Re: INFO: rcu detected stall in do_idle

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: 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 <juri.lelli@gmail.com>
Subject: Re: INFO: rcu detected stall in do_idle
Date: Tue, 16 Oct 2018 16:03:22 +0200	[thread overview]
Message-ID: <20181016140322.GB3121@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <alpine.DEB.2.21.1810161516430.7787@nanos.tec.linutronix.de>

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?