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Date:   Tue, 16 Oct 2018 17:36:08 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Juri Lelli <juri.lelli@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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
Subject: Re: INFO: rcu detected stall in do_idle
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On 16/10/18 16:45, Thomas Gleixner wrote:
> On Tue, 16 Oct 2018, Juri Lelli wrote:
> > 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?
> 
> If the cap is active, then yes. But you want to use the actual
> configuration not the default.

Sure.

Although DEADLINE bandwidth is "replicated" across the CPUs of a domain,
so we can still admit a while(1) on multi-CPUs domains. Mmm, guess we
should be able to fix this however if we limit also the per-task maximum
bandwidth considering rt_runtime/rt_period.