linux-kernel - Re: [PATCH 0/5] Fix FIFO-99 abuse

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Message-ID: <20190802140854.ixq4cmo5nsfdaj24@e107158-lin.cambridge.arm.com>
Date:   Fri, 2 Aug 2019 15:08:54 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/5] Fix FIFO-99 abuse

On 08/02/19 14:41, Peter Zijlstra wrote:
> On Fri, Aug 02, 2019 at 11:26:12AM +0100, Qais Yousef wrote:
> 
> > Yes a somewhat enforced default makes more sense to me. I assume you no longer
> > want to put the kthreads that just need to be above OTHER in FIFO-1?
> 
> I'm not sure, maybe, there's not that many of them, but possibly we add
> another interface for them.

By the way, did you see this one which is set to priority 16?

https://elixir.bootlin.com/linux/v5.3-rc2/source/drivers/gpu/drm/msm/msm_drv.c#L523

> 
> > While at it, since we will cram all kthreads on the same priority, isn't
> > a SCHED_RR a better choice now? I think the probability of a clash is pretty
> > low, but when it happens, shouldn't we try to guarantee some fairness?
> 
> It's never been a problem, and aside from these few straggler threads,
> everybody has effectively been there already for years, so if it were a
> problem someone would've complained by now.

Usually they can run on enough CPUs so a real clash is definitely hard.

I'm trying to collect data on that, if I find something interesting I'll share
it.

> 
> Also; like said before, the admin had better configure.

I agree. But I don't think an 'admin' is an easily defined entity for all
systems. On mobile, is it the SoC vendor, Android framework, or the
handset/platform vendor/integrator?

In a *real* realtime system I think things are better defined. But usage of RT
tasks on generic systems is the confusing part. There's no real ownership and
things are more ad-hoc.

> 
> Also also, RR-SMP is actually broken (and nobody has cared enough to
> bother fixing it).

If you can give me enough pointers to understand the problem I might be able to
bother with it :-)

Thanks

--
Qais Yousef