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 <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240413141746.GA10008@redhat.com

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 2a262d3ecb3da4964f42cecccc5f44479d5ea5f1..5891e715f00d028b0d2f4fd157e99f319b899a1d 100644 (file)
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type,
 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)) {
@@ -138,7 +139,8 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
        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) {
@@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
                }
        }
 
+       if (cpumask_empty(non_housekeeping_mask))
+               goto free_housekeeping_staging;
+
        if (!housekeeping.flags) {
                /* First setup call ("nohz_full=" or "isolcpus=") */
                enum hk_type type;