linux-kernel - Re: [PATCH] sched/rt: RT_RUNTIME

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Date:   Mon, 7 Nov 2016 12:22:21 -0600
From:   Clark Williams <williams@...hat.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Christoph Lameter <cl@...ux.com>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

On Mon,  7 Nov 2016 09:17:55 +0100
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> The rt throttling mechanism prevents the starvation of non-real-time
> tasks by CPU intensive real-time tasks. In terms of percentage,
> the default behavior allows real-time tasks to run up to 95% of a
> given period, leaving the other 5% of the period for non-real-time
> tasks. In the absence of non-rt tasks, the system goes idle for 5%
> of the period.
> 
> Although this behavior works fine for the purpose of avoiding
> bad real-time tasks that can hang the system, some greed users
> want to allow the real-time task to continue running in the absence
> of non-real-time tasks starving. In other words, they do not want to
> see the system going idle.
> 
> This patch implements the RT_RUNTIME_GREED scheduler feature for greedy
> users (TM). When enabled, this feature will check if non-rt tasks are
> starving before throttling the real-time task. If the real-time task
> becomes throttled, it will be unthrottled as soon as the system goes
> idle, or when the next period starts, whichever comes first.
> 
> This feature is enabled with the following command:
>   # echo RT_RUNTIME_GREED > /sys/kernel/debug/sched_features
> 

I'm still reviewing the patch, but I have to wonder why bother with making it a scheduler feature?

The SCHED_FIFO definition allows a fifo thread to starve others because a fifo task will run until it yields. Throttling was added as a safety valve to allow starved SCHED_OTHER tasks to get some cpu time.  Adding this unconditionally gets us a safety valve for throttling a badly written fifo task, but allows the fifo task to continue to consume cpu cycles if it's not starving anyone. 

Or am I missing something that's blazingly obvious?

Clark

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