linux-kernel - Re: INFO: rcu detected stall in do

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Date:   Fri, 19 Oct 2018 13:39:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...il.com>,
        syzbot <syzbot+385468161961cee80c31@...kaller.appspotmail.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
        nstange@...e.de, syzkaller-bugs@...glegroups.com, henrik@...tad.us,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: INFO: rcu detected stall in do_idle

On Thu, Oct 18, 2018 at 01:08:11PM +0200, luca abeni wrote:
> Ok, I see the issue now: the problem is that the "while (dl_se->runtime
> <= 0)" loop is executed at replenishment time, but the deadline should
> be postponed at enforcement time.
> 
> I mean: in update_curr_dl() we do:
> 	dl_se->runtime -= scaled_delta_exec;
> 	if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) {
> 		...
> 		enqueue replenishment timer at dl_next_period(dl_se)
> But dl_next_period() is based on a "wrong" deadline!
> 
> 
> I think that inserting a
>         while (dl_se->runtime <= -pi_se->dl_runtime) {
>                 dl_se->deadline += pi_se->dl_period;
>                 dl_se->runtime += pi_se->dl_runtime;
>         }
> immediately after "dl_se->runtime -= scaled_delta_exec;" would fix the
> problem, no?

That certainly makes sense to me. The only remaining issue would then be
placing a limit on the amount of times we can take that loop; which, as
you propose in a later email; can be done separately as a limit on
runtime.