Re: [PATCH 5/5] spi: Reduce kthread priority

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Peter,

On Thu, Aug 1, 2019 at 2:12 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Aug 01, 2019 at 01:27:03PM +0200, Geert Uytterhoeven wrote:
> > On Thu, Aug 1, 2019 at 1:18 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > The SPI thingies request FIFO-99 by default, reduce this to FIFO-50.
> > >
> > > FIFO-99 is the very highest priority available to SCHED_FIFO and
> > > it not a suitable default; it would indicate the SPI work is the
> > > most important work on the machine.
> > >
> > > Cc: Benson Leung <bleung@xxxxxxxxxxxx>
> > > Cc: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> > > Cc: Guenter Roeck <groeck@xxxxxxxxxxxx>
> > > Cc: Mark Brown <broonie@xxxxxxxxxx>
> > > Cc: linux-spi@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > > ---
> > >  drivers/platform/chrome/cros_ec_spi.c |    2 +-
> > >  drivers/spi/spi.c                     |    2 +-
> > >  2 files changed, 2 insertions(+), 2 deletions(-)
> > >
> > > --- a/drivers/platform/chrome/cros_ec_spi.c
> > > +++ b/drivers/platform/chrome/cros_ec_spi.c
> > > @@ -706,7 +706,7 @@ static int cros_ec_spi_devm_high_pri_all
> > >                                            struct cros_ec_spi *ec_spi)
> > >  {
> > >         struct sched_param sched_priority = {
> > > -               .sched_priority = MAX_RT_PRIO - 1,
> > > +               .sched_priority = MAX_RT_PRIO / 2,
> >
> > include/linux/sched/prio.h says:
> >
> >  * Priority of a process goes from 0..MAX_PRIO-1, valid RT
> >  * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
> >  * tasks are in the range MAX_RT_PRIO..MAX_PRIO-1. Priority
> >  * values are inverted: lower p->prio value means higher priority.
> >
> > So the new 50 is actually a higher priority than the old 99?
> >
> > Given I'm far from an RT expert, I must be missing something?
> > Thanks!
>
> Ah; you found the confusion ;-)
>
> https://lkml.kernel.org/20190617122448.GA3436@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

/me adds /r after org/
Thanks!

> But basically, user-space prio is [1-99], while in-kernel prio is
> [0-98]. The above is user prio (it basically uses the
> sched_setscheduler() syscall).
>
> So 50 really is lower than 99.

IC.

BTW, what about having a #define for MAX_RT_PRIO / 2?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds