linux-kernel - Re: [PATCH 5/5] spi: Reduce kthread priority

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Date:   Thu, 1 Aug 2019 13:27:03 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Guenter Roeck <groeck@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH 5/5] spi: Reduce kthread priority

Hi Peter,

On Thu, Aug 1, 2019 at 1:18 PM Peter Zijlstra <peterz@...radead.org> 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@...omium.org>
> Cc: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> Cc: Guenter Roeck <groeck@...omium.org>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: linux-spi@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  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!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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