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Date:   Tue, 16 Oct 2018 16:41:28 +0200
From:   Juri Lelli <juri.lelli@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        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,
        juri.lelli@redhat.com
Subject: Re: INFO: rcu detected stall in do_idle
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On 16/10/18 16:03, Peter Zijlstra wrote:
> On Tue, Oct 16, 2018 at 03:24:06PM +0200, Thomas Gleixner wrote:
> > It does reproduce here but with a kworker stall. Looking at the reproducer:
> > 
> >   *(uint32_t*)0x20000000 = 0;
> >   *(uint32_t*)0x20000004 = 6;
> >   *(uint64_t*)0x20000008 = 0;
> >   *(uint32_t*)0x20000010 = 0;
> >   *(uint32_t*)0x20000014 = 0;
> >   *(uint64_t*)0x20000018 = 0x9917;
> >   *(uint64_t*)0x20000020 = 0xffff;
> >   *(uint64_t*)0x20000028 = 0;
> >   syscall(__NR_sched_setattr, 0, 0x20000000, 0);
> > 
> > which means:
> > 
> >   struct sched_attr {
> >   	 .size		= 0,
> > 	 .policy	= 6,
> > 	 .flags		= 0,
> > 	 .nice		= 0,
> > 	 .priority	= 0,
> > 	 .deadline	= 0x9917,
> > 	 .runtime	= 0xffff,
> > 	 .period	= 0,
> >   }
> > 
> > policy 6 is SCHED_DEADLINE
> > 
> > That makes the thread hog the CPU and prevents all kind of stuff to run.
> > 
> > Peter, is that expected behaviour?
> 
> Sorta, just like FIFO-99 while(1);. Except we should be rejecting the
> above configuration, because of the rule:
> 
>   runtime <= deadline <= period
> 
> Juri, where were we supposed to check that?

Not if period == 0.

https://elixir.bootlin.com/linux/latest/source/kernel/sched/deadline.c#L2632
https://elixir.bootlin.com/linux/latest/source/kernel/sched/deadline.c#L2515

Now, maybe we should be checking also against the default 95% cap?

Best,

- Juri