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.
