linux-kernel - Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case

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Date:   Tue, 14 Apr 2020 12:29:01 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     luca abeni <luca.abeni@...tannapisa.it>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for
 !fit case

On 04/09/20 20:43, Dietmar Eggemann wrote:
> On 09.04.20 16:55, Qais Yousef wrote:
> > Hi Luca
> > 
> > On 04/09/20 15:00, luca abeni wrote:
> 
> [...]
> 
> >> There is already a tunable for the SCHED_DEADLINE admission test
> >> (/proc/sys/kernel/sched_rt_{runtime,period}_us, if I understand well
> >> what you are suggesting). The problem is that it is not easy to find a
> >> value for this tunable that guarantees the hard respect of all
> >> deadlines.
> > 
> > I don't think it's similar to what I was referring to. But my knowledge about
> > DL could have failed me to fully appreciate what you're saying.
> > 
> > This tunable for RT prevents a single task from using 100% CPU time. I think
> > DL uses it in a similar manner.
> > 
> > What I meant by overcommiting, is allowing more DL tasks than the system can
> > guarantee to meet their deadlines.
> > 
> > For example, in the context of capacity awareness, if you have 2 big cores, but
> > 4 DL tasks request a bandwidth that can only be satisfied by the big cores,
> > then 2 of them will miss their deadlines (almost) consistently, IIUC.
> 
> __dl_overflow() now uses
> 
>     X = cap_scale(dl_b->bw, rd_capacity(cpu)) instead of X = cpus
> 
> in
> 
>     dl_b->bw * X < dl_b->total_bw - old_bw + new_bw;
> 
> 
> As an example, Arm64 Hikey64 with 4 LITTLE and 4 big CPUs uses now
> (4x462 + 4x1024)/1024 = 5944/1024 ~ 5,80 instead of 8 CPUs.
> 
> On mainline, the rt-app tests:
> 
> "tasks" : {
>  "thread0" : {
>   "policy" : "SCHED_DEADLINE",
>   "instance" : 8,
>   "timer" : {"ref" : "unique0", "period" : 16000, "mode" : "absolute" },
>   "run" : 10000,
>   "dl-runtime" :11000,
>   "dl-period" : 16000,
>   "dl-deadline" : 16000
>   }
> }
> 
> is admitted on this board whereas with the patchset some of the tasks
> are rejected:
> 
> [rt-app] <crit> [7] sched_setattr returned -1
> sched_setattr: failed to set deadline attributes: Device or resource busy
> 
> ---
> 
> IMHO, one of the 3 places where DL Admission Control hooks into is:
> 
>     __sched_setscheduler -> sched_dl_overflow() ->__dl_overflow()
> 
> [...]

Thanks Dietmar! I should have looked at the code with more intent.

--
Qais Yousef