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

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Message-ID: <20161107190620.1b629d82@utopia>
Date:   Mon, 7 Nov 2016 19:06:20 +0100
From:   Luca Abeni <luca.abeni@...tn.it>
To:     Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        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 19:03:08 +0100
Tommaso Cucinotta <tommaso.cucinotta@...up.it> wrote:

> On 07/11/2016 14:51, Daniel Bristot de Oliveira wrote:
> > Hi Tommaso,
> 
> Hi,
> 
> I'm cc-ing Luca for GRUB et al., pls find a few further notes below...
Thanks Tommaso! I've seen the email on the linux-rt-users mailing list,
and I'll reply there about GRUB...



			Thanks,
				Luca


> 
> > On 11/07/2016 11:31 AM, Tommaso Cucinotta wrote:
> >> as anticipated live to Daniel:
> >> -) +1 for the general concept, we'd need something similar also for
> >> SCHED_DEADLINE
> >
> > Resumed: the sum of the runtime of deadline tasks will not be
> > greater than the "to_ratio(global_rt_period(),
> > global_rt_runtime())" - see init_dl_bw(). Therefore, DL rq will not
> > be throttle by the RT throttling mechanism.
> >
> > Extended: RT tasks' throttling aims to bound, for all CPUS of a
> > domain - when RT_RUNTIME_SHARING sharing is enabled; or per-rq -
> > when RT_RUNTIME_SHARING is disabled; the amount of time that RT
> > tasks can run continuously, in such way to provide some CPU time
> > for non-real-time tasks to run. RT tasks need this global/local
> > throttling mechanism to avoid the starvation of non-rt tasks
> > because RT tasks do not have a limited runtime - RT task (or
> > taskset) can run for an infinity runtime.
> >
> > DL tasks' throttling has another meaning. DL tasks' throttling aims
> > to avoid *a* DL task for running for more than *its own*
> > pre-allocated runtime.
> 
> sure, and having an option to let it run for longer, if there's
> nothing else running in the system, is still interesting for pretty
> much similar reasons to those being discussed in this thread ...
> 
> > The sum of allocated runtime for all DL tasks will not to be greater
> > than RT throttling enforcement runtime. The DL scheduler admission
> > control already avoids this by limiting the amount of CPU time all
> > DL tasks can consume (see init_dl_bw()). So, DL tasks are avoid ind
> > the "global" throttling on before hand - in the admission control.
> >
> > GRUB might implement something <<similar>> for the DEADLINE
> > scheduler. With GRUB, a deadline tasks will have more runtime than
> > previously set/granted.....
> 
> yes, the main difference being: GRUB will let a DL task run for longer
> than its own runtime, but still let it starve anything below (RT as
> well as OTHER tasks); perhaps Luca (cc) has some further comment on
> this...
> 
> Thanks,
> 
> 	T.
> 
> > But I am quite sure it will still be bounded by the sum
> > of the already allocated DL runtime, that will continue being
> > smaller than "to_ratio(global_rt_period(), global_rt_runtime())".
> >
> > Am I missing something?
> >
> >> -) only issue might be that, if a non-RT task wakes up after the
> >> unthrottle, it will have to wait, but worst-case it will have a
> >> chance in the next throttling window
> >
> > In the current default behavior (RT_RUNTIME_SHARING), in a domain
> > with more than two CPUs, the worst case easily become "infinity,"
> > because a CPU can borrow runtime from another CPU. There is no
> > guarantee for minimum latency for non-rt tasks. Anyway, if the user
> > wants to provide such guarantee, they just need not enable this
> > feature, while disabling RT_RUNTIME_SHARING (or run the non-rt task
> > as a deadline task ;-))
> >
> >> -) an alternative to unthrottling might be temporary class
> >> downgrade to sched_other, but that might be much more complex,
> >> instead this Daniel's one looks quite simple
> >
> > Yeah, decrease the priority of the task would be something way more
> > complicated and prone to errors. RT tasks would need to reduce its
> > priority to a level higher than the IDLE task, but lower than
> > SCHED_IDLE...
> >
> >> -) when considering also DEADLINE tasks, it might be good to think
> >> about how we'd like the throttling of DEADLINE and RT tasks to
> >> inter-relate, e.g.:
> >
> > Currently, DL tasks are limited (in the bw control) to the global RT
> > throttling limit...
> >
> > I think that this might be an extension to GRUB... that is
> > extending the current behavior... so... things for the future - and
> > IMHO it is another topic - way more challenging.
> >
> > Comments are welcome :-)
> >
> > -- Daniel
> >
> 
>