       if (err)
> > >               dev_err(dev, "Can't set cros_ec high pri priority: %d\n", err);
> > >       return err;
> > > --- a/drivers/spi/spi.c
> > > +++ b/drivers/spi/spi.c
> > > @@ -1589,11 +1589,9 @@ EXPORT_SYMBOL_GPL(spi_take_timestamp_pos
> > >   */
> > >  static void spi_set_thread_rt(struct spi_controller *ctlr)
> > >  {
> > > -     struct sched_param param = { .sched_priority = MAX_RT_PRIO / 2 };
> > > -
> > >       dev_info(&ctlr->dev,
> > >               "will run message pump with realtime priority\n");
> > > -     sched_setscheduler(ctlr->kworker_task, SCHED_FIFO, &param);
> > > +     sched_set_fifo(ctlr->kworker_task);
> > >  }
> > >
> > >  static int spi_init_queue(struct spi_controller *ctlr)
> > >
> > >