Re: [tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

[Oleg Nesterov <oleg@redhat.com>]

On 04/28, Ingo Molnar wrote:
>
> * tip-bot2 for Oleg Nesterov <tip-bot2@linutronix.de> wrote:
>
> > Another corner case is "nohz_full=0" on a machine with a single CPU or with
> > the maxcpus=1 kernel argument. In this case non_housekeeping_mask is empty
> > and tick_nohz_full_setup() makes no sense. And indeed, the kernel hits the
> > WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> >
> > And how should the kernel interpret the "nohz_full=" parameter? It should
> > be silently ignored, but currently cpulist_parse() happily returns the
> > empty cpumask and this leads to the same problem.
> >
> > Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> > and do nothing in this case.
>
> So arguably the user meant NOHZ_FULL to be turned off - but it is de-facto
> already turned off by the fact that there's only a single CPU available,
> right?

Or the user passes the empty "nohz_full=" mask on a multi-CPU machine.

In both cases (before this patch) housekeeping_setup() calls
tick_nohz_full_setup(non_housekeeping_mask) which sets
tick_nohz_full_running = true even if tick_nohz_full_mask is empty.

This doesn't look right to me and triggers the "this should not happen"
warning in tick_sched_do_timer().

But let me repeat, I know nothing about nohz/etc.

Oleg.