linux-kernel - Re: [PATCH] Documentation/scheduler/sched-rt-group: Update SCHED DEADLINE references

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Date:   Thu, 1 Feb 2018 11:53:45 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/scheduler/sched-rt-group: Update SCHED
 DEADLINE references

Hi Daniel,

On 01/02/18 11:31, Daniel Bristot de Oliveira wrote:
> The documentation was mentioning the "future SCHED EDF" as the
> solution for fine-grained control of deadline/period. This patch
> updates this citing the (now) existing SCHED_DEADLINE.
> 
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Luca Abeni <luca.abeni@...tannapisa.it>
> Cc: Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
> Cc: Claudio Scordino <claudio@...dence.eu.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> 
> ---
>  Documentation/scheduler/sched-rt-group.txt | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt
> index d8fce3e78457..ed763c243914 100644
> --- a/Documentation/scheduler/sched-rt-group.txt
> +++ b/Documentation/scheduler/sched-rt-group.txt
> @@ -153,7 +153,7 @@ There is work in progress to make the scheduling period for each group
>  
>  The constraint on the period is that a subgroup must have a smaller or
>  equal period to its parent. But realistically its not very useful _yet_
> -as its prone to starvation without deadline scheduling.
> +as its prone to starvation.
>  
>  Consider two sibling groups A and B; both have 50% bandwidth, but A's
>  period is twice the length of B's.
> @@ -168,16 +168,7 @@ This means that currently a while (1) loop in A will run for the full period of
>  B and can starve B's tasks (assuming they are of lower priority) for a whole
>  period.
>  
> -The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
> -full deadline scheduling to the linux kernel. Deadline scheduling the above
> -groups and treating end of the period as a deadline will ensure that they both
> -get their allocated time.
> -
> -Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
> -the biggest challenge as the current linux PI infrastructure is geared towards
> -the limited static priority levels 0-99. With deadline scheduling you need to
> -do deadline inheritance (since priority is inversely proportional to the
> -deadline delta (deadline - now)).
> -
> -This means the whole PI machinery will have to be reworked - and that is one of
> -the most complex pieces of code we have.
> +Nowadays it is possible to use the deadline scheduler (SCHED_DEADLINE) to
> +allocate runtime/period for tasks. The deadline scheduler does not suffer the
> +side effects explained above. For more information about the deadline
> +scheduler, you should read Documentation/scheduler/sched-deadline.txt.

True. But it doesn't have support for groups yet (while this doc seems
to refer to groups). Maybe add a paragraph about this?

Thanks,

- Juri