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Date:   Fri, 19 Oct 2018 15:14:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     luca abeni <luca.abeni@santannapisa.it>
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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On Thu, Oct 18, 2018 at 12:33:32PM +0200, luca abeni wrote:
> 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 was talking about a global !root max-u, but yes the cgroup max-u makes
definite sense as well.

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

Probably; yes. The asymmetry is unfortunate of course.

> > That is, something like:
> > 
> >   TICK_NSEC/2 < period < 10*TICK_NSEC
> 
> As written above I would not enforce a maximum period.

I'm confused: 'period < 10*TICK_NSEC' reads like a max to me.

(irrespective of the argument on wether the max should be HZ related;
and I think you and Juri made good argument for it not to be)

> > 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?)

It depends on how strict we want to enforce the no-interference rule.
The smaller we make this, the less accurate we enforce, the worse the
interference between tasks.

Note that we're only talking about a default; and HZ=100 is daft in any
case.

> 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?

Agreed.

> > 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)

Agreed, that makes sense.