sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

housekeeping_setup() checks cpumask_intersects(present, online) to ensure
that the kernel will have at least one housekeeping CPU after smp_init(),
but this doesn't work if the maxcpus= kernel parameter limits the number of
processors available after bootup.

For example, a kernel with "maxcpus=2 nohz_full=0-2" parameters crashes at
boot time on a virtual machine with 4 CPUs.

Change housekeeping_setup() to use cpumask_first_and() and check that the
returned CPU number is valid and less than setup_max_cpus.

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.

Signed-off-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Reviewed-by: Phil Auld <[e-mail]>
Acked-by: Frederic Weisbecker <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/sched/isolation.c

···
 static int __init housekeeping_setup(char *str, unsigned long flags)
 {
 	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
+	unsigned int first_cpu;
 	int err = 0;
 
 	if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) {
···
 	cpumask_andnot(housekeeping_staging,
 		       cpu_possible_mask, non_housekeeping_mask);
 
-	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
+	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
+	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
 		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
 		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
 		if (!housekeeping.flags) {
···
 				"using boot CPU:%d\n", smp_processor_id());
 		}
 	}
+
+	if (cpumask_empty(non_housekeeping_mask))
+		goto free_housekeeping_staging;
 
 	if (!housekeeping.flags) {
 		/* First setup call ("nohz_full=" or "isolcpus=") */