linux-kernel - Re: [PATCH 4/5] media/ivtv: Reduce default FIFO priority

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Message-ID: <7970f0e30d1eb83e7067225d07b923863bf1ac50.camel@silverblocksystems.net>
Date:   Thu, 01 Aug 2019 08:24:22 -0400
From:   Andy Walls <andy@...verblocksystems.net>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, Andy Walls <awalls@...metrocast.net>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/5] media/ivtv: Reduce default FIFO priority

Hi Peter:

On Thu, 2019-08-01 at 13:13 +0200, Peter Zijlstra wrote:
> The ivtv driver creates a FIFO-99 thread by default, reduce this to
> FIFO-1.
> 
> FIFO-99 is the very highest priority available to SCHED_FIFO and
> it not a suitable default; it would indicate the ivtv work is the
> most important work on the machine.

ivtv based boards are legacy, convential PCI boards.  At this point,
these old boards are generally installed in boxes dedicated to video
capture (e.g. MythTV setups) or boxes dedicated to capturing VBI
information, like closed captioning, for business intelligence.

For boxes dedicated to video or VBI capture, the ivtv work may very
well be close to the most important work on the machine, to avoid
dropping video frames or VBI data.


> FIFO-1 gets it above all OTHER tasks, which seems high enough lacking
> better justification.

I agree that FIFO-99 is the wrong default level.

However, in my opinion, threads responsible for real time data
acquisition should have higher priority than the other kernel driver
threads normally running at FIFO-50.

How about FIFO-51 as the default?

Regards,
Andy

> Cc: Andy Walls <awalls@...metrocast.net>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Cc: linux-media@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  drivers/media/pci/ivtv/ivtv-driver.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/drivers/media/pci/ivtv/ivtv-driver.c
> +++ b/drivers/media/pci/ivtv/ivtv-driver.c
> @@ -738,7 +738,7 @@ static void ivtv_process_options(struct
>   */
>  static int ivtv_init_struct1(struct ivtv *itv)
>  {
> -	struct sched_param param = { .sched_priority = 99 };
> +	struct sched_param param = { .sched_priority = 1 };
>  
>  	itv->base_addr = pci_resource_start(itv->pdev, 0);
>  	itv->enc_mbox.max_mbox = 2; /* the encoder has 3 mailboxes (0-
> 2) */
> 
>