cpu/hotplug: Add debug printks for hotplug callback failures

CPU hotplug callbacks can fail and cause a rollback to the previous
state. These failures are silent and therefore hard to debug.

Add pr_debug() to the up and down paths which provide information about the
error code, the CPU and the failed state. The debug printks can be enabled
via kernel command line or sysfs.

[ tglx: Adopt to current mainline, massage printk and changelog ]
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Link: https://lore.kernel.org/r/20210409055316.1709-1-dongli.zhang@oracle.com

kernel/cpu.c

@@ -692,6 +692,10 @@ static int cpuhp_up_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
 
 	ret = cpuhp_invoke_callback_range(true, cpu, st, target);
 	if (ret) {
+		pr_debug("CPU UP failed (%d) CPU %u state %s (%d)\n",
+			 ret, cpu, cpuhp_get_step(st->state)->name,
+			 st->state);
+
 		cpuhp_reset_state(st, prev_state);
 		if (can_rollback_cpu(st))
 			WARN_ON(cpuhp_invoke_callback_range(false, cpu, st,
@@ -1091,6 +1095,9 @@ static int cpuhp_down_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
 
 	ret = cpuhp_invoke_callback_range(false, cpu, st, target);
 	if (ret) {
+		pr_debug("CPU DOWN failed (%d) CPU %u state %s (%d)\n",
+			 ret, cpu, cpuhp_get_step(st->state)->name,
+			 st->state);
 
 		cpuhp_reset_state(st, prev_state);