linux-kernel - Re: WARNING in enqueue_task

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Message-ID: <20181119134349.GA2119@localhost.localdomain>
Date:   Mon, 19 Nov 2018 14:43:49 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     luca abeni <luca.abeni@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>,
        syzbot <syzbot+119ba87189432ead09b4@...kaller.appspotmail.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, mingo@...hat.com,
        syzkaller-bugs@...glegroups.com, x86@...nel.org,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: WARNING in enqueue_task_dl

On 19/11/18 13:52, Peter Zijlstra wrote:
> On Mon, Nov 19, 2018 at 01:07:18PM +0100, luca abeni wrote:
> 
> > > On Sun, 18 Nov 2018, syzbot wrote:
> 
> > > > WARNING: CPU: 1 PID: 6351 at kernel/sched/deadline.c:628
> > > > enqueue_task_dl+0x22da/0x38a0 kernel/sched/deadline.c:1504
> > 
> > Here, it looks like a task is invoking sched_setattr() to become
> > SCHED_DEADLINE when dl_boosted is set...
> > 
> > Is this possible / correct?
> 
> Possible, clearly. Correct, only in so far as that it is not a malformed
> program, but it is very poor design to actually trigger this (of course
> the fuzzer doesn't care about that).
> 
> > If this (sched_setattr() with dl_boosted set) should not be possible,
> > then we have a bug that we need to investigate...
> > 
> > Otherwise, I suspect we can just remove the WARN_ON at line 628 of
> > deadline.c
> 
> I wonder why we put that WARN in there to begin with... git-blame gives
> us:
> 
>   98b0a8578050 ("sched/deadline: Remove useless parameter from setup_new_dl_entity()")
> 
> So the problem seems to be that if we're boosted, we should maybe not be
> using our own (newly set) parameters, but those of the donor task.
> 
> Specifically, our 'suboptimal' deadline inheritance scheme 'requires' us
> to use the inherited deadline, not our own. So in that respect I think
> the WARN is valid, although I'm not sure what, apart from actually
> finishing that PE patch-set we can do about it just now.

Mmm, but, as it was written in the comment that was removed by 295d6d5
("sched/deadline: Fix switching to -deadline"), I was still expecting
that for a boosted task setup_new_dl_entity() shouldn't be called.
Wonder if this is another manifestation of the problems we have with
clocks. Need to think more about it.