[PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full

[Oleg Nesterov]

Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not
include the boot CPU, this is no longer true after the commit 08ae95f4fd3b
("nohz_full: Allow the boot CPU to be nohz_full").

However after another commit aae17ebb53cd ("workqueue: Avoid using isolated
cpus' timers on queue_delayed_work") the kernel will crash at boot time in
this case; housekeeping_any_cpu() returns an invalid cpu nr until smp_init()
paths bring the 1st housekeeping CPU up.

Change housekeeping_any_cpu() to check the result of cpumask_any_and() and
return smp_processor_id() in this case. Yes, this is just the simple and
backportable workaround which fixes the symptom, but smp_processor_id() at
boot time should be safe at least for type == HK_TYPE_TIMER, this more or
less matches the tick_do_timer_boot_cpu logic.

We should not worry about cpu_down(); tick_nohz_cpu_down() will not allow
to offline tick_do_timer_cpu (the 1st online housekeeping CPU).

Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")
Reported-by: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20240402105847.GA24832@xxxxxxxxxx/
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 Documentation/timers/no_hz.rst |  7 ++-----
 kernel/sched/isolation.c       | 11 ++++++++++-
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
index f8786be15183..7fe8ef9718d8 100644
--- a/Documentation/timers/no_hz.rst
+++ b/Documentation/timers/no_hz.rst
@@ -129,11 +129,8 @@ adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
 online to handle timekeeping tasks in order to ensure that system
 calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
 (This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
-user processes to observe slight drifts in clock rate.)  Therefore, the
-boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
-"nohz_full=" mask that includes the boot CPU will result in a boot-time
-error message, and the boot CPU will be removed from the mask.  Note that
-this means that your system must have at least two CPUs in order for
+user processes to observe slight drifts in clock rate.) Note that this
+means that your system must have at least two CPUs in order for
 CONFIG_NO_HZ_FULL=y to do anything for you.
 
 Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 373d42c707bc..2a262d3ecb3d 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type)
 			if (cpu < nr_cpu_ids)
 				return cpu;
 
-			return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
+			cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
+			if (likely(cpu < nr_cpu_ids))
+				return cpu;
+			/*
+			 * Unless we have another problem this can only happen
+			 * at boot time before start_secondary() brings the 1st
+			 * housekeeping CPU up.
+			 */
+			WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
+				     type != HK_TYPE_TIMER);
 		}
 	}
 	return smp_processor_id();
-- 
2.25.1.362.g51ebf55