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Date:   Thu, 18 Oct 2018 12:33:32 +0200
From:   luca abeni <luca.abeni@santannapisa.it>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Juri Lelli <juri.lelli@gmail.com>,
        syzbot <syzbot+385468161961cee80c31@syzkaller.appspotmail.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>, mingo@redhat.com,
        nstange@suse.de, syzkaller-bugs@googlegroups.com, henrik@austad.us,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
        Claudio Scordino <claudio@evidence.eu.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: INFO: rcu detected stall in do_idle
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Hi Peter,

On Thu, 18 Oct 2018 11:48:50 +0200
Peter Zijlstra <peterz@infradead.org> wrote:
[...]
> > So, I tend to think that we might want to play safe and put some
> > higher minimum value for dl_runtime (it's currently at 1ULL <<
> > DL_SCALE). Guess the problem is to pick a reasonable value, though.
> > Maybe link it someway to HZ? Then we might add a sysctl (or
> > similar) thing with which knowledgeable users can do whatever they
> > think their platform/config can support?  
> 
> Yes, a HZ related limit sounds like something we'd want. But if we're
> going to do a minimum sysctl, we should also consider adding a
> maximum, if you set a massive period/deadline, you can, even with a
> relatively low u, incur significant delays.

I agree with this.


> And do we want to put the limit on runtime or on period ?

I think we should have a minimum allowed runtime, a maximum allowed
runtime, a minimum allowed period and a (per-user? per-control
group?) maximum allowed utilization.

I suspect having a maximum period is useless, if we already enforce a
maximum runtime.


> That is, something like:
> 
>   TICK_NSEC/2 < period < 10*TICK_NSEC

As written above I would not enforce a maximum period.


> 
> and/or
> 
>   TICK_NSEC/2 < runtime < 10*TICK_NSEC

I think (but I might be wrong) that "TICK_NSEC/2" is too large... I
would divide the tick for a larger number (how many time do we want to
allow the loop to run?)

And I think the maximum runtime should not be TICK-dependent... It is
the maximum amount of time for which we allow the dealdine task to
starve non-deadline tasks, so it should be an absolute time, not
something HZ-dependent... No?


> Hmm, for HZ=1000 that ends up with a max period of 10ms, that's far
> too low, 24Hz needs ~41ms. We can of course also limit the runtime by
> capping u for users (as we should anyway).

Regarding capping u for users: some time ago, with Juri we discussed
the idea of having per-cgroup limits on the deadline utilization... I
think this is a good idea (and if the userspace creates a cgroup per
user, this results in per-user capping - but it is more flexible in
general)


				Luca