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

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Message-ID: <20161108195015.GP3117@twins.programming.kicks-ass.net>
Date:   Tue, 8 Nov 2016 20:50:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Subject: Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

On Tue, Nov 08, 2016 at 08:29:49PM +0100, Daniel Bristot de Oliveira wrote:
> 
> 
> On 11/08/2016 07:05 PM, Peter Zijlstra wrote:
> >> > 
> >> > I know what we want to do, but there's some momentous problems that
> >> > need to be solved first.
> > Like what?
> 
> The problem is that using RT_RUNTIME_SHARE a CPU will almost always
> borrow enough runtime to make a CPU intensive rt task to run forever...
> well not forever, but until the system crash because a kworker starved
> in this CPU. Kworkers are sched fair by design and users do not always
> have a way to avoid them in an isolated CPU, for example.
> 
> The user then can disable RT_RUNTIME_SHARE, but then the user will have
> the CPU going idle for (period - runtime) at each period... throwing CPU
> time in the trash.

So why is this a problem? You really should not be running that much
FIFO tasks to begin with.

So I'm willing to take out (or at least default disable
RT_RUNTIME_SHARE). But other than this, this never really worked to
begin with. So it cannot be a regression. And we've lived this long with
the 'problem'.

And that means this is a 'feature' and that means I say no.

We really should be doing the right thing here, not make a bigger mess.