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Date:   Thu, 18 Oct 2018 12:38:05 +0200
From:   luca abeni <luca.abeni@santannapisa.it>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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 Juri,

On Thu, 18 Oct 2018 12:10:08 +0200
Juri Lelli <juri.lelli@redhat.com> wrote:
[...]
> > 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.
> > 
> > And do we want to put the limit on runtime or on period ?
> > 
> > That is, something like:
> > 
> >   TICK_NSEC/2 < period < 10*TICK_NSEC
> > 
> > and/or
> > 
> >   TICK_NSEC/2 < runtime < 10*TICK_NSEC
> > 
> > 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).  
> 
> I also thought of TICK_NSEC/2 as a reasonably safe lower limit

I tend to think that something larger than "2" should be used (maybe
10? I mean: even if HZ = 100, it might make sense to allow a runtime
equal to 1ms...)


> that will implicitly limit period as well since
> 
>    runtime <= deadline <= period

I agree that if we end up with TICK_NSEC/2 for the runtime limit then
explicitly enforcing a minimum period is not needed.


> Not sure about the upper limit, though. Lower limit is something
> related to the inherent granularity of the platform/config, upper
> limit is more to do with highest prio stuff with huge period delaying
> everything else; doesn't seem to be related to HZ?

I agree


			Luca